An Introduction to Transducers and Instrumentation

Citation preview

An Introduction to Transducers and Instrumentation

Curriculum Manual IT02

©2007 LJ Create. This publication is copyright and no part of it may be adapted or reproduced in any material form except with the prior written permission of LJ Create.

Lesson Module: 17.50 Version 0 Issue: ME706/F

IT02 An Introduction to Transducers and Instrumentation Curriculum Manual Addendum Sheet

Addendum Sheet

Please note that the following warning label has now been added to the D1750 trainer. This is to indicate the area of moving parts, and that fingers should be kept clear.

Keep fingers clear of all moving parts

Technical Publications Department LJ Create

IT02 An Introduction to Transducers and Instrumentation Curriculum Manual Contents

Chapter

Contents

Pages

Introduction

............................................................................................. i - iv

Basic Control Systems

Chapter 1

Basic Control Systems Equipment and Terms Used ..........1 - 16

Input Transducers

Chapter 2

Positional Resistance Transducers ...................................17 - 32

Chapter 3

Wheatstone Bridge Measurements ...................................33 - 52

Chapter 4

Temperature Sensors ........................................................53 - 76

Chapter 5

Light Measurement...........................................................77 - 98

Chapter 6

Linear Position or Force Applications............................99 - 114

Chapter 7

Environmental Measurement .......................................115 - 126

Chapter 8

Rotational Speed or Position Measurement .................127 - 152

Chapter 9

Sound Measurements ...................................................153 - 162

Output Transducers

Chapter 10

Sound Output................................................................163 - 172

Chapter 11

Linear or Rotational Motion.........................................173 - 190

Display Devices

Chapter 12

Display Devices............................................................191 - 208

Signal Conditioning Circuits

Chapter 13

Signal Conditioning Amplifiers ...................................209 - 234

Chapter 14

Signal Conversions.......................................................235 - 252

Chapter 15

Comparators, Oscillators and Filters ............................253 - 270

Chapter 16

Mathematical Operations .............................................271 - 290

Closed Loop Control Systems

Chapter 17

Control System Characteristics ....................................291 - 300

Chapter 18

Practical Control Systems ............................................301 - 334

An Introduction to Transducers and Instrumentation IT02 Contents Curriculum Manual

Appendices

Appendix A

Using a Multimeter ...................................................... 335 - 340

Appendix B

The Oscilloscope ......................................................... 341 - 362

IT02 An Introduction to Transducers and Instrumentation Curriculum Manual Introduction

Introduction

Introduction This comprehensive course of study is based on a single panel Transducer and Instrumentation Trainer, the DIGIAC 1750. The D1750 unit provides examples of a full range of input and output transducers, signal conditioning circuits and display devices. The unit is self-contained and enables the characteristics of many individual devices to be investigated, building to form complete closed loop systems. As each item is introduced there is a description of the principles of the device, together with practical exercises to illustrate its characteristics and applications. The treatment is non-mathematical and little previous knowledge is assumed, although it is expected that students will have a basic knowledge of electrical circuits and units, and electronic components and devices. It is the intention that at the end of this course the student will, with the knowledge gained, be able to select suitable components and interconnect them to form required closed-loop systems. Although the course has been laid out progressively it is sometimes necessary to make use of a device before a full investigation has been carried out. For instance, in order to investigate any input transducer, an input signal may be needed. This signal may be provided by one of the output transducers not yet covered. Also signal conditioning and display devices will be needed from an early stage. In the event of any difficulty, it is recommended that the student should skip forward to the relevant section to obtain further information.

i

An Introduction to Transducers and Instrumentation IT02 Introduction Curriculum Manual

Test Instruments A digital multimeter will be required when working through this module. The meter must have ranges to cover at least: DC voltage: DC current: Resistance:

200mV to 20V 1mA to 100mA 10Ω to 10MΩ

To complete the exercises you will need to be familiar with connecting, setting the range and obtaining readings from multimeters. If you are not familiar with the use of these instruments please refer first to Appendix A before carrying out any exercises. Note that some of the exercises in Chapter 4 (Temperature Measurement) will require the use of two multimeters. Some examinations of voltage waveforms will be required using an oscilloscope. You will be expected to be able to make the necessary adjustments and settings to obtain time related sketches of the waveforms examined. Recommendations for the settings of the various controls will be given where appropriate. Again, if you are not familiar with this instrument or the applications of it, please refer to Appendix B before attempting the relevant exercise. A function generator will be required to provide sinewave and square wave inputs to some circuits. This should have a range of frequencies covering at least 10Hz 1MHz, and output of 20Vp-p (with an internal attenuator to allow amplitude settings), and an output impedance of 50 Ω. The output lead should be terminated in standard 4mm banana plugs for ease of connection directly to the D1750 Trainer panel.

ii

IT02 An Introduction to Transducers and Instrumentation Curriculum Manual Introduction

The Module Power Supplies The D1750 Transducer and Instrumentation Trainer contains all of the power supplies needed to make it operate. You can switch these power supplies ON and OFF with the Power Supplies switch located on the rear panel.

Making Circuit Connections During each Practical Exercise in this manual, you will be asked to make circuit connections using the 4mm Patching Cords. Whenever you make (or change) circuit connections, it is good practice to always do so with the Power Supplies switch in the OFF position. You should switch the Power Supplies ON only after you have made, and checked, your connections. Remember that the Power Supplies switch must be ON in order for you to be able to make the observations and measurements required in the Exercise. At the end of each Exercise, you should return the 'Power Supplies' switch to the 'OFF' position before you dismantle your circuit connections.

Your Workstation Depending on the laboratory environment in which you are working, your workstation may, or may not, be computer managed. This will affect the way that you use this laboratory manual. If you are in any doubt about whether your workstation is computer managed, you should consult your instructor.

iii

An Introduction to Transducers and Instrumentation IT02 Introduction Curriculum Manual

Using this Manual at a Computer Managed Workstation In order to use this curriculum manual at a computer managed workstation you will require one of the following items: ♦ ♦

D3000 Hand-held Data Terminal. A personal computer (PC) that has been installed with computer managed student workstation software.

If you are working in a computer managed environment for the first time, you should first read the operating information that has been provided with your computer managed workstation. This tell you how to: ♦ ♦ ♦ ♦

Log onto the management system and request work. Make responses to questions in a computer managed environment. Hand in your work when completed. Log off at the end of your work session.

Whenever you see the symbol in the left-hand margin of this Curriculum Manual, you are required to respond to questions using your computer managed workstation. You should also record your responses so that you can review them at any time in the future. The following D3000 Lesson Module is available for use with this Curriculum Manual: D3000 Lesson Module 17.50

Using this Manual at a Workstation that is

not Computer

Managed

Whenever you see the symbol in the left-hand margin of this Curriculum Manual, you are required to answer a question. If your workstation is not computer managed, you should record your answer so that it can be subsequently marked by your instructor.

Good luck with your Studies.

iv

IT02 Basic Control Systems Equipment and Terms Used Curriculum Manual Chapter 1

Chapter 1 Basic Control Systems Equipment and Terms Used

Objectives of this Chapter

Having studied this Chapter you will be able to: 

State the difference between open loop and closed loop systems.



Write the expression for the overall gain of a negative feedback closed loop system.



Calculate the overall gain of a negative feedback closed loop system from given information.



List the basic components of a closed loop system and explain their functions.



Explain the meaning of terms associated with control system equipment.

1

Basic Control Systems Equipment and Terms Used IT02 Chapter 1 Curriculum Manual

1.1

Open Loop System Figure 1.1 represents a block diagram of an open loop system. A reference input, or command signal, is fed to an actuator which operates on the controlled variable to produce an output.

Reference I/P (Command Signal)

Actuator

Controlled Variable

O/P

Fig 1.1

The output magnitude depends on the magnitude of the reference input signal but the actual output magnitude for a particular input may not remain constant but may vary due to changes within or exterior to the system. For example, in a simple room heating application, a heater set for a certain output will result in a certain room temperature. The actual temperature will depend on the ambient temperature outside the room and also whether the doors and windows are open or closed.

2

IT02 Basic Control Systems Equipment and Terms Used Curriculum Manual Chapter 1

1.2

Closed Loop System Figure 1.2 shows a basic block diagram of a closed loop control system. With this system, the output magnitude is sensed, fed back and compared with the desired value as represented by the reference input. Any error signal is fed to the actuator to vary the controlled variable to reduce this error. Reference I/P Error Detector

Feedback signal

Actuator

Controlled Variable

O/P

Sensor

Fig 1.2

The system thus tends to maintain a constant output magnitude for a fixed magnitude input reference signal. The feedback signal is effectively subtracted from the reference signal input to obtain the error signal and hence the system is referred to as a negative feedback system. The magnitude of the reference signal required for a particular output magnitude for a closed loop system will be greater than that required for open loop operation because the negative feedback reduces the overall gain of the system.

3

Basic Control Systems Equipment and Terms Used IT02 Chapter 1 Curriculum Manual

1.3

Gain in an Open Loop System

Gain G

Input Vi

Output Vo

Fig 1.3

Output Vo = G Vi

1.4

Gain = G

Gain in a Closed Loop System Error (Vi - HVo)

Input Vi

Feedback (HVo)

Gain G

Output Vo

Attenuator H

Fig 1.4

H = the fraction of the output fed back to the input The error signal = Vi - HVo The output Vo = G(Vi - HVo) = GVi - GHVo Vo + GHVo = GVi Vo(1 + GH) = GVi Vo G = Vi 1 + GH i.e.

4

Gain =

G 1 + GH

IT02 Basic Control Systems Equipment and Terms Used Curriculum Manual Chapter 1

The gain is therefore reduced, and, if the gain G is very large, the formula simplifies to:G 1 Gain = = GH H If the gain of the amplifier (G) is high then the overall system gain is dependent only on the feedback fraction H.

1.5

Examples

1.5a

(i)

An amplifier has a gain (G) of 15 and a feedback loop with an attenuation 1 fraction (H) of . 30 Vo The loop gain of the system will be: Vi 15 15 G = = = 10 1 1 1 + GH 1 + 15 1+ 30 2

(ii)

An amplifier with a gain of 100 has 10% negative feedback (H = 0.1). Vo The loop gain of the system will be: Vi G 100 = = 100 = 9.1 1 + GH 1 + 100 × 0.1 1 + 10 1 Note that = 10, which is very nearly the same as the loop gain. H

An amplifier with a gain (G) of 20 has a feedback fraction (H) of 0.15. The loop gain of the system will be:

a 3

b

4

c

5

d

6.67

5

Basic Control Systems Equipment and Terms Used IT02 Chapter 1 Curriculum Manual

1.6

Practical Closed Loop Control System Figure 1.5 shows a block diagram of a practical closed loop control system. This shows signal conditioning blocks in the signal paths between the error detector and the actuator and between the sensor and the error detector.

Reference I/P

Error Detector

Signal Conditioning Signal Conditioning

Actuator

Controlled Variable

O/P

Sensor

Signal Conditioning

Display

Fig 1.5

It also shows a display which indicates the magnitude of the output variable and includes a signal conditioning block in the display path. Signal conditioning may consist of signal amplification, attenuation or linearising, waveform filtering or modification, conversion from analog to digital form, or may be a matching circuit. These may be necessary to convert the output from one circuit into a form suitable for the input to the following circuit, or to improve the system accuracy.

6

IT02 Basic Control Systems Equipment and Terms Used Curriculum Manual Chapter 1

1.7

Controlled Variables For a particular industrial process there may be more than one controlled variable and each of the controlled variables will have its own closed loop control system. The controlled variable may be:-

Position (angular or linear) Temperature Pressure Flow rate Humidity Speed (angular or linear) Acceleration Light level Sound level The control system may operate using pneumatic, hydraulic or electrical principles and the sensors used for the measurement of the controlled variable must provide an output signal in a form suitable for the system in use. This will normally involve a conversion from one energy system to another and devices used to accomplish this energy conversion are referred to a TRANSDUCERS. Sensors and actuators are both forms of transducer, sensors representing input transducers and actuators representing output transducers. The DIGIAC 1750 unit is an electrical system and includes a full range of sensors, actuators, signal conditioning circuits and display devices. Used with this manual, the unit will introduce the student to the basic principles and characteristics of a comprehensive range of transducers and their application to practical closed loop control systems.

7

IT02 Basic Control Systems Equipment and Terms Used Curriculum Manual Chapter 1

1.8

Glossary of Terms - Transducers Transducer:

A device which converts information from one energy system to another.

Sensor:

A device which senses, or measures, the magnitude of system variables. Normally also convert thetransducers. measured quantity into another energy system andthey hence they are also

Actuator:

A device which accepts an input in one system and converts it into another energy system, which is normally mechanical. These devices are also transducers.

Specification:

Data specifying the performance capabilities and requirements of equipment.

Accuracy:

The error present in a measurement as compared to the true value of the quantity.

Sensitivity:

The ratio of the output of a device compared to the magnitude of the input quantity.

Resolution:

The largest change in the input that produces no detectable change in the output; for example, the degree to which a system can distinguish

Range: Bandwidth:

between adjacent values or settings. A statement of the values over which the device can be used and within which the accuracy is within the stated specification. The range of input signal frequencies over which a device or circuit is capable of being operated while providing an output within its stated specification.

Transfer function: The mathematical relationship between two variables that are related. Normally the relationship between the input and output of a system. Linear:

A relationship between two quantities that have a constant ratio; for example, a graphical straight line relationship.

Non linear:

A relationship between two quantities that cannot be described by a linear relationship.

Linearity: Response time:

A measure of the deviation of a measurement from an ideal straight line response of the same measurement over the same range. The time taken for the output to reach, or be within a rated percentage of, a new final value, after the input has been changed.

9

Basic Control Systems Equipment and Terms Used IT02 Chapter 1 Curriculum Manual

1.9

Glossary of Terms - Signal Conditioning Circuits Amplifier:

A circuit having an input and output that are related linearly and with the output greater than the input. The circuit may operate on both DC and AC circuits

Offset:

For amplifier, input zero,these the output may not be zero. ThisaisDC referred to aswith the the offset. With amplifiers, a control is provided and labeled: "Offset" or "Set Zero" to set the output to zero with the input zero, before the amplifier is used.

Gain:

The ratio of output to input for a circuit.

Attenuator:

A circuit having an input and an output that are related linearly and having an output less than the input.

AC Amplifier:

An amplifier that will amplify alternating signals only.

Differential Amplifier: A voltage amplifier having two inputs and where the output voltage magnitude is proportional to the difference in voltages between the two inputs.

10

Summing Amplifier:

A voltage amplifier having multiple inputs, the output being proportional to the sum of the various applied inputs.

Inverter:

A voltage amplifier having the polarity of the output the reverse of the input. The output magnitude may be the same as the input (gain of -1), or there may be voltage gain associated with the polarity reversal.

Power Amplifier:

An amplifier with a large current output capability.

Buffer Amplifier:

An amplifier having unity gain (output = input), and having a high input impedance and a low output impedance.

Comparator:

A circuit having two inputs A & B and an output that can be in one of two possible states depending on the magnitudes of the inputs. With input A greater than B, the output will be in one state (possibly high voltage). With input A less than B, the output will be in the alternative state (low voltage).

Oscillator:

A circuit producing an alternating output at a particular frequency.

Alarm Oscillator:

A circuit having an input and an output. With the input magnitude below a certain level, the output is zero. When the input exceeds the threshold the output is an alternating voltage.

IT02 Basic Control Systems Equipment and Terms Used Curriculum Manual Chapter 1

The transfer characteristic of a non-linear device for increasing input voltages may be different from the characteristic for decreasing input voltages. The result is a 'hysteresis loop,' as shown in figure 1.7(a) below. For a switching circuit, the term 'hysteresis' normally refers to the input switching voltages. The input voltage to cause switching for rising input voltages is arranged to be higher

Hysteresis:

than that to produce switching for falling input voltages (see figure 1.7(b) below). The difference between the input voltages is referred to as the hysteresis.

Output

Output for fallinginput

Rising Voltage I/P Output for risinginput

Hysteresis Voltage

Switchin Levels

Falling Voltage

Input (a)

(b)

Fig 1.7

Latch:

A circuit having two possible output states depending on the magnitude of the input voltage. When operated with the input level sufficient to change the output to its alternative state, the output is held (or latched) in this state irrespective of the subsequent magnitude of the input voltage.

Filter:

Circuit designed to allow signals of a selected frequency range to pass through and stop all others.

Low Pass Filter:

A circuit allowing low frequency signals to pass while blocking the passage of higher frequencies.

High Pass Filter:

A circuit allowing high frequency signals to pass while blocking the passage of lower frequencies.

11

Basic Control Systems Equipment and Terms Used IT02 Chapter 1 Curriculum Manual

12

Band Pass Filter:

A circuit allowing signals over a selected range of frequencies to pass while blocking the passage of signals at both lower and higher frequencies.

Full-Wave Rectifier:

A circuit converting an alternating waveform into a unidirectional or DC waveform.

V/F Converter:

A circuit converting a DC input voltage to an alternating voltage, the frequency being dependent on the magnitude of the DC input voltage.

F/V Converter:

A circuit converting an alternating input voltage to a direct voltage output, the output voltage magnitude being proportional to the frequency of the input voltage.

V/I Converter:

A circuit converting an direct input voltage into an output current, the current magnitude depending on the input voltage.

I/V Converter:

A circuit converting an input current into an output voltage, the voltage magnitude being dependent on the magnitude of the input current.

Integrator:

A circuit having an output voltage that is proportional to the product (input voltage x time).

Differentiator:

A circuit having an output voltage that is proportional to the rateof-change of the input voltage.

Sample and Hold:

A circuit with input and output. In the sample state, the output voltage is equal to and follows any changes in the input voltage. In the hold state, the output voltage is held at the value of the input signal at the instant the "hold" signal was initiated.

Ultrasonic:

A signal at a frequency above the normal audio range and hence inaudible to the human ear (normally >16kHz).

IT02 Basic Control Systems Equipment and Terms Used Curriculum Manual Chapter 1

Student Assessment 1

Reference I/P 1

2

Actuator

4

3

O/P

Sensor

5 Practical Closed Loop Control System

6 Fig 1

1.

2.

3.

4.

In Fig 1 block numbered 1 is a:

a

signal conditioner

b

display

c

controlled variable

d

error detector

In Fig 1 block numbered 4 is a:

a

signal conditioner

b

display

c

controlled variable

d

error detector

In Fig 1 block numbered 6 is a:

a

signal conditioner

b

display

c

controlled variable

d

error detector

A closed loop control system has an open loop gain G and a negative feedback factor H. The expression for the overall gain of the system with feedback applied is: H G 1+ GH a b 1 + GH c d 1+ GH 1 + GH G

Continued ...

13

Basic Control Systems Equipment and Terms Used IT02 Chapter 1 Curriculum Manual

Student Assessment 1 Continued ...

5.

An electrical control system produces an output of 50V for a reference input of 1V under open loop conditions. If a fraction 0.1 of the output is fed back as negative feedback to the input, the input reference voltage now required to produce the same output of 50V is:

a 6.

7.

8.

b

8.33V

c

6V

a

waveform modification

b

error detection

c

amplification

d

attenuation

The term accuracy refers to the:

a

conversion of energy from one form to another

b

largest change of input that produces no change in the output

c

error present in a measurement compared to the true value

d

mathematical relationship between the input and output of a system

A device has an output that is variable in 250 equal increments and has a maximum output of 10V. The resolution of the device is:

0.25V

b

0.1V

c

0.04V

d 0.4V

A filter which passes all signals at frequencies above 10kHz is called a:

a

high pass filter

b

band pass filter

c

low pass filter

10. An example of a band pass filter response is one which will:

14

d 5V

Which of the following is NOT a function of a signal conditioning circuit?

a 9.

10V

a

pass all signals at frequencies below 10kHz

b

pass all signals at frequencies above 12kHz

c d

pass all signals at frequencies below 10kHz and above 12kHz pass all signals at frequencies below 12kHz and above 10kHz

d

band stop filter

IT02 Basic Control Systems Equipment and Terms Used Curriculum Manual Chapter 1

Student Assessment 1 Continued ...

11. An electrical comparator is a circuit with:

a

one input and one output

b

two inputs and one output

c

one input and two outputs

d

two inputs and two outputs

12. A comparator with hysteresis is one which will:

a

hold the output state after the input voltages are removed

b

change output state at precisely the same differential between the input voltages

c

change output state at different rising input voltage to falling input voltage

d

change output state if the input signals are within the correct range of frequencies

13. A comparator with latch is one which will:

a

hold the output state after the input voltages are removed

b

change output state at precisely the same differential between the input voltages

c

change output state at different rising input voltage to falling input voltage

d

change output state if the input signals are within the correct range of frequencies

14. An open-loop control system is one in which:

a

there is not feedback from output back to input

b

negative feedback is applied from output to input

c

positive feedback is applied from output to input

d

there is no connection between input and output

15

Basic Control Systems Equipment and Terms Used IT02 Chapter 1 Curriculum Manual

16

IT02 Curriculum Manual

Positional Resistance Transducers Chapter 2

Chapter 2 Positional Resistance Transducers

Objectives of this Chapter

Having studied this Chapter you will be able to: 

Describe the basic construction of rotary and slider variable resistors.



State that the resistance section may be either a carbon track or wirewound.



Describe the difference between a logarithmic and a linear track.



Draw the basic characteristics of output voltage against variable control setting.



Describe the effect on the output voltage of loading the output circuit.



Compare the application of a carbon track variable resistor to the wirewound type.

Equipment Required for

• •

DIGIAC 1750 Transducer and Instrumentation Trainer. 4mm Connecting Leads.

this Chapter

•

Digital Multimeter.

17

Positional Resistance Transducers Chapter 2

2.1

IT02 Curriculum Manual

Variable Resistor Construction A variable resistor consists of a "track" having a fixed overall resistance with a "wiper" which can be moved to make contact with any point along the track. In the carbon type, the total track resistance is varied by adjusting the proportion of non-conducting material to carbon in the compound during manufacture. This will produce a track of constant resistance along its length, so that any section of the track will have the same resistance as any other similar section. The track will be linear. Variable resistors intended for use in audio applications, where subjective appreciation of sound amplitude (loudness) is proportional to logarithmic scales, are made with similar logarithmic (non-linear) scales. The resistance along the track is not a linear relationship, increasing with the square of the rotation of the spindle, or movement of the slide wiper (R ∝ S2, where S is the setting of the wiper). A close approximation is made to the ideal logarithmic characteristic by using three or four sections of track with different resistance slopes. Non-Linear variable resistors are not suitable as positional transducers and are therefore not included on the DIGIAC 1750 Trainer facilities. The track can be laid out on a rotary or a straight base, as in Fig 2.1. CarbonTracks

Wiper

Carbon Track

Wirewound Carbon Track

Wiper

Wiper

Wire Coil

Metal strip Connections

Slider Type

Connections

Connections

Rotary Types

Fig 2.1

For higher power applications the track may be wire wound, with the wiper making contact with the top edge of a coil of resistance wire.

18

IT02 Curriculum Manual

2.2

Positional Resistance Transducers Chapter 2

Linear Variable Resistor Characteristics A variable resistor can be used to provide a variable voltage. A steady voltage is applied across the ends of the fixed track. The wiper then picks off a variable voltage at the contact point with the track (with respect to the end of the track). Used in this way the variable resistor is called a potentiometer.

+5V

10 9 8 7 6 5 4 3

0V

+5V

Variable Contact Setting

O/P

O/P

2 1

0V

1 2 3 4 5 6 7 891 0 Variable Contact Setting

Fig 2.2

With a dual polarity voltage source, the polarity and magnitude of the output voltage will depend on the direction of movement of the wiper from its central position, as shown in Fig 2.3.

+5V

0V

10 9 8 7 6 5 4 3

-5V

2 1

+5V

Variable Contact Setting

O/P 0V 1 2 3 4 5 6 7 891 0 Variable Contact Setting

O/P -5V

Fig 2.3

Note that the position of the variable resistor spindle (or slider) setting is indicated by the output voltage from the potentiometer.

19

Positional Resistance Transducers Chapter 2

2.3

IT02 Curriculum Manual

Practical Exercise Variation of Output Voltage with Setting of Rotary Potentiometer C +12V

CARBON TRACK 5 4

B

Digital Meter

C

6 7

3

8

2

9

B A

10

1

V

100k

V 0V 0V

Digital Meter +12V

A

SchematicDiagram

PhysicalLayoutDiagram

Fig 2.4 

Locate the 100k Ω variable resistor on the DIGIAC 1750 Trainer (bottom left-hand corner). Connect the circuit as shown in Fig 2.4 using the power supply facilities at the bottom of the panel and the 20V DC range of a digital multimeter.



Set the 100k Ω rotary resistor control fully counter-clockwise to setting 1 as shown in Fig 2.4. Note that the dial is not marked with numbers on the printed panel. These numbers have been shown in Fig 2.4 to make it easier to follow these instructions and collate results.



After ensuring that the voltage adjustment is correctly set switch ON the power supply (switch on the rear of the unit just above the main power socket).



Note the output voltage as indicated on the digital multimeter and record in Table 2.1.

Control Setting

1

Output Voltage Table 2.1

20

2 V



V

3 V

4 V

5 V

6 V

7 V

V

8 V

9

10

V

Set the rotary control to "2" and repeat the reading, recording the result in again Table 2.1.

IT02 Curriculum Manual

Positional Resistance Transducers Chapter 2



Repeat the reading and recording for all other settings of the rotary control.



From the results recorded in Table 2.1 plot the characteristic of the 100k Ω variable resistor on graticule of Graph 2.1 below. 12 11 10 Output 9 Voltage (volts) 8

7 6 5 4 3 2 1 0

1

2

3

4

5

6

7

8 91 0 resistor setting

Graph 2.1 Characteristic of a Linear Rotary Carbon Potentiometer

Note that it is not easy to be precise with your setting of the variable resistor and this may result in the plotted points not following a smooth relationship. You should draw the best compromise to show the characteristic as you believe that it should be. At the ends of the track the wiper comes into contact with the terminal connections to the track, causing non-linearity at both ends. From setting 2 through setting 9 the variation of voltage should be fairly linear. Voltage across this section (V9 - V2) = Voltage per division ( 2.3a

V9

− V2

9-2

)=

V V

Enter your voltage per division. 

Switch OFF the power supply.

21

Positional Resistance Transducers Chapter 2

2.4

IT02 Curriculum Manual

Practical Exercise Variation of Output Voltage with Setting of Slide Potentiometer C

SLIDE

+5V

C B

1

B

V

-5V

2

4

5

6

7

8

9

10

A

10k

V

Digital Meter

Digital Meter

0V

A

3

-5V

0V

SchematicDiagram

+5V

PhysicalLayoutDiagram

Fig 2.5 

The 10k Ω slide potentiometer on the DIGIAC 1750 Trainer is just above the rotary potentiometers. Connect the circuit as shown in Fig 2.5 using the power supply facilities at the bottom of the panel and the 20V DC range of your digital multimeter.



Set the 10k Ω slide resistor control to the left to setting 1 as shown in Fig 2.5. Note that the marked numbers are again not on the printed panel.



Switch ON the power supply.



Note the output voltage as indicated on the digital multimeter and record in Table 2.2.

Control Setting

1

Output Voltage

2 V

V

3 V

4 V

5 V

6 V

7 V

V

8 V

9

10

V

Table 2.2  

22

Set the control to "2" and repeat the reading. Repeat the readings for all other settings of the slide control, recording the result in Table 2.2.

IT02 Curriculum Manual



Positional Resistance Transducers Chapter 2

From the results recorded in Table 2.2 plot the characteristic of the 10k Ω slide resistor with dual polarity supply on graticule of Graph 2.2 below. +5

Output +4 Voltage (volts) +3 +2 +1 0

-1 -2 -3 -4 -5

1

2

3

4

5

6

7

8

9

10

resistor setting

Graph 2.2 Characteristic of a Linear Slide Carbon Potentiometer 



Switch OFF theand power supplysupply and remove potentiometer the power panels.the connections between the slide Use the digital multimeter on a suitable range (20k Ω) to measure the resistance between terminal A and wiper B with the wiper set to position 9: Resistance R9 =



k

Move the wiper to position 2 and repeat the resistance measurement: Resistance R2 =

k

Resistance between settings 9 & 2 = R 9 - R2 = Voltage between settings 9 & 2 = V9 - V2 = Voltage per k 2.4a

=

V9 -V=2 (R 9 -R 2 )k Ω

k V

V/k

Enter your voltage per k

23

Positional Resistance Transducers Chapter 2

2.5

IT02 Curriculum Manual

Effect of Loading Consider a 10kΩ variable resistor connected to a 10V supply with the wiper in its central position. There will be a resistance of 5k Ω from the wiper to each end of the track (Fig 2.6(a)). If a 5kΩ fixed resistor is connected across the output then it will be in parallel with the lower half of the potentiometer (Fig 2.6(b)) and will draw current through the upper half of the potentiometer. This causes a higher voltage drop across the upper half of the track than the lower half (Fig 2.6(c)).

+10V

+10V

5k Ω

+10V

5k Ω

6.67V

5k Ω

3.33V O/P

5V O/P

5k Ω 0V

5k Ω

5k Ω

0V

(a)

2.5k Ω

3.33V

0V

(b)

(c)

Fig 2.6

Another way of looking at this is that the shunting effect of the 5k Ω load resistor is to reduce the total resistance of the lower half to 2.5k Ω (Fig 2.6(c)). Only one third of the applied voltage will be dropped across the lower half and two thirds across the upper. The variations of resistance as the wiper is moved will be quite complex and the voltage at the output will be non-linear.

24

IT02 Curriculum Manual

2.6

Positional Resistance Transducers Chapter 2

Practical Exercise Effect of Loading on the Potentiometer Output Voltage With the power supply switched OFF and no connections made to any components, measure the resistance of the 100k Ω rotary variable resistor between contact A and the wiper as it is set to the marked points on its scale. Use a suitable scale (200k Ω) on your digital multimeter and record the results in Table 2.3 overleaf in the row marked "Load Resistance".



The 100kΩ resistor is to be used as a load resistance across the output of a 10k Ω position sensing variable resistor. Connect the circuit as shown in Fig 2.7 but initially leave out the lead from contact C of the 100k Ω resistor to contact B of the 10k Ω so that the load is not connected across the output.



C +12V 5

CARBON TRACK 6 C

WIREWOUND TRACK 6 5 C

7

4

Omit lead at first

7

4

3

8

B

3

8

B

2

9

A

2

9

A

B 10k Ω

1

10

1

100k

10

V

Digital Meter

100k Ω 0V

0V

V Digital Meter

10k +12V

A

SchematicDiagram

Physical LayoutDiagram

Fig 2.7 

Switch the power supply ON and adjust the 10k Ω rotary resistor to give an output of 6V. Do not re-adjust this setting during the rest of this exercise.



Set the 100k Ω resistor fully clockwise (10) and connect the missing lead from contact C of the 100kΩ resistor to contact B of the 10k Ω so that the load is connected across the output of the positional sensor (10kΩ resistor).



Note the output voltage and record in Table 2.3.

25

Positional Resistance Transducers Chapter 2

Control Setting

10

Output Voltage

9 V

Load Resistance

8

V

kΩ

7

V

kΩ

IT02 Curriculum Manual

V

kΩ

6

5

V kΩ

V

4 V

kΩ

3

V

kΩ

V

kΩ

2

1

V kΩ

kΩ

kΩ

Table 2.3 

Change the setting of the 100k load resistor and record the effect as the load resistor is set to each marked position in Table 2.3.



From the information in Table 2.3, plot the characteristic of Output Voltage against Load Resistance on the graticule of Graph 2.3 below: 7 Output 6 Voltage (volts) 5

4 3 2 1 0

10

20

30

40

50

60

70

80

90

100

Load Resistance (k

Graph 2.3

C

MOVING COIL METER

+12V CARBON TRACK 6 5 C 7

4

8

3

B 10k Ω

9

2 1

V 100k Ω 0V

26

A

10k

Digital Meter

+

V 0V

0V

+12V

A

SchematicDiagram

Fig 2.8

Digital Meter

10

B

5 -10

PhysicalLayoutDiagram

L J

0

5 +10

IT02 Curriculum Manual

Positional Resistance Transducers Chapter 2

Do not alter the setting of the 10k

resistor.

With the Load Resistance (100k Ω resistor) removed from circuit connect the panel mounted Moving Coil Meter as in Fig 2.8 and switch ON the power supply.





Note the effect on the output voltage reading of having the analog type meter connected in circuit as well as the digital multimeter. Multimeter voltage reading with the Moving Coil Meter connected =

Compare this reading with the results on the characteristic curve of Graph 2.3 and read off the graph the loading resistance presented by the Moving Coil Meter to the output:



Loading resistance of the Moving Coil Meter = 2.6a

V

k

Enter your value of the loading resistance of the Moving Coil Meter in k .

What you have observed here is a problem which can be very misleading if you are not aware of the difficulties of using a low impedance meter to take measurements in a high impedance circuit. The problem can be overcome by using a Buffer Amplifier. MOVING COIL METER 5

CARBON TRACK 6 C

5

BUFFER #1

7

4

-10

3

8

2

9 1

I/P A

+10

+ +VIN

-

10k

0V 0V

5

O/P

B

10

0

+12V

V

L J

Fig 2.9 

Modify the circuit to include Buffer #1 as in Fig 2.9 and note the effect on the output voltage as indicated by both meters.

Output voltage = 2.6b 

V (digital)

V (analog)

Enter your value of analog output voltage with Buffer #1 in circuit in volts.

Switch OFF the power supply. 27

Positional Resistance Transducers Chapter 2

2.7

IT02 Curriculum Manual

Resolution Resolution has been defined as the largest change in the input which does not cause a change in the output. Alternatively it can be defined as the smallest change in input which does cause a change in output. For the carbon track resistor this value is very small the individual particles of carbon are tiny and variations of resistance can besince considered to be infinitely small. The resolution for a wirewound resistor is not so good, since, as the wiper is moved, it has to jump from one turn of the wire coil to the next. The output voltage therefore increases in steps equal to the applied voltage divided by the number of turns if the wiper only makes contact with one turn at a time. Wiper

1

2

3

4

5

(a)

1

2

3

4

5

(b)

1

2

3

4

5

(c)

Fig 2.10

This may not be quite the case, since the wiper may make contact with two or more turns at once as in Fig 2.10(b). The mathematical treatment of this will depend on the thickness of the wire (power rating) and the size of the wiper contact (current rating). Multi-turn wirewound tracks will largely overcome this problem.

2.8

Comparison of Carbon with Wirewound Track Carbon

Cheap Good Resolution Can be made miniature

28

Wirewound

High Current Ratings Durability (Reliability)

IT02 Curriculum Manual

2.9

Positional Resistance Transducers Chapter 2

Practical Exercise Servo Potentiometer A special positional potentiometer is mounted on the experiment board which has a very large arc of turning, approaching 360 °. It is called a Servo Potentiometer.

SERVO POTENTIOMETER

+5V

2.2kΩ

O/P

20kΩ V

-5V

0V

O/P

Schematic Diagram

V

Layout Diagram

0V

Fig 2.11

To bring the potentiometer scale into contact with the drive wheel on the shaft, press and release the mounting at the point arrowed in Fig 2.11. The potentiometer can then be turned manually with the shaft, using one of the large wheels, such as the Hall Effect Sensor Disk. The potentiometer can be turned directly from the dial, manually, if preferred. The ±5V input voltages to the Servo Potentiometer are connected internally. 

Connect a digital multimeter on the 20V DC range to the output of the potentiometer as shown in Fig 2.11.



Turn the potentiometer to find the maximum positive output voltage position. Note the value of this voltage and the angle, as given on the potentiometer dial, in the first column of Table 2.4 overleaf.

29

Positional Resistance Transducers Chapter 2

150

Control Dial Setting Output Voltage

V

V

120

V

90

V

IT02 Curriculum Manual

60

V

360 0

30

V

V

330 -30

V

300 -60

270 -90

V

V

240 -120

V

210 -150

V

V

Table 2.4 

Rotate the dial in steps of 30 ° clockwise from the maximum voltage position (beginning with 150°), noting the output voltage at each step and recording the values in Table 2.4.



At the final step note the angle from the dial setting and the value of the maximum negative voltage setting.



From the information recorded in Table 2.4, draw the characteristic of the output voltage/dial setting of the Servo Potentiometer on the graticule provided below:

Output Voltage

+5 +4 +3 +2 +1

-180 -150 -120 -90 -60 -30 30 60 90 120 150 180 -1 -2

Dial Setting

-3 -4 -5

Graph 2.4 Characteristic of the Servo Potentiometer

2.9a

30

Enter the dial setting in degrees for the maximum positive output voltage.

IT02 Curriculum Manual

Positional Resistance Transducers Chapter 2

Student Assessment 2 1.

2.

A log. scale (logarithmic) potentiometer is most suitable for use as a:

a

linear positional transducer

b

linear voltage controller

c

sound volume controller

d

rotary positional transducer

A variable resistor is stated to be wirewound. It is most likely to be:

a

linear because the resistance will be constant along the wire's length

b

linear as long as it is of the slide (not rotary) type

c

logarithmic because of the inductance of the coiled wire

d

non-linear because of the difficulty of making wire of a constant resistance 5

6 7

4

8

3

9

2 10

1

3.

A linear potentiometer (as in Fig 1 above) has control settings of 1-10 and is connected to a +9V, 0, -9V supply, with the -9V connection made to the end marked setting "1". The voltage at the setting marked "6" will be:

a 4.

-1V

b

0V

c

+1V

d +1.8V

A 10k linear potentiometer is connected to a 12V supply. With the output circuit loaded by a 5k load, the output voltage in the mid position (A) and maximum setting (B) will be:

a 5.

Fig 1

A = 6V, B = 12V b

A = 3V, B = 12V c

A = 4V, B = 8V

d A = 4V, B = 12V

A wirewound variable resistor consists of 1200 turns of wire. Assuming that the wiper only makes contact with one turn at any time, and the applied voltage across the track is 10V, the resolution in terms of the output voltage will be:

a

8.3mV

b

10mV

c

12mV

d 16.7mV

Continued...

31

Positional Resistance Transducers Chapter 2

IT02 Curriculum Manual

Student Assessment 2 Continued...

100kΩ A=1

10V

A

B

100kΩ

EMFSource

BufferAmplifier

Fig 2

6.

7.

The circuit of Fig 2 consists of a source of EMF of 10V with an output resistance of 100k . The Buffer Amplifier has a gain of 1 and an input impedance of 100k and is not initially connected to the source. What voltage would you expect to get at: A - with a digital multimeter with an input resistance of 10M (A1), A - with a moving coil meter with a resistance of 10k (A2), and - using the same moving coil meter as in A2 above but connected to the supply via the buffer amplifier (B):

a

A1 = 10V, A2 = 1V, B = 10V

b

A1 = 9.9V, A2 = 0.9V, B = 10V

c

A1 = 9.9V, A2 = 0.91V, B = 5V

d

A1 = 10V, A2 = 1V, B = 0.83V

Which of the following is NOT an advantage of the carbon track variable resistor when compared with the wirewound type?

a

32

cost

b

reliability

c

small size

d resolution

IT02 Curriculum Manual

Wheatstone Bridge Measurements Chapter 3

Chapter 3 Wheatstone Bridge Measurements

Objectives of this Chapter

Equipment Required for this Chapter

Having studied this Chapter you will be able to: 

State the principles of the basic Wheatstone Bridge circuit for resistance measurement.



Describe the term "null balance".



State and apply the expression for calculating an unknown resistance from the Bridge values at balance.



Discuss the factors affecting the resolution and accuracy of measurements.



Discuss the reason for the three-wire resistance circuit.



Apply null methods to voltage measurements.



Make resistance and voltage measurements using the DIGIAC 1750 facilities.

• • •

DIGIAC 1750 Transducer and Instrumentation Trainer. 4mm Connecting Leads. Digital Multimeter.

33

Wheatstone Bridge Measurements Chapter 3

3.1

IT02 Curriculum Manual

Wheatstone Bridge Circuit Fig 3.1 shows the basic Wheatstone Bridge circuit, consisting of four resistors and a sensitive center zero meter connected to a DC source.

I I2

I1 R1 DC Supply

Im

R3

R2 G R4

Fig 3.1

R1, R2 & R3 are accurate, close tolerance, resistors. R3 is variable and calibrated over its full range. R4 is the unknown resistor to be measured.

3.2

Null Balance During measurement, R3 is adjusted until there is no current (Im) flowing in the galvanometer circuit. The galvanometer current is zero or "null". Under these conditions, the bridge is said to be "balanced". Hence the term "null balance". The purpose of the galvanometer is to "detect"the presence of the null condition. From the known values of R1, R2 & R3 at balance, the value of R4 can be calculated from:R2 R4 = x R3 R1 The ratio of the values of resistors R2:R1 sets the range, so that values of the unknown resistor R4 which are larger or smaller than the variable resistor R3 can be measured. There is no limit to the range of values which can be measured. Any inaccuracy in the values of the ratio arm resistors R1 & R2, and also in the standard variable resistor R3, will result in errors in the measured value of R4. Since no current flows in the "null introduced by this part of the circuit.

34

detector"branch

at balance no error can be

IT02 Curriculum Manual

3.3

Wheatstone Bridge Measurements Chapter 3

Deriving the Formula With no current in the galvanometer circuit, the voltages at either end of it must be the same. This means that the voltages across R1 & R2 must be the same and similarly those across R3 & R4. With no current in the galvanometer, the current in R1 must be the same as that in R3 and the current in R2 must equal that in R4. If current I1 flows in R1 & R3 and current I 2 flows in R2 & R4:I1R1 = I2R2..................................................................... (i) I1R3 = I2R4.................................................................... (ii) Dividing:

(i) ÷ (ii) I 1 R1

=

I1 R3 ∴

R1

I 2 R4 =

R3 ∴

R4

I 2 R2

=

R2 R4 R2 R1

× R3

The unknown resistance R4 depends on the ratio R2:R1 and the value of R3 at balance. The resistors R1 and R2 are normally referred to as the "ratio arms"of the bridge. Note 1. The value of the supply voltage or the magnitude of the currents flowing in the resistors does not affect the result. This means that the supply voltage need not be stabilized, and that the circuit currents can be kept to low values for a component where the self heating effect of the current flowing could affect the result. 2. The galvanometer current accuracy is unimportant, since, under balanced

conditions, the current in it is zero. The main characteristics required for the galvanometer are a low resistance and a high sensitivity so that a small deviation of voltage from zero produces a large scale reading.

35

Wheatstone Bridge Measurements Chapter 3

3.4

IT02 Curriculum Manual

The Three Wire Resistance Measuring Circuit With some resistance transducer circuits, the transducer may be situated a relatively large distance from the bridge circuit, and the resistance of the connecting leads may be significant and could affect the results. For these situations the three wire connection arrangement is used.

R1

R2

DC Supply

G

R1

Long leads 2-wire

R3

DC Supply

R2 G

Long leads 3-wire

R3

R4

(a)

R4

(b)

Fig 3.2

Fig 3.2 (a) shows the circuit with a resistance transducer R4 situated remotely from the bridge and connected via two wires. The resistance of these wires will be included in the measurement of R4. Fig 3.2 (b) shows the three wire arrangement. One of the wires to the transducer is now included in the R2 circuit and the other is in the R4 circuit. The resistance of both circuits will therefore be increased equally and the effect on the balance condition will be minimized, provided that the resistances of R2 and R4 are of similar magnitudes. The extra wire in the galvanometer circuit will have no effect on the reading, since there is no current flowing in it at the balance condition.

36

IT02 Curriculum Manual

3.5

Wheatstone Bridge Measurements Chapter 3

The DIGIAC 1750 Facilities Fig 3.3 shows the Wheatstone Bridge layout provided with the DIGIAC 1750 unit.

WHEATSTONE BRIDGE

Fine Setting

D

unlock

12k

C

lock 3

A OUT

B

Coarse Setting

Switch

IN 1V

0V

Rx

Fig 3.3

A high quality 10-turn potentiometer fulfills the functions of the resistors R1 & R3 for resistance, or a potentiometer for voltage measurements. The track resistance of 10kΩ has a maximum non-linearity of 0.25%. The "Fine" dial is calibrated 0 - 100 in steps of 2, and the "Coarse" reading is calibrated 0 - 10, thus enabling readings to be estimated from the dial with a discrimination of 1:1000, representing a resolution of 10Ω. Reading the dial: If the number in the window (coarse setting) is 3 and the fine setting is on 74, then the dial reading is 374. The resistance between the 0V terminal and A (the wiper) is 10Ω x 374 = 3.74kΩ.

A close-tolerance 12kΩ resistor (R2) and an unknown resistor Rx (R4) are provided for resistance measurement. A switch open circuits the unknown resistor Rx to allow the measurement of other unknown resistors which can be connected between socket C and the 0V terminal. An accurate standard voltage of 1V is available at socket B. The moving coil meter can be used as a center zero indicating instrument. Since it is arranged as a 10V voltmeter its sensitivity is insufficient for a direct application as a galvanometer. This problem can be overcome by using a differential amplifier followed by a high gain DC amplifier from the signal conditioning circuits.

37

Wheatstone Bridge Measurements Chapter 3

3.6

IT02 Curriculum Manual

Practical Exercise Measurement of Resistance Fig 3.4 shows the layout diagram required for setting up the null detector.

DIFFERENTIAL AMPLIFIER O/P B

-

A

+

A-B

MOVING COIL METER

5

AMPLIFIER #2 I/ P

-10

O/ P

0

5 +10

+ -

.5

+

.4

OFFSET

.7

.8

.3

1 100 10

.6

.9

.2

GAIN COARSE

.1

1.0

0V

L J

GAIN FINE

Fig 3.4

Initially the amplifier and meter configuration which forms the sensitive galvanometer must be set up so that zero input produces zero output when the gain is set to maximum. 

Connect the meter and amplifiers as shown in Fig 3.4 with the + & - inputs to the Differential Amplifier short circuited so that the input is zero. Set the Amplifier #2 GAIN COARSE control to 10 and the GAIN FINE to 1.0.



Switch the power supply ON and adjust the OFFSET control so that the moving coil meter indicates approximately zero. Then set the GAIN COARSE control to 100 and re-adjust the OFFSET control for zero output precisely.

You will find that this adjustment is very sensitive. That is why you were instructed to obtain an approximate setting with the gain set to 10 first. Note The setting of the offset control may require adjustment as the temperature of the unit varies during use and it is advisable to use the above procedure to check and re-adjust as necessary at regular intervals.

38

IT02 Curriculum Manual

Wheatstone Bridge Measurements Chapter 3

WHEATSTONE BRIDGE D

+5V DIFFERENTIAL AMPLIFIER

12k

C A B

3

OUT

B

IN 1V

A-B

A

Rx

0V

O/P

+

MOVING COIL METER

5

AMPLIFIER #2 I/ P

-10

O/ P

0

5 +10

+ -

.5

+

.4 1 100 10

OFFSET

GAIN COARSE

.6 .7

.8

.3

.9

.2 .1

0V

1.0

L J

GAIN FINE

Fig 3.5 

With the switch on the Wheatstone bridge circuit set to IN (connecting the unknown resistor in circuit) set the Amplifier #2 GAIN COARSE control to 10 and connect the circuit as shown in Fig 3.5.



Adjust the control of the 10-turn variable resistor so that the moving coil meter reading is approximately zero, then set the GAIN COARSE control to 100. Finally adjust the 10-turn resistor control accurately for zero meter (null) reading to balance the bridge.

Reading the dial: If the number in the window (coarse setting) is 3 and the fine setting is on 74, then the dial reading is 374. 

Note the resistor dial reading (overleaf).

This represents the resistance R3 in the theoretical circuit considered earlier.

39

Wheatstone Bridge Measurements Chapter 3

IT02 Curriculum Manual

=

Dial reading Resistance R3 = 10 x dial reading

=

Resistance R1 = 10,000 - R3

=

Resistance R2 = 12,000Ω Unknown resistance Rx =

3.6a

R2 R1

x

R3

=

Enter your value for the unknown resistor Rx in k

Carry out further resistance measurements on the 10kΩ slide variable resistor to obtain familiarity with the equipment and its adjustments as follows: 



Set the Wheatstone Bridge switch to OUT to remove the unknown resistor Rx from the circuit. Connect the 10k Ω Slide variable resistor terminals A & B to the Wheatstone Bridge circuit connections C & 0V. With the 10k

Ω

resistor control set to maximum, measure its resistance as

follows:1. Check that the amplifier offset is set correctly and adjust if necessary.





2.

With Amplifier #2 GAIN COARSE control set to 10, obtain an approximate balance by adjusting the 10-turn resistor.

3.

Set Amplifier #2 GAIN COARSE control to 100 and obtain final balance. Note the dial reading and enter the value in Table 3.1.

Repeat the procedure to measure the resistance of the 10k Ω resistor for all settings from 9 through 1, recording the dial readings at balance in Table 3.1. Calculate the resistance corresponding with each reading, recording the results in Table 3.1. R2 is still 12kΩ.

Note: Since the quoted accuracy of the 10-turn variable resistor is 0.25%, this represents 1 part in 400. There is no reason for giving results to any more than four significant figures. 

40

Switch OFF the power supply.

IT02 Curriculum Manual

Wheatstone Bridge Measurements Chapter 3

10kΩ Resistor Setting

Dial Reading at Balance

R3 (10 x Dial)

10

R1 (10kΩ - R3)

R4 =

R2 R1

x

R3

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

9 8 7 6 5 4 3 2 1 Table 3.1

3.6b

Enter your value for the 10k

variable resistor at the setting 5 in k .

C

1kΩ B A Fig 3.6

Note that a 1kΩ resistor is connected in series with the wiper of all potentiometers on the D1750 Trainer. This prevents damage to the potentiometer in the event of back-driving the output with a voltage, which could otherwise cause a heavy current to flow as the wiper is moved towards terminal A.

41

Wheatstone Bridge Measurements Chapter 3

3.7

IT02 Curriculum Manual

Measurement of Voltage Method 1 A calibrated variable resistor, standard voltage source and galvanometer are required, these being connected as shown in Fig 3.7.

+ Unknown Voltage

G

Rt

-

R

+ -

Standard Voltage

Fig 3.7

The position of the slider of the variable resistor is adjusted until the circuit is balanced with no current flowing in the galvanometer. Under these conditions, the voltage across the R section of the variable resistance is equal to the value of the standard voltage supply. The unknown voltage is proportional to the total resistance of the variable resistor Rt and the section resistance R, and can be calculated from:Unknown voltage =

Rt R

x

Standard voltage

The method has disadvantages:1.

The unknown voltage source is loaded by the variable resistor and hence the voltage may be affected.

2.

The method only allows measurement of voltages greater than the standard voltage.

This method of measuring potential is the srcin of the term "potentiometer" for a variable resistor. Early models of this measuring instrument were made of a highly accurate, close tolerance, resistance wire which was stretched between terminals on a scaled background. It was known as a Slide-Wire Potentiometer.

42

IT02 Curriculum Manual

3.8

Wheatstone Bridge Measurements Chapter 3

Practical Exercise Measurement of Voltage Using Method 1 WHEATSTONE BRIDGE D

WIREWOUND TRACK 6 5 C

DIFFERENTIAL

7

4 3

8

2

9 1

C

B

B

0V

V

OUT

B

-

IN

A

+

A-B

10k 1V

0V

AMPLIFIER O/P

A 3

A

10

12k

Rx

+5V

MOVING COIL METER AMPLIFIER #2

5

I /P

-

O/P .5

+

.4 1 100 10

OFFSET

GAIN COARSE

.6

0

5 +10

+

.7

.3

.8

.2

.9 .1

-10

1.0

GAIN FINE

0V

L J

Fig 3.8 

First set the OFFSET control of Amplifier #2 using the same procedure used in Practical Exercise 3.6: Switch ON the power supply and with the Differential Amplifier inputs shorted together and Amplifier #2 GAIN FINE set to 1.0, adjust the OFFSET for approximately zero output with the GAIN COARSE set to 10. Adjust finally for zero with the GAIN COARSE set to 100.



Connect the circuit as shown in Fig 3.8 and set the switch on the Wheatstone Bridge circuit to OUT to disconnect the 12k Ω ratio arm resistor and the unknown resistor Rx from the circuit.



Set the Amplifier #2 GAIN COARSE to 10 and set the output from the 10k Ω wirewound resistor to 4V as indicated by the digital meter. This represents the "unknown" voltage.



Adjust the 10-turn resistor for approximate balance and then obtain final balance with Amplifier #2 GAIN COARSE set to 100.

43

Wheatstone Bridge Measurements Chapter 3



IT02 Curriculum Manual

Note the dial reading at balance, enter the value in Table 3.2 and calculate the value of the unknown voltage from:Unknown voltage =

1000 Dialreadin 1000

= Dialreadin 

x

x

Standard voltage

1V

Repeat the procedure with the "unknown" voltage input set to each of the values indicated in Table 3.2, recording the readings and calculating the voltages for each value. "Unknown" Voltage 4.0 3.5 3.0 2.5 2.0 1.5 1.0

Dial Reading at Balance

Calculated Voltage V V V V V V V

Table 3.2

3.8a

Enter your dial reading with the "unknown" voltage set to 2.5V.

The method has the disadvantage of loading the unknown voltage source and this can be demonstrated as follows:



44

Set the "unknown " voltage to 2.0V and obtain balance conditions. Now remove the connection from the output of the wirewound resistor (socket B) to the Wheatstone bridge (socket D) and note the revised value of the unknown voltage as indicated by the digital voltmeter.

IT02 Curriculum Manual

Wheatstone Bridge Measurements Chapter 3

"Unknown" Voltage:

3.8b

When connected to the bridge =

V

Disconnected from the bridge =

V

Enter your value of the "Unknown" Voltage when disconnected from the bridge in V.

3.9

Measurement of Voltage Method 2 This method requires an additional DC source of voltage with a magnitude exceeding the maximum value of the unknown voltages to be measured and another variable resistor Rs. The schematic diagram is shown in Fig 3.9.

Rs

+ G

Rt R

Standard + Voltage

+

Unknown Voltage

Fig 3.9

For measurement of voltages less than the standard voltage , the slider of the variable resistor is set to its maximum position and, with the galvanometer connected to the standard voltage source, the value of Rs is adjusted until there is no current flowing in the galvanometer and the circuit is balanced.

45

Wheatstone Bridge Measurements Chapter 3

IT02 Curriculum Manual

The full resistance Rt is then calibrated to represent the value of the standard voltage. To measure an unknown voltage, the galvanometer is connected to the unknown voltage and the slider position is again adjusted for circuit balance. The section R at balance represents the magnitude of the unknown voltage. Unknown voltage =

R Rt

x

Standard voltage

For the measurement of voltages higher than the standard voltage , the variable resistor can be calibrated against the standard voltage with the slider set to a position lower than the maximum setting. This setting will now represent a magnitude equal to the standard voltage. Balance with an unknown voltage is obtained as before and the unknown voltage calculated from :Unknown voltage =

R(unknown connected) R(standardconnected)

x

Standard voltage

With this method, no current is taken from the unknown voltage source at balance and hence the circuit is not loaded. The voltage obtained should therefore be accurate, within the limits of accuracy of the variable resistor.

46

IT02 Curriculum Manual

Wheatstone Bridge Measurements Chapter 3

3.10 Practical Exercise Measurement of Voltage Using Method 2 You should be familiar with the procedures for initially setting the amplifier offset and balancing the bridge circuit by now. Instructions for the procedures will not therefore be repeated in this exercise. Measurement of Voltages Less Than the Standard Voltage. 

Carry out the OFFSET initializing procedure and then connect the circuit as indicated in Fig 3.10, using the 100k Ω variable resistor as Rs (Fig 3.9) in the supply circuit of the additional DC source. Note that the output of the 10kΩ wirewound variable resistor is not connected initially. This will be used as the source of the "unknown" voltage.

WHEATSTONE BRIDGE D

AMPLIFIER #2

12k

C

I/P

DIFFERENTIAL AMPLIFIER O/P

A

-

OUT

3

B

B

-

A

+

IN 1V

0V

O/ P .5

+

Rx

OFFSET

.8

.3

.9

.2

GAIN COARSE

.6 .7

.4 1 100 10

A-B

1.0

.1

GAIN FINE

MOVING COIL METER 5

CARBON TRACK 6 C

WIREWOUND TRACK 6 5 C

7

4

8

2

9 1

B A

10

100k

5

7

4

3

3

8

2

9 1

10

-10

B A

10k

5 +10

+ 0V

0V

0

+5V

L J

V

Fig 3.10 

Set the 10-turn resistor to its maximum setting (1000) and adjust the setting of the 100kΩ resistor for balanced conditions, i.e. null indication on the moving coil (M.C.) meter. Set Amplifier #2 GAIN COARSE control to 10 initially and then finally to 100 during the balancing.

47

Wheatstone Bridge Measurements Chapter 3

IT02 Curriculum Manual

When completed, the 10-turn resistor has been calibrated so that full scale reading of 1000 represents a voltage of 1.000V. 

 

Replace the 1.0V reference voltage source (from the Wheatstone Bridge circuit) with the “unknown” voltage output of the 10kΩ wirewound variable resistor, by moving the lead that is connected to socket A of the Differential Amplifier FROM socket B of the Wheatstone Bridge circuit TO socket B of the 10kΩ wirewound variable resistor. Set the "unknown" voltage to 0.25V as indicated on the digital multimeter. Adjust the control of the 10-turn resistor for balance and note the dial reading for this balance condition. This reading will represent the unknown voltage directly in mV. Record the value in Table 3.3 and compare with the reading indicated by the digital multimeter.

"Unknown" Voltage Input

0.25V

Dial Reading at Balance

0.40V

mV

0.60V

mV

0.70V

mV

mV

0.80V mV

0.95V mV

Table 3.3 

Repeat the procedure for other "unknown" voltage inputs given in Table 3.3. 1000 Dial 900 Setting

800 700 600 500 400 300 200 100 0

Graph 3.1

48

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7 0.8 0.9 1.0 "Unknown" Voltage

IT02 Curriculum Manual

Plot the characteristic of Dial Reading against "unknown" input voltage on the graticule provided.



3.10a

Wheatstone Bridge Measurements Chapter 3

Read from your Graph 3.1 and enter the Dial Setting corresponding to an input voltage of 0.53V. Measurement of Voltages Greater Than the Standard Voltage. 

Remove the lead from socket C of the 10k Ω wirewound resistor to socket B of the 100kΩ resistor to remove the 1V supply.



Replace the 100k Ω resistor used for calibration with the 10k Ω slider unit and apply the +12V supply to this and the 10kΩ wirewound instead of the +5V.



Set the control dial of the 10-turn resistor to setting 0100 and connect the A socket of the Differential Amplifier back to socket B of the Wheatstone Bridge as shown in Fig 3.10.



Adjust the 10k Ω slider resistor control setting for bridge balance. When completed, the 10-turn resistance has been calibrated so that a dial reading of 0100 represents a voltage of 1.00V and a maximum dial reading of 1000 will represent a voltage of 10V.



Remove the 1.0V reference voltage source from socket A of the Differential Amplifier and connect the "unknown" voltage from socket B of the 10kΩ wirewound resistor to socket A of the Differential Amplifier.



Apply various "unknown" voltages in the range 0 - 10V to the circuit . Note the dial reading for balance for each input voltage setting and enter the values in Table 3.4.

"Unknown" Voltage Input Dial Reading at Balance Measured Voltage Table 3.4

(Volts)

1

2

V

3

V

4

V

6

V

8

V

9

V

V

49

Wheatstone Bridge Measurements Chapter 3

IT02 Curriculum Manual

Loading Effect 

Set the "unknown" input voltage to 5V and note the voltage change on the digital meter when the lead to the Differential Amplifier is removed. "Unknown" Voltage:

3.10b

When Connected to the bridge

=

V

Disconnected from the bridge

=

V

Enter the apparent change of the "Unknown" Voltage in mV when the amplifier is disconnected from the bridge.

The slight loading effect is due to the input resistance of the Differential Amplifier.

Notes:

........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ 50

IT02 Curriculum Manual

Wheatstone Bridge Measurements Chapter 3

Student Assessment 3

1000Ω +1V

100Ω G

0V Rx Unknown

853Ω

Fig 1

1.

2.

For the circuit of Fig 1, the name of the circuit is:

a

wirewound potentiometer

b

Wheatstone Bridge

c

slide wire potentiometer

d

carbon track variable resistor

When the circuit of Fig 1 is balanced, the value of the unknown resistor Rx is:

a 3.

4.

b

853

Ω

c

1172

Ω

d 8.53k

Ω

For the circuit of Fig 1, if the supply voltage was increased to +2V the effect on the balance condition would be to:

a

double the resistance value

b

half the resistance value

c

half the error in the measurement

d

make no change at all

For the circuit of Fig 1, which of the following components would NOT affect the accuracy of the measurement?

a 5.

85.3 Ω

100 Ω resistor

b

853 Ω resistor

c

1000 Ω resistor

d

galvanometer G

A resistance transducer is situated at a distance from the measuring bridge and is connected to it via just two wires, each of which has a resistance of 10 . If the resistance of the transducer is 120 , the bridge reading will be:

a

110 Ω

b

120

Ω

c

130

Ω

d 140

Ω

Continued ...

51

Wheatstone Bridge Measurements Chapter 3

IT02 Curriculum Manual

Student Assessment 3 Continued ... 6.

If the circuit connection described in question 5 used a three-wire system, the bridge reading would be:

a

110 Ω

b

Ω

120

c

Ω

130

d 140

Ω

Rs

7438Ω

+

+ G

V1

V1

2562 Ω

1.5V

+ -

R

Fig 2

7.

V2

V3

Fig 3

Fig 2 voltage, Fig 3 resistance both voltage

b d

Fig 3 voltage, Fig 2 resistance both resistance

When the circuit in Fig 2 is balanced, the value of V1 will be:

a 9.

+ +

The circuits of Fig 2 and Fig 3 are for measuring:

a c 8.

G

Rt

5.85V

b

4.35V

c

3.9V

d 0.52V

The circuit of Fig 3 is calibrated against a standard voltage (V2) of 1V to a dial setting of 0200. The dial has a discrimination of 1:1000. To be able to measure the maximum unknown voltage the value of the additional supply (V1) must be:

a

1V

c

=2V

d >5V

10. Which of the following is NOT an advantage of the circuit of Fig 3 compared to that of Fig 2?

a c

52

measures higher and lower voltages does not load the voltage source tested

b d

measures resistance draws no current from the source tested

IT02 Curriculum Manual

Temperature Measurement Chapter 4

Chapter 4 Temperature Measurement

Objectives of this Chapter

Equipment Required for this Chapter

Having studied this Chapter you will be able to: 

Describe the characteristics of an IC temperature sensor.



Describe the construction and characteristics of a Platinum RTD resistance transducer.



Describe the construction and characteristics of an NTC thermistor.



Discuss the characteristics of NTC thermistor bridge circuits.



Describe the construction and characteristics of a thermocouple.



Deduce temperatures from a voltage reading across a transducer.

• • •

DIGIAC 1750 Transducer and Instrumentation Trainer. 4mm Connecting Leads. 2 x Digital Multimeters.

• •

Stopwatch (not supplied). Scientific Calculator (not supplied).

53

Temperature Measurement Chapter 4

4.1

IT02 Curriculum Manual

The DIGIAC 1750 Temperature Transducer Facilities Fig 4.1 shows the layout of the temperature transducer facilities of the DIGIAC 1750 unit. The active transducers are mounted within a clear plastic enclosure which contains a heater.

REFERENCE THERMOCOUPLE (+ LM335)

TYPE 'K' THERMOCOUPLE

IC TEMP SENSORS EXT.

O/P

O/P

+ CLEAR PLASTIC ENCLOSURE

INT.

REF +

+

B

HEATER

O/P

O/P A NTCTHERMISTORS HEATER ELEMENT

PLATINUMR.T.D. I/P

Fig 4.1

The heated enclosure is provided to raise the temperature of the sensor transducers to allow measurements to be taken during experiments. In the case of the NTC thermistors and the thermocouples, an additional, separate unit is mounted outside the heated enclosure. The externally mounted sensors are made available for comparison between ambient (room) temperature and the temperature within the enclosure. The externally mounted "K" type thermocouple is contained within a package in contact with an IC temperature sensor (LM335) to act as a thermometer with voltage output. This will be used in many of the experiments as the reference (REF) thermometer. Note: It is important to give the unit time to cool down between the experiments. This will allow the enclosure to return to ambient temperature.

54

IT02 Curriculum Manual

4.2

Temperature Measurement Chapter 4

The IC Temperature Sensor This is an integrated circuit containing 16 transistors, 9 resistors and 2 capacitors contained in a transistor type package. The device reference number is LM335 and it provides an output of 10mV/°K. Measurements of the output voltage therefore indicate the temperature directly in degrees Kelvin (°K). For example, at a temperature of 20°C (293°K) the output voltage will be 2.93V. The circuit arrangement provided with the IC Temperature Sensor on the DIGIAC 1750 unit is shown in Fig 4.2. +5V

1kΩ

1kΩ Ext. O/P Int.

LM 355 0V

Socket for connection of external LM335

Fig 4.2

A 2-pin socket is provided for the connection of an external LM335 unit if desired. Note An LM335 unit is mounted on the Type "K" Thermocouple panel, external to the heated enclosure and fitted in a heat sink together with another type "K" thermocouple, its output being available from the REF socket on that panel. The output from this can be used as an indication of the ambient temperature outside the heated enclosure, and that from the INT. socket in Fig 4.2 indicates the temperature within the heated enclosure.

The output from the REF socket does not give an accurate value of the room (ambient) temperature when the heater is in use, due mainly to heat passing along the PCB by conduction from the heater. An LM335 remotely mounted or some other method is necessary if accurate measurement of ambient temperature is required.

55

Temperature Measurement Chapter 4

4.3

IT02 Curriculum Manual

Practical Exercise Characteristics of an LM335 IC Temperature Sensor TYPE 'K' THERMOCOUPLE

IC TEMP SENSORS EXT.

O/P

O/P

+

INT.

REF +

+

B

O/P

O/P

V

A N T C T H E R M IS T O R S

P L A T IN U M R . T . D .

HEATER ELEMENT

I/P

+1 2V

0V

Fig 4.3 

Connect a voltmeter to the circuit (as shown in Fig 4.3), switch the power supply ON and note the output voltage, this (x100) representing the ambient temperature in °K. Record the value in Table 4.1.



Connect the +12V supply to the heater input socket and note the voltage reading every minute until the value stabilizes. Record the values in Table 4.1. (Note °C = °K - 273.)

Time (minutes)

0

Voltage

1 V V V

2 V

3 V

4 V

5 V

6 V

°K

Temperature

°C Table 4.1

4.3a

Enter your temperature reading in C after 5 minutes. 

56

Switch OFF the power supply.

7

8

9 V

10 V

V

IT02 Curriculum Manual

Temperature Measurement Chapter 4

Exercise 4.3 illustrates the characteristics of the LM335 transducer, indicates the maximum temperature rise possible using the heater supplied at 12V, and also gives you an idea of the time scale required for the unit to reach stable conditions.

4.4

The Platinum RTD (Resistance Temperature Detector) Transducer Laser Trimmed Platinum Film

Ceramic Substrate

Gold Contact Plates Connections

Fig 4.4

The construction of the Platinum RTD Transducer is shown in Fig 4.4, consisting of a thin film of platinum deposited on a ceramic substrate and having gold contact plates at each end that make contact with the film. The platinum film is trimmed with a laser beam to cut a spiral for a resistance of 100Ω at 0°C. The resistance of the film increases as the temperature increases. It has a positive temperature coefficient (PTC). The increase in resistance is linear, the relationship between resistance change and temperature rise being 0.385Ω/°C. Rt = Ro + 0.385t

where

Rt = resistance at temperature t°C Ro = resistance at 0°C (= 100Ω)

Normally, the unit would be connected to a DC supply via a series resistor and the voltage developed across the transducer is measured. The current flow through the transducer will then cause some self heating, the temperature rise due to this being of the order of 0.005°C/mW dissipated in the transducer.

57

Temperature Measurement Chapter 4

IT02 Curriculum Manual

The very simple electrical circuit arrangement of the DIGIAC 1750 unit is as shown in Fig 4.5. O/P

RTD

0V

Fig 4.5

The white dot signifies that this is a PTC, not NTC (negative temperature coefficient) type of resistor which would have a black dot. In the practical exercise you will connect the platinum RTD in series with a high resistance to a DC supply and measure the voltage drop across it. Due to the small variation of resistance, the current change will be negligible and the voltage drop across the transducer will be directly proportional to its resistance.

Notes: ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................

58

IT02 Curriculum Manual

4.5

Temperature Measurement Chapter 4

Practical Exercise Characteristics of a Platinum RTD Transducer

IC TEMP SENSORS

SLIDE

C

EXT. B

O/P INT. +

1

2

3

4

5

6

7

8

9

10

10k

A

+

O/P

V PLATINUM R.T.D. HEATER ELEMENT

I/P

0V

+5 V

Fig 4.6 

Set the slider of the 10k Ω carbon resistor to mid-way and connect the circuit as shown in Fig 4.6, with the digital multimeter set to its 200mV or 2V DC range.



Switch ON the power supply and adjust the slider control of the 10k Ω resistor so that the voltage drop across the platinum RTD is 108mV (0.108V) as indicated by the digital multimeter.

This calibrates the platinum RTD for an assumed ambient temperature of 20°C, since the resistance of the RTD at 20°C will be 108Ω. Note that the voltage reading across the RTD in mV is the same as the RTD resistance in Ω, since the 0.108 current flowing must be = 1mA. 108 Note: If the ambient temperature differs from 20°C, the voltage can be set to the correct value for this ambient temperature if desired:

1.

Set the voltmeter to its 20V range and measure the INT output from the IC Temperature Sensor to obtain the ambient temperature: Voltage x 100 = temperature in °K Temperature in °C = °K - 273

2.

RTD resistance = 100 + 0.385 x °C. Set the voltage drop across the RTD for this value.

59

Temperature Measurement Chapter 4

IT02 Curriculum Manual



Connect the voltmeter, set to its 20V DC range, to the INT output of the IC Temperature Sensor. This represents the RTD temperature (voltage x 100 = temperature in °K). Record the temperature in the first column of Table 4.2.



Connect a second voltmeter, set to its 200mV range, to measure the voltage output from the RTD transducer. This voltage (in mV) is equal to the RTD resistance (in Ω). Record the resistance in the first column of the table.



Connect the +12V supply to the Heater Element input and record the RTD temperature (in °K) and RTD resistance (in Ω) after each of the times given in the table.

Time (minutes)

0

1

2

3

4

5

6

7

8

9

10

°K

RTD Temperature

°C

RTD Resistance

Ω

Ω

ΩΩΩΩΩΩΩ

Ω

Ω

Table 4.2 

Convert the RTD Temperature into °C (°K - 273) and add to Table 4.2.



Plot the graph of RTD resistance ( Ω) against temperature (°C) on the axes provided. Extend your graph down to cover 0°C. 130 128 126 RTD 124 Resistance 122 120 118 116 114 112 110 108 106 104 102 100 98 0

Graph 4.1

60

10

20

30

40 50 60 RTD Temperature o C

70

IT02 Curriculum Manual

4.5a

Temperature Measurement Chapter 4

Enter the total change in the resistance of the RTD Transducer over the temperature range 20-50 C in

4.5b

Is the resistance/temperature characteristic linear?

Yes

4.5c

.

or

No

Enter your estimated (extrapolated) resistance of the RTD Transducer from the graph at 0 C.

During the exercise, the current flowing was of the order of 1mA. Since the applied voltage was +5V, the total circuit resistance was therefore of the order of 5kΩ (which you can see from the setting of the 10k Ω slider resistor). The variation of resistance of the RTD Transducer therefore had little effect on the circuit current and hence the voltage drop across it represented the resistance value reasonably accurately. The current of 1mA in the RTD represents a very low power dissipation in the RTD. The self-heating effect would produce a temperature rise of 0.02°C. 

Calculate the power dissipation in the RTD Transducer at a temperature of 50°C when the standard circuit current of 1mA flows in it.

Power dissipation in the RTD Transducer =

4.5d

W

Enter your calculated value of the power dissipated in the RTD Transducer in W. 

Switch OFF the power supply.

61

Temperature Measurement Chapter 4

4.6

IT02 Curriculum Manual

The NTC (Negative Temperature Coefficient) Thermistor The thermistor (thermally sensitive resistor) is manufactured with the intention that its value will change with temperature. Unlike a normal resistor, a large coefficient of resistance (change of resistance with temperature) is desirable. Some are made with resistance which increases with temperature (positive temperature coefficient, PTC) or decreases (negative temperature coefficient (NTC). They are made in rod, disc or bead form. The construction of a typical NTC thermistor is shown in Fig 4.7(a), consisting of an element made from sintered oxides of metals such as nickel, manganese and cobalt, with contacts made to each side of the element.

+5V Th1

active disc element

B O/P A

Th2 Contacts

0V

Construction

Electrical Circuit

(a)

(b)

Fig 4.7

As the temperature of the element increases, its resistance falls, the resistance/ temperature characteristic being non-linear. The resistance of the thermistors provided with the DIGIAC 1750 unit is of the order of 5kΩ at an ambient temperature of 20°C (293°K). Two similar units are provided, one being mounted inside the heated enclosure. This is connected to the +5V supply and designated A. The other is mounted outside the heated enclosure. It is connected to the 0V (ground) line and is designated B. The circuit arrangement is shown in Fig 4.7(b).

62

IT02 Curriculum Manual

4.7

Temperature Measurement Chapter 4

Practical Exercise Characteristics of an NTC Thermistor The resistance of the NTC thermistor varies over a wide range for the temperature range available within the heated enclosure. For this reason the method used to measure the resistance in Exercise 4.5 cannot be used this time. If resistance readings are to be taken at regular intervals of 1 minute, the readings must be obtained very quickly. The method selected connects the thermistor in series with a calibrated resistor to the +5V supply. For each reading, the variable resistor is adjusted until the voltage at the junction of the thermistor and resistor is half of the supply voltage. For this setting there will be the same voltage drop across the thermistor and the resistor and, since the same current flows in each, their resistances must be equal. Hence the value of the resistance read from the calibrated resistor scale is the same as the resistance of the thermistor.

+5V n.t.c. 1k Ω

Calibrated Variable

2.5V

V +

0V

+

Schematic Diagram B

WHEATSTONE BRIDGE D

O/P A 12k

C A

OUT

B

3

NTC THERMISTORS HEATER ELEMENT

I/P

IN 1V

V

Rx

0V

Fig 4.8

63

Temperature Measurement Chapter 4

IT02 Curriculum Manual



Connect the circuit as shown in Fig 4.8. Set the switch on the Wheatstone bridge circuit to OUT to disconnect the 12kΩ and Rx resistors from the circuit, and set the calibrated variable resistor dial reading to approximately 500.



Switch the power supply ON and adjust the resistor control until the voltage indicated by the voltmeter is 2.5V.



Connect a second voltmeter, set to the 20V DC range, to measure the INT output of the IC Temperature Sensor. This represents the temperature (voltage x 100 = temperature in °K).



Record the values of temperature (in °K) and dial reading in the first column of Table 4.3.

Time (minutes)

0

1

2

3

4

5

6

7

8

9

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

kΩ

10

°K

Temperature (from IC Transducer)

°C

Dial Reading for 2.5V Thermistor Resistance (10 x Dial reading + 1k Ω) Table 4.3

kΩ



Connect the +12V supply to the Heater Element input socket and, at oneminute intervals, note the value of the temperature (in °K) and the dial reading to produce 2.5V across the resistance. Record in Table 4.3.



Convert the temperature measurements into °C (°K - 273) and add to the table.



For each of the dial readings recorded in Table 4.3, calculate the thermistor resistance and record in the table. Note: There is a 1kΩ resistor in the output lead of the variable resistance, so the thermistor resistance will be 10 x Dial reading + 1k .



Plot the graph of thermistor resistance against temperature on the axes provided.

Due to the shape of the response characteristic, the device is not suitable for applications where an accurate indication of temperature is required.

64

IT02 Curriculum Manual

Temperature Measurement Chapter 4

7k Ω

6k Ω Thermistor Resistance 5k Ω

4k Ω

3k Ω

2k Ω

1k Ω 0

10

20

30

40 50 60 Temperature o C

70

Graph 4.2

4.7a

From your graph, enter the resistance of your thermistor at a temperature of 35 in k .

Thermistors are used in very many electronic circuit applications for the control of currents and voltages as equipment temperatures vary. As transducer sensors they are more suitable for applications in protection and alarm circuits where an indication of temperature threshold is required. Some thermistors are available which have a rapid change of resistance when the temperature exceeds a certain value. 

Switch OFF the power supply.

65

Temperature Measurement Chapter 4

4.8

IT02 Curriculum Manual

Two Thermistor Bridge Circuits When used for alarm or protection circuits, two thermistors would normally be used, these being connected in a bridge circuit as shown in Fig 4.9.

R

Th1 A

DC Supply

O/P B

R

Th2

Fig 4.9

The two resistors R have the same resistance as the "cold" resistance of the thermistors. When cold, there will be no output at the connections AB because the bridge will be balanced under this condition. As the temperature rises, the resistance of both thermistors will decrease. The potential of connection A will rise and that of connection B will fall, giving a larger output than would be obtained with a circuit using only one thermistor.

4.9

Practical Exercise Characteristics of NTC Bridge Circuits Two bridge circuits will be investigated, one containing only one thermistor (Th1) and the other, two.

RV2

DC Supply

66

Th1 (A)

Digital Multimeter

V

RV1

Th2 (B)

Fig 4.10

10kΩ

10kΩ

RV3

10kΩ

IT02 Curriculum Manual

Temperature Measurement Chapter 4

Since the three branches to be used are all in parallel (Fig 4.10) they can be connected at the beginning and brought into operation simply by moving the null detector (digital multimeter). Note that the second thermistor (Th2) is not contained within the heated enclosure and will therefore not be subjected to the same heating effect as Th1. The circuit will not be as efficient as can be expected from one in which both thermistors are mounted in the same temperature environment. Variable resistors, RV2 & RV3 are adjusted to balance the branch "cold" resistances (approximately 5kΩ) to give 2.5V at the center-tap, and RV1 is also adjusted for 2.5V at the wiper. The circuit will then be ready for heating measurements.

WHEATSTONE BRIDGE D 12k

C A

TYPE 'K' THERMOCOUPLE OUT

B

3

-

IN 1V

0V

O/P +

Rx

REF + SLIDE

B 1

2

3

4

5

6

+

C

7

8

9

10

NTC THERMISTORS

7

4

8

2

9 10

10k

HEATER ELEMENT

I/P

V

3

1

A

A

10k

WIREWOUND TRACK 6 5 C

+5V

B O/P

B A 0V

Fig 4.11

Th1, the 10kΩ 10-turn resistor (RV3) and the 10k Ω wirewound resistor (RV1) form the bridge circuit with one active thermistor. Th1, the 10kΩ 10-turn resistor (RV3), Th2 and the 10kΩ carbon slide resistor (RV2) form the bridge with two active thermistors.

67

Temperature Measurement Chapter 4



Connect the circuit as shown in Fig 4.11 and set the switch on the Wheatstone Bridge circuit to OUT.



Switch the power supply ON and adjust the 10k Ω wirewound resistor (RV1) so that the voltmeter reading is 2.5V. The fixed branch of the bridge is now set for center balance.



Connect the voltmeter between socket B of the 10k Ω wirewound resistor (RV1) and NTC thermistor A. Adjust the 10kΩ 10-turn resistor (RV3) on the Wheatstone Bridge circuit for a voltage reading of zero.



Now connect the voltmeter between socket B of the 10k Ω wirewound resistor (RV1) and NTC thermistor B. Adjust the 10kΩ carbon slider resistor (RV2) for an output voltage of zero.



Both bridges are now set for zero output with the thermistors at ambient temperature.



Connect the voltmeter, set to the 20V DC range, to measure the INT output of the IC Temperature Sensor. This represents the temperature (voltage x 100 = temperature in °K). Record the value in the first column of Table 4.4.

Time (minutes) Temperature (IC Temperature Transducer) Bridge Output

IT02 Curriculum Manual

0

1

2

3

4

5

6

7

8

9

10

°K °C

1 active NTC 2 active NTC

V V V

V

V

V

V

V

V V V

V V V

V

V

V

V

V

V V V

Table 4.4

68



Connect a second voltmeter, set to the 2V DC range, between socket A of the NTC and socket B of the 10k Ω wirewound resistor (RV1). This is the bridge output for 1 active NTC thermistor. Record your measured voltage in the table.



Move one of the connections for the second voltmeter from the 10k Ω wirewound resistor (RV1), to socket B of the 10kΩ slide resistor (RV2). This is the bridge output for 2 active NTC thermistors. Again record your measured voltage in the table.

IT02 Curriculum Manual

Now connect the 12V supply to the heater input and, at one-minute intervals, record the temperature (in °K) from the first voltmeter, and the bridge outputs for the circuit with 1 active NTC and 2 active NTC thermistors (by changing the connections for the second voltmeter as you did before).





Temperature Measurement Chapter 4

Draw graphs of output voltage against temperature for the two bridge circuits on the same axes provided (Graph 4.3): 2.4 2.2 Output Voltage 2.0 (volts) 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0

0

10

20

30

40 50 60 Temperature o C

70

Graph 4.3

4.9a

From your graph, enter the output voltage for the one transducer bridge at a temperature of 50 in V.

4.9b

From your graph, enter the output voltage for the two transducer bridge at a temperature of 50 in V.

Note that the output with two active thermistors is greater than that with only one thermistor. However, if both active thermistors were at the same temperature, the output voltage would be twice that for one active thermistor. 4.9c

Is the relationship linear ?

Yes 

or

No

Switch OFF the power supply. 69

Temperature Measurement Chapter 4

IT02 Curriculum Manual

4.10 Type "K" Thermocouple Chromel Wire Spot-welded "Hot" junction Alumel Wire

"Cold" junction

Output Voltage

Fig 4.12

Fig 4.12 shows the construction of a thermocouple, consisting of two wires of different materials joined by welding together at one end. For the type "K" thermocouple the two materials are alumel and chromel. With this arrangement, when the ends that are joined together are heated, an output voltage is obtained between the other two ends. The ends that are joined together are referred to as the "hot" junction and the other ends are the "cold" junction. The magnitude of the output voltage depends on the temperature difference between the "hot" and "cold" junctions and on the materials used. For the type "K" thermocouple the output voltage is fairly linear over the temperature range 0-100°C and of magnitude 40.28 µV/°C difference between the "hot" and "cold" junctions. Two thermocouples are provided with the DIGIAC 1750 unit, one being mounted within the heated enclosure, this being the active unit which will have its "hot" and "cold" junctions at different temperatures in operation. The unit an is mounted outside the heated enclosure incorporatedofinthe a heat other sink with LM335 IC Temperature Sensor so thatand theistemperature "cold" junction of the active thermocouple can be measured.

70

IT02 Curriculum Manual

Temperature Measurement Chapter 4

The second thermocouple is connected in series with the first with the wires of the same material connected together. This ensures that the connections to the output circuit are made from the same material which eliminates the possibility of an EMF being introduced into the circuit by connections between different materials. The second thermocouple does not contribute to the output voltage because its "hot" and "cold" junctions are maintained at the same temperature. The circuit arrangement is as shown in Fig 4.13.

+5V

Active Thermocouple

Ref

+

LM 335

O/P

Inactive Thermocouple

-

0V

Fig 4.13

Due to the low output voltage of the thermocouple, amplification is required. An amplifier gain of 200 will give readings within one range of the digital multimeter. During operation, the temperature of the "cold" junction varies, due mainly to heat conduction from the heater along the PCB and the junction is in effect "floating". This is a common occurrence with thermocouple installations where the thermocouple leads are short. To overcome the problem, extra leads of the same material or different materials having the same thermoelectric properties are used to extend the "cold" junction to a point where a steady temperature can be maintained. These cables are referred to as "compensating cables".

71

Temperature Measurement Chapter 4

IT02 Curriculum Manual

4.11 Practical Exercise Characteristics of a "K" Type Thermocouple INSTRUMENTATION AMPLIFIER O/P

TYPE 'K' THERMOCOUPLE

O/P

+

B

-

A

+

x100 AMPLIFIER O/P I/P

A-B

+100V IN

REF

+

AMPLIFIER #1 O/P

I/P

-

HEATER ELEMENT

.5

+

.4 1 100 10

I/P

GAIN COARS E

.7 .8

.3

.9

.2 .1

OFFSET

.6

1.0

GAIN FINE V

0V

Fig 4.14 



Connect the circuit as shown in Fig 4.14, set the voltmeter to the 2V DC range and set Amplifier #1 GAIN COARSE to 10 and GAIN FINE to 0.2. Switch the power supply ON and set the Amplifier #1 OFFSET control as follows: 1) Temporarily disconnect the inputs to the Instrumentation Amplifier. 2) Short circuit together the Instrumentation Amplifier input connections. 3) Adjust the OFFSET control for zero indication on the voltmeter.



Re-connect the Thermocouple outputs to the Instrumentation Amplifier as shown in Fig 4.14. The measured voltage should still be zero with the "hot" and "cold" junctions at the same temperature.



Connect a second voltmeter, set to the 20V DC range, to the INT. socket of the IC Temperature Sensor. This represents the hot junction temperature inside the heated enclosure (voltage x 100 = temperature in °K). Record this in Table 4.5.



72

Reconnect the second voltmeter to the REF output socket in the Type ‘K’ Thermocouple block. This represents the cold junction temperature outside the enclosure (voltage x 100 = temperature in °K). Record this in Table 4.5.

IT02 Curriculum Manual

Temp. °K

Temperature Measurement Chapter 4

0

1

2

3

4

mV

mV

mV

mV

mV

5

6

7

8

9

10

mV

mV

mV

mV

Hot Junction (INT.) Cold Junction (REF) Difference

Thermocouple O/P

mV

mV

Table 4.5 



Connect the +12V supply to the heater and at 1 minute intervals, record the thermocouple output voltage (displayed on the first voltmeter), and the voltages representing the hot and cold thermocouple junction temperatures (by changing the connections for the second voltmeter as you did before). Construct the graph of thermocouple output voltage against temperature difference between the "hot" and "cold" junctions on the axes provided.

320 300 280 260 240 220

Output 200 Voltage (mV) 180 160 140 120 100 80 60 40 20 0

0

5

10

15

20

25

30

35

Temperature Differenceo C

Graph 4.4

73

Temperature Measurement Chapter 4

4.11a

Is the characteristic linear?

Yes

4.11b

IT02 Curriculum Manual

or

No

Deduce from your graph and enter the relationship in mV/ C. 

Switch OFF the power supply.

The actual value of the transfer characteristic will depend on the gain provided by the amplifier system at the settings used, which can be adjusted to calibrate the system as desired.

Notes: ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................

74

IT02 Curriculum Manual

Temperature Measurement Chapter 4

Student Assessment 4 1.

The output voltages you would expect to obtain from an LM335 Temperature Transducer at temperatures of (i) 0 C, (ii) 50 C & (iii) -20 C, are:

a c 2.

6.

119 Ω

b

138

Ω

c

150

5.78°C

b

15.2 °C

c

The resistance of an NTC thermistor is 5k resistance will be:

a 5.

(i) 0V, (ii) +0.50V, (iii) -0.20V (i) 2.73V, (ii) 3.23V, (iii) 2.53V

less than 5k Ω

b

equal to 5k Ω

at 0 C and 138

Ω

A platinum RTD Transducer has resistance of 100 temperature when its resistance is 115.2 will be:

a 4.

b d

A platinum RTD Transducer has resistance of 100 resistance at 50 C will be:

a 3.

(i) 0V, (ii) -0.50V, (iii) +0.20V (i) 2.93V, (ii) 3.43V, (iii) 2.73V

d 188

at 0 C and 138

30.4 °C

at 100 C. Its

Ω at 100 C. The

d 40 °C

at 20 C. If its temperature is i ncreased, its

c

greater than 5k Ω d infinite

A thermocouple gives an output of 40µV/ C difference in temperature between the "hot" and "cold" junctions. The output voltages expected for the junction temperatures of (i) "cold" 0 C "hot" 50 C (ii) "cold" 20 C "hot" 70 C (i) "cold" 50 C "hot" 50 C will be:

a

(i) 2V, (ii) 2V, (iii) 0V

b

(i) 2mV, (ii) 2.8mV, (iii) 2mV

c

(i) 2mV, (ii) 2mV, (iii) 0V

d

(i) 200 µV, (ii) 280µV, (iii) 0V

A thermocouple gives an output of 40µV/ C difference in temperature between the "hot" and "cold" junctions. The amplifier gain required to enable the thermocouple circuit to produce an output of 1V for a temperature difference of 100 C between the "hot" and "cold" junctions is:

a

25

b

40

c

250

d 500

Continued ...

75

Temperature Measurement Chapter 4

IT02 Curriculum Manual

Student Assessment 4 Continued ...

4kΩ

+5V

Th1 O/P

0V Th2

4kΩ

Fig 1

7.

8.

The characteristics of the devices Th1 & Th2 shown in Fig 1 are such that as the temperature is increased:

a

the voltage across them will rise

b

their resistance will increase

c

their resistance will fall

d

the current through them will decrease

Two thermistors Th1 & Th2 having resistance 4k

at 20 C and 1k

at 60 C are

connected in the bridge circuit shown in Fig 1. The output voltage from the circuit at a temperature of 20 C will be:

a 9.

0V

b

2.5V

c

4V

d 5V

Two thermistors Th1 & Th2 having resistance 4k at 20 C and 1k at 60 C are connected in the bridge circuit shown in Fi g 1. The output voltage from the circuit at a temperature of 60 C will be:

a

1V

b

2V

c

3V

d 4V

10. The characteristics of an IC Temperature Sensor give an output voltage of:

a

76

1mV/°C

b

10mV/ °K

c

100mV/°C

d 1V/ °K

IT02 Curriculum Manual

Light Sensors Chapter 5

Chapter 5 Light Sensors

Objectives of this Chapter

Equipment Required for this Chapter

Having studied this Chapter you will be able to: 

Discuss the characteristics of a filament lamp.



Describe the construction and characteristics of a photovoltaic cell.



Describe the construction and characteristics of a phototransistor.



Describe the construction and characteristics of a photoconductive cell.



Describe the construction and characteristics of a PIN photodiode.

• • • •

DIGIAC 1750 Transducer and Instrumentation Trainer. 4mm Connecting Leads. Digital Multimeter. Opaque box to cover the clear plastic enclosure.

77

Light Sensors Chapter 5

5.1

IT02 Curriculum Manual

The DIGIAC 1750 Opto-Transducer Facilities Fig 5.1 shows the arrangement of the opto-electronic (light) transducers provided on the DIGIAC 1750 Trainer. The opto-sensors are contained within a clear plastic enclosure and can be illuminated by a lamp which is placed centrally. CLEAR PLASTIC ENCLOSURE

P.I.N. PHOTODIODE

PHOTOVOLTAIC CELL

O/P

O/P

+

+

O /P

LAMP

PHOTOCONDUCTIVE CELL LAMP FILAMENT

O /P

PHOTOTRANSISTOR I/P

Fig 5.1

All semiconductor devices are sensitive to light falling upon them. That is why the devices (diodes, transistors, IC's) are contained within opaque encapsulations, to prevent light getting at the active materials. With some devices, the main effect of light irradiation will be to increase their conductivity (reduce their resistance). In others either an EMF is generated or currents are released to flow in an external circuit.

78

IT02 Curriculum Manual

5.2

Light Sensors Chapter 5

The Incandescent Lamp The light source to be used in the experiments is a tungsten filament lamp. The filament glows more brightly as the power feeding the lamp is increased. Two factors will be affected as the lamp voltage is increased: 1.

The temperature of the filament is proportional to the input power. Power varies with the square of the voltage, and is also affected by the resistance of the lamp, which increases as the filament temperature increases (it has a positive temperature coefficient).

2.

The spectral response of the lamp varies with the filament temperature. At low temperatures the light is in the infra-red region of the visible spectrum and the light output gradually increases in frequency (red → orange → yellow . . . ) as the temperature is raised.

These factors make it difficult to be too precise about the response of the sensors which will be investigated. In order to determine the response of the filament lamp an acceptable reference must be established. The photovoltaic cell is a linear device, the output short circuit current being directly proportional to the luminous flux (lux) being received.

79

Light Sensors Chapter 5

5.3

IT02 Curriculum Manual

Practical Exercise The Filament Lamp

MOVING COIL METER

0 5

-10

5

+10

+ P . I. N . P H O T O D IO D E

-

L J

0V

P H O T O V O L T A IC C E L L

O/P

O/P +

+

O/ P WIREWOUND TRACK 6 5 C

POWER AMPLIFIER

7

4 3

8

2

9 1

O/P

B

O/ P

PHOTOCONDU CTIV E CELL

I/P

I/P

Digital Multimeter

A

10

P HOTOTRA NSISTOR

A

LAMP FILAMENT

10k

+12V

0V

Fig 5.2

Lamp filament voltage (volts) Lamp filament current (mA) Lamp powerfilament (mW) Lamp resistance ( Ω) Table 5.1

80



Connect the circuit as shown in Fig 5.2 with the digital multimeter connected as an ammeter on the 200mA range in between the power amplifier and the lamp filament socket. Switch ON the power supply.



Set the 10k Ω wirewound resistor to minimum for zero output voltage (on the moving coil meter) from the power amplifier.



Take readings of lamp filament current as indicated on the digital multimeter as the lamp voltage is increased in 1V steps. Record the results in Table 5.1. 0

1

2

3

4

5

6

7

8

9

10

IT02 Curriculum Manual

Light Sensors Chapter 5



Calculate the corresponding values of lamp filament power (V xI) and resistance (V÷I), recording the results in Table 5.1.



Plot the graphs of lamp power and resistance against applied voltage on the graticule provided. 800

200

750

190

700

180

Lamp 650 Power (mW) 600

170

550

150

500

140

450

130

400

120

350

110

300

100

250

90

200

80

150

70

100

60

50

50

Lamp Resistance

160

0 0

1

2

3

4

40 5 6 7 8 9 10 Lamp Voltage (volts)

Graph 5.1

5.3a

From your graph estimate and enter the lamp voltage necessary to give a power dissipation of 250mW.

5.3b

From your graph estimate and enter the resistance of the lamp filament when the applied voltage is 4.5V.



Switch OFF the power supply.

81

Light Sensors Chapter 5

5.4

IT02 Curriculum Manual

Photovoltaic Cell A photovoltaic cell is one which generates an EMF when light falls onto it.

P-type Depletion Layer

N-type

Fig 5.3

One of the regions is made very thin (about one millionth of a meter, 1µm). Light can easily pass through this without much loss of energy. When the light reaches the junction, at the depletion layer, it is absorbed and the released energy creates hole-electron pairs which diffuse across the junction. The thin layer, which is only lightly doped, rapidly becomes saturated and charge carriers can be released into an external circuit to form a current, pushed around the circuit by the force (electro-motive force, EMF, electron-moving-force) of the surplus of charge carriers released by the energy absorbed. -200

-150 Anode Current -100 (microamps) -50

10,000 lux

2,000 lux

Anode Voltage (volts)

Symbol

+0.5

Fig 5.4

Note that the anode current is shown as negative because the internal current inside any source of EMF must flow with opposite polarity to the external current, the electrons arriving at the anode returning to the cathode inside the photo-cell.

82

IT02 Curriculum Manual

Light Sensors Chapter 5

The lux referred to in Fig 5.4 is the unit of incident light (light arriving at the cell).

O/P

0V Circuit Arrangement

Fig 5.5

Characteristics of Photovoltaic Cell Type MS5B

Open circuit voltage (in sunlight)

500mV

Short circuit current (in sunlight)

10mA

Peak spectral response wavelength 840nm (IR) Note: IR = infra red Response time

10 µ s

Table 5.2

If the output of the cell is short circuited there will be no output voltage at all, since this will all be dropped internally across the resistance of the cell. The short circuit output current obtained will vary from zero to maximum according to the incident light. The device can be used either as a voltage source or as a current source and is inherently a linear device. To increase the output voltage, cells may be connected in series. Parallel connection allows a greater current to be drawn. When used as an energy source they are known as Solar Cells. Note:

For the characteristic to be linear it is necessary for the light output of the lamp to be of constant light frequency (spectral color) and for the light output (in lux) to be directly proportional to the power input.

83

Light Sensors Chapter 5

5.5

IT02 Curriculum Manual

Practical Exercise The Photovoltaic Cell

MOVING COIL METER

0

5

5 +10

-10 +

P .I .N . P H O T O D I O D E

-

L J

0V

P HO T O V O LT A I C C E LL

O/P

O/P

+

+

O /P WIREWOUND TRACK 6 5 C

POWER AMPLIFIER

7

4

8

3

1

PH OT OC O ND UC TI V E C E LL

A

PH O TO TRA NS IS T OR

LAMP FILAMENT

I/P

I/P

9

2

O/P

B

O/P

A

10

10k

+12V

0V

Fig 5.6

Lamp filament voltage (volts) Short Circuit Output Current Open Circuit Output Voltage Table 5.3

84



Connect the circuit as shown in Fig 5.6 with the digital multimeter (ammeter) on the 2mA range to measure the short circuit current between the Photovoltaic Cell output and Ground. Fit an opaque box over the Clear Plastic Enclosure to exclude all ambient light.



Switch ON the power supply and set the 10k Ω wirewound resistor to minimum for zero output voltage from the power amplifier.



Take readings of Photovoltaic Cell Short Circuit Output Current as indicated on the digital multimeter as the lamp voltage is increased in 1V steps. Record the results in Table 5.3. 0

1

2

3

4

5

6

7

8

9

10

µA

µA

µA

µA

µA

µA

µA

µA

µA

µA

µA

V

V

V

V

V

V

V

V

V

V

V

IT02 Curriculum Manual

Light Sensors Chapter 5



Switch OFF the power supply, set the multimeter as a voltmeter to read the Open Circuit Output Voltage. Switch ON the power supply and repeat the readings, adding the results to Table 5.3.



Plot the graphs of Photovoltaic Cell Short Circuit Output Current and Open Circuit Output Voltage against Lamp filament voltage on the graticule provided. 1100

1.10

1050

1.05

1000

1.00

950

0.95

900 Photovoltaic Cell 850 Short Circuit Output Current 800 (µA)

0.90

750

0.75

700

0.70

650

0.65

600

0.60

550

0.55

500

0.50

450

0.45

400

0.40

350

0.35

300

0.30

250

0.25

200

0.20

150

0.15

100

0.10

50

0.05

Photovoltaic Cell Open Circuit Output Voltage 0.80 (volts)

0.85

0 0

1

2

3

0 4 5 6 7 8 9 10 Lamp Filam ent Voltage (volts)

Graph 5.2

85

Light Sensors Chapter 5

5.5a

IT02 Curriculum Manual

From your graph estimate and enter the short circuit output current in A when the Lamp filament voltage is 7.5V.

5.5b

Are the graphs linear?

Yes 

5.6

or

No

Switch OFF the power supply.

The Phototransistor The construction and circuit used are shown in Fig 5.7. The device is an NPN three layer semiconductor device similar to a normal transistor, the regions being called emitter (e), base (b) and collector (c).

+V

c Incident Light

N P N Lens

Case

Incident Light

eb

R Output c

b

e 0V

Fig 5.7

The device differs from the normal transistor in allowing light to fall onto the base region, focused there by a lens. The circuit connection is shown in Fig 5.7, the collector being connected to the positive of a DC supply via a load resistor R. The base connection is not used in this circuit but is available for biasing to change the threshold level. With no light falling on the device there will be a small leakage current flowing due to thermally generated hole-electron pairs and the output voltage from the circuit will be slightly less than the supply voltage due to the voltage drop across the load resistor R.

86

IT02 Curriculum Manual

Light Sensors Chapter 5

When light falls on the base region the leakage current increases. With the base connection open circuit, this current flows out via the base-emitter junction and is amplified by normal transistor action to give a large change in the collector leakage current. With increased current flowing in the load resistor R, the output voltage reduces and is dependent on the light falling on the device. Vout = V - Iceo R where: V = Supply voltage, Iceo = Collector leakage current, R = Collector load resistance. Fig 5.8 shows the circuit arrangement for the DIGIAC 1750 unit.

O/P

c b

e 0V

Fig 5.8

The main characteristics of the device are: Type

MEL12

Collector Current

Dark

100nA

(Vce = 5V)

Typical ambient

3.5mA

Table 5.4

87

Light Sensors Chapter 5

5.7

IT02 Curriculum Manual

Practical Exercise Characteristics of a Phototransistor

MOVING COIL METER

5

0

P . I . N. P HO T O D I O D E

P HO T O V O L T A I C C E LL

O/P

O/P

5 +10

-10

+

+

+

O/ P

O /P

-

L J

0V

PH O TO CO NDUCT IV E C ELL

PH O T OT RA NSI ST O R

LAMP FILAMENT

I/P SLIDE

WIREWOUND TRACK 6 5 C

C

V

B

POWER AMPLIFIER

7

4 3

8

2

9 1

O/P

B

1

2

3

4

5

6

7

8

9

10

A

10k

I/P A

10

10k

+12V

0V

+5V

Fig 5.9

88



Connect the circuit as shown in Fig 5.9 and set the 10k Ω carbon slider control to minimum setting (1) so that the Phototransistor load resistance is approximately 1kΩ (protection resistor only).



Connect the digital multimeter on the 20V DC range to measure the Phototransistor output voltage. Fit the opaque box over the Clear Plastic Enclosure to exclude all ambient light.



Switch ON the power supply and set the 10k Ω wirewound resistor to minimum for zero output voltage from the power amplifier.



Take readings of Phototransistor output voltage as indicated on the digital multimeter as the lamp voltage is increased in 1V steps. Record the results in Table 5.5.

IT02 Curriculum Manual

Lamp filament voltage (volts) Phototransistor Output Voltage

0

Light Sensors Chapter 5

1 V

2 V

3 V

4 V

5 V

6 V

7 V

8 V

9 V

10 V

V

Table 5.5 

Plot the graph of Phototransistor Output Voltage against Lamp filament voltage on the graticule provided.

5.5 5.0 Phototransistor 4.5 Output Voltage 4.0 (V) 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0

0

1

2

3 4 5 6 7 8 9 10 Lamp Filament Voltage (volts)

Graph 5.3

5.7a

From your graph estimate and enter the filament input voltage when the Phototransistor output voltage is 2.5V.

5.7b

As the filament input voltage increases the phototransistor output voltage 'levels out' at approximately:

a 4.5-5.5V 

b

3-4V

c

1.5-2.5V

d

0-1.0V

Switch OFF the power supply.

89

Light Sensors Chapter 5

5.8

IT02 Curriculum Manual

The Photoconductive Cell, LDR Fig 5.10 shows the basic construction of a photoconductive cell, consisting of a semiconductor disc base with a gold overlay pattern making contact with the semiconductor material. The circuit arrangement for the DIGIAC 1750 unit is also shown.

O/P

Cadmium Sulfide disk

Contact

Contact 0V Gold Contact Fingers

Circuit Arrangement

Fig 5.10

The resistance of the semiconductor material between the gold contacts reduces when light falls on it. With no light on the material, the resistance is high. Light falling on the material produces hole-electron pairs of charge carriers and reduces the resistance. Out of the various semiconductor materials available, a cadmium sulfide photoconductive cell is used on the DIGIAC 1750 unit because it responds to light with a range of wavelengths similar to those of the human eye (400-700nm). An alternative name for this device is the Light Dependent Resistor, LDR.

Cell Resistance Peak Spectral Response

Dark 1MΩ

Ambient (typ.) 400Ω 530nm

Table 5.6

When is removed from This the device, the hole-electron slow totime. reform and thelight response is sluggish. is indicated by the large pairs fallingare response

90

IT02 Curriculum Manual

5.9

Light Sensors Chapter 5

Practical Exercise Characteristics of a Photoconductive Cell

MOVING COIL METER

5

0

P . I . N. P HO T O D I O D E

P H O T O V O LT A I C C E L L

O/P

O/P

5 +10

-10

+

+

+

O/P

O /P

-

L J

0V

PH O TO C ONDUCT IV E C E LL

PHOT OT RANSI ST O R

LAMP FILAMENT

I/P

V SLIDE WIREWOUND TRACK 6 5 C

C B

POWER AMPLIFIER

7

4 3

8

2

9 1

O/P

B

1

2

3

4

5

6

7

8

9

10

A

10k

I/P A

10

10k

+12V

0V

+5V

Fig 5.11 

Connect the circuit as shown in Fig 5.11 and set the 10k Ω carbon slider control to setting 3 so that the Photoconductive Cell load resistance is approximately 3kΩ.

+5V

3kΩ

0V

Output LDR

Fig 5.12 

Connect the digital multimeter on the 20V DC range to measure the Photoconductive Cell output voltage. Fit the opaque box over the Clear Plastic Enclosure to exclude all ambient light.



Switch ON the power supply and set the 10k Ω wirewound resistor to minimum for zero output voltage from the power amplifier.

91

Light Sensors Chapter 5

IT02 Curriculum Manual



Take readings of Photoconductive Cell output voltage as indicated on the digital multimeter as the lamp voltage is increased in 1V steps. Record the results in Table 5.7.

Lamp filament voltage (volts)

0

Photoconductive Cell Output

1 V

2 V

3 V

4

5

V

V

6 V

7 V

8 V

9 V

10 V

V

Table 5.7 

Plot the graph of Photoconductive Cell Output Voltage against Lamp filament voltage on the graticule provided.

5.5 5.0 Photoconductive 4.5 Cell Output 4.0 Voltage (V) 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0

0

1

2

3 4 5 6 7 8 9 10 Lamp Filament Voltage (volts)

Graph 5.4

5.9a

From your graph estimate and enter the lamp filament voltage when the circuit output voltage is 3V. 

92

Switch OFF the power supply.

IT02 Curriculum Manual

Light Sensors Chapter 5

5.10 The PIN Photodiode Fig 5.13 shows the construction of the PIN photodiode.

Light

+

Light

N

Lens Depletion Layers Hole

Intrinsic (I) Region

I P

Electron

Electron/Hole pairs generated in the I region Contacts

Fig 5.13

This differs from a standard PN photodiode by having a layer of intrinsic (pure) silicon, the I region, between the normal P and N regions. The main improvement of the introduction of the I region is a reduction in the capacitance of the junction, resulting in a faster response time which can be as high as 0.5ns. The device can be operated in one of two ways: (a) as a photovoltaic cell, measuring the voltage output, and (b) by amplifying the output current and converting it into a voltage.

O/P

0V

Sensitivity

0.55A/W

Current Characteristic

2856KnA/lx

Response Time

3.5ns

Peak Spectral Response

850nm (IR)

Characteristics of a BPX65 PIN Diode

Fig 5.14

Fig 5.14 shows the circuit arrangement and characteristics for the PIN Diode mounted on the DIGIAC 1750 unit.

93

Light Sensors Chapter 5

IT02 Curriculum Manual

5.11 Practical Exercise Characteristics of a PIN Photodiode

MOVING COIL METER

5

0

-10

P .I .N . P HO T O D I O D E

P H O TO V O LT A I C C E L L

O/P

O/P

5 +10 +

+

+

O/ P

O/ P

-

L J

0V

P HOTO CO N DU CTIVECE LL

CURRENT AMPLIFIER WIREWOUND TRACK 6 5 C 7

4 3

8

2

9 1

10

P HO TO TRAN SISTO R

LAMP FILAMENT

I/P AMPLIFIER #1 I/ P

POWER AMPLIFIER O/P

B

O /P

O/P I/P 104 I IN

-

+

I/P

.4 1 100 10

A

10k

.5

+12V

OFFSET

GAIN COARSE

.6 .7 .8

.3

.9

.2 .1

1.0

GAIN FINE

V 0V

Fig 5.15

94



Connect the circuit as shown in Fig 5.15, using the Current Amplifier to measure the current output of the PIN Photodiode.



Use the digital multimeter on the 20V DC range to measure the output voltage of Amplifier #1. Fit the opaque box over the Clear Plastic Enclosure to exclude all ambient light.



Switch ON the power supply and set the 10k Ω wirewound resistor to minimum for zero output voltage from the power amplifier.



Set the GAIN COARSE of Amplifier #1 to 10 and set the GAIN FINE to 1.0. Check that the OFFSET is giving zero output for zero input and adjust if necessary.



Take readings of Amplifier #1 output voltage as indicated on the digital multimeter as the lamp voltage is increased in 1V steps. Record the results in Table 5.8 in the row labeled PIN Photodiode Current Amp. O/P.

IT02 Curriculum Manual

Lamp filament voltage (volts) PIN Photodiode

0

Current Amp. O/P

Light Sensors Chapter 5

1

2

3

4

5

6

7

8

9

10

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

PIN Photodiode Output Voltage Table 5.8 

Transfer the Current Amplifier input and output connections to the Buffer Amplifier, so that the Buffer Amplifier replaces the Current Amplifier in the circuit. This will allow you to measure the output voltage of the PIN Photodiode.



Take readings of PIN Photodiode amplified Output Voltage as the lamp voltage is again increased in 1V steps. Record the results in Table 5.8 in the row labeled PIN Photodiode Output Voltage.



Plot the graphs of PIN Photodiode Current Amplifier Output Voltage and Buffered Output Voltage against Lamp filament voltage on the graticule provided.

4.5 4.0

PIN Photodiode Output Voltage 3.5 (V) 3.0

2.5 2.0 1.5 1.0 0.5 0

0

1

2

3 4 5 6 7 8 9 10 Lamp Filament Voltage (volts)

Graph 5.5

95

Light Sensors Chapter 5

5.11a

IT02 Curriculum Manual

From your graph, estimate and enter the lamp filament voltage when the circuit output voltage is 2V with the Current Amplifier connected.

5.11b

From your graph, estimate and enter the lamp filament voltage when the circuit output voltage is 2V with the Buffer Amplifier connected.

5.11c

Are the two graphs similar shapes?

Yes 

or

No

Switch OFF the power supply.

Notes: ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................

96

IT02 Curriculum Manual

Light Sensors Chapter 5

Student Assessment 5 1.

2.

3.

4.

As the input voltage is varied, the light output from a filament lamp is:

a

directly proportional to the input voltage

b

of constant color temperature for tungsten

c

a non-linear relationship

d

reduced as the voltage is increased

The Solar Cell is a form of:

a

photovoltaic cell

b

light dependent resistor

c

photoconductive cell

d

phototransistor

A photovoltaic cell gives an output of 0.5V for a certain illumination level and is capable of a current output of 5mA. With two identical units connected (i) in series or (ii) in parallel the output capability will be:

a

(i) V = 0.5V, (ii) I = 5mA

b

(i) V = 1.0V, (ii) I = 5mA

c

(i) V = 0.5V, (ii) I = 10mA

d

(i) V = 1.0V, (ii) I = 10mA

A phototransist is connected tocollector a 10V DC supplyisvia a 2kTheload resistor. For one level of ambientor illumination the current 2mA. collector voltage will be:

a 5.

b

4V

c

6V

d 8V

For the phototransistor in question 4 above, if the level of ambient illumination is doubled and assuming that the device has a linear relationship, you would expect the collector voltage to be:

a 6.

2V

2V

b

4V

c

6V

d 8V

The Light Dependent Resistor (LDR) is a form of:

a

photovoltaic cell

b

PIN photodiode

c

photoconductive cell

d

phototransistor

Continued ...

97

Light Sensors Chapter 5

IT02 Curriculum Manual

Student Assessment 5 Continued ... 7.

8.

9.

As the level of illumination on a photoconductive cell is increased:

a

its resistance is increased

b

a voltage is generated

c

the junction temperature is increased

d

its resistance is reduced

The greatest advantage of a PIN photodiode over an ordinary photodiode is:

a

simpler construction

b

less semiconductor regions

c

greater capacitance

d

faster response times

Examining the characteristics of all of the opto-sensors seems to indicate that:

a

there is no similarity between any of them

b

the lamp has very little light output at low driving voltages

c

all of the circuits give an increasing output voltage as the illumination is increased

d

all of the circuits gave a decreasing output voltage as the illumination was increased

10. The characteristics of the PIN photodiode are most similar to those of the:

98

a

photovoltaic cell

b

tungsten filament lamp

c

photoconductive cell

d

phototransistor

IT02 Curriculum Manual

Linear Position or Force Applications Chapter 6

Chapter 6 Linear Position or Force Applications

Objectives of this Chapter

Having studied this Chapter you will be able to: 

Describe the construction, principle and characteristics of a Linear Variable Differential Transformer (LVDT).



Describe the construction and characteristics of a linear variable capacitor.



Describe the construction and characteristics of a strain gauge.

Equipment Required for this Chapter

•

• • •

DIGIAC 1750 Transducer and Instrumentation Trainer. 4mm Connecting Leads. Digital Multimeter. Oscilloscope.

99

Linear Position or Force Applications Chapter 6

6.1

IT02 Curriculum Manual

The Linear Variable Differential Transformer (LVDT) The construction and circuit arrangement of an LVDT are as shown in Fig 6.1. It consists of three coils mounted on a common former and having a magnetic core that is movable within the coils.

Secondaries

Primary Coil Former

A

Primary A

B

B

Motion

Core

Magnetic Core

Secondaries Connections

Fig 6.1

The center coil is the primary and is supplied from an AC supply. The coils on either side are secondary coils and are labeled A & B in Fig 6.1. Coils A & B have equal number of turns and are connected in series opposing so that the output voltage is the difference between the voltages induced in the coils. Fig 6.2 shows the output obtained for different positions of the magnetic core.

AC Input A

AC Input B

A

C or e

A

Cor e

Output

+

Output

+

I/P

+

I/P

I/P

0

0

0

-

-

-

+

+

O/P

+

O/P

0

O/P

0

-

0

-

(a) Fig 6.2

B

Core

Output

100

AC Input B

-

(b)

(c)

IT02 Curriculum Manual

Linear Position or Force Applications Chapter 6

With the core in its central position as shown in Fig 6.2(b) there should be equal voltages induced in coils A & B by normal transformer action and the output voltage would be zero. In practice this ideal condition is unlikely to be found, but the output voltage will reduce to a minimum. With the core moved to the left as shown in Fig 6.2(a), the voltage induced in coil A (Va) will be greater than that induced in coil B (Vb). There will therefore be an output voltage Vout = (Va - Vb) and this voltage will be in phase with the input voltage as shown. With the core moved to the right as shown in Fig 6.2(c) the voltage induced in coil A (Va) will be less than that induced in coil B (V b) and again there will be an output voltage Vout = (Va - Vb) but in this case the output voltage will be antiphase with the input voltage. Movement of the core from its central (or neutral) position produces an output voltage. This voltage increases with the movement from the neutral position to a maximum value and then may reduce for further movement from this maximum setting. Note that the phase will remain constant on either side of the neutral position. There is no gradual change of phase, only an abrupt reversal when passing through the neutral position. An amplitude only measurement of the output voltage, such as that provided by a meter, gives an indication of movement from the neutral position but will not indicate the direction of that movement. Used in conjunction with a phase detector, an output can be obtained that is dependent on both magnitude and direction of movement from neutral position. The oscilloscope gives both phase and magnitude indications. Fig 6.3 shows the circuit arrangement and device characteristics of the DIGIAC 1750 unit.

Turns per coil

I/P O/P

75

Inductance per coil 68 µ H Output Voltage

10mV/mm from neutral

Mechanical Travel 15mm 0V

LVDT Characteristics

Fig 6.3

101

Linear Position or Force Applications Chapter 6

6.2

IT02 Curriculum Manual

Practical Exercise Characteristics of a Linear Variable Differential Transformer

LVDT

A.C. AMPLIFIER O/P

VARIABLE CAPACITOR

40kHz FILTER

FULL WAVE RECTIFIER

O/P

I/P

I/P

O/P I/P VIN

I/P

O /P

I /P

V

10 1000 100

O/ P GAIN

0V

40kHz OSCILLATOR

MOVING COIL METER

O/P 5

AMPLIFIER #1 I /P

-10

O/ P

0

5 +10

+ -

.5

+

.4 1 100 10

OFFSET

GAIN COARSE

.6 .7

.8

.3

.9

.2 .1

1.0

0V

L J

GAIN FINE

Fig 6.4

In this exercise you will measure the rectified output using the digital multimeter on the 20V DC range and also amplify and measure it using the M.C. analog meter, as this gives a better impression of the variation of output voltage with core position.

102



Connect the circuit as shown in Fig 6.4 with the digital multimeter on the 2V DC range to monitor the output of the Full-Wave Rectifier. Switch ON the power supply.



Set the A.C. Amplifier gain to 1000.



Set the GAIN COARSE control of Amplifier #1 to 100 and GAIN FINE control to 0.2. Check that the OFFSET control is set for zero output with zero input and adjust if necessary.



Adjust the core position by rotating the operating screw to the neutral position. This will give minimum output voltage. Note the value of this voltage from the digital multimeter and record in Table 6.1.



Rotate the core control screw in steps of 1 turn for 4 turns in the clockwise direction (when viewing the control from the left-hand side of the D1750 unit) and record your results in Table 6.1. Then turn the control screw in the counter clockwise direction, again recording the results in Table 6.1.

IT02 Curriculum Manual

Linear Position or Force Applications Chapter 6

Core position (turns from neutral)

Output Voltage

-4

Digital meter

-3

-2

-1

0

+1

+2

+3

+4

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

V

Analog meter

Table 6.1 

Plot the graph of output voltage from the analog meter readings against core position on the axes provided.

10 Output 9 Voltage (volts) 8

7 6 5 4 3 2 1 0 -4

-3 -2 -1 0 +1 +2 +3 Core Position (turns from neutral)

+4

Graph 6.1

6.2a

6.2b

Enter your minimum voltage reading from the digital multimeter in mV.

Enter your voltage reading from the M.C. analog meter when the core is turned 2 turns out (-2) from the neutral position in V.



Switch OFF the power supply.

103

Linear Position or Force Applications Chapter 6

LVDT

IT02 Curriculum Manual

VARIABLE CAPACITOR

A.C. AMPLIFIER O/P

40kHz FILTER

I/P

O/P I/P

I/P

O/ P

I/ P

10 1000

O /P

GAIN 100 40kHz OSCILLATOR

TP1

0V

TP2

0V

O/P

CH .1

C H .2

OSCILLOSCOPE

Fig 6.5 

Change the circuit to that shown in Fig 6.5 to observe the effect of the polarity ofchange in the output. Note panel that test are provided at the bottom the DIGIAC 1750 Trainer for points connection of oscilloscope probes.

104



Note: for the LVDT considered here, unless the two secondary coils are identical, there will be non-perfect coupling between each secondary coil and the primary coil, resulting in a frequency-dependent phase shift in the output voltage (relative to the input voltage).



Set up the oscilloscope as follows: lock the timebase to CH.1, trigger selector to AC CH.1 Amplifier on AC input, 50mV/div CH.2 Amplifier on AC input, 0.5V/div timebase to 5µs/div position both traces on the center horizontal line of the display



Switch ON the power supply and vary the core position through its full range and observe the effect on the output voltage as seen on CH.2 of the oscilloscope display.



Adjust the timebase fine control to give 1½ cycles of displayed waveform.

IT02 Curriculum Manual



Linear Position or Force Applications Chapter 6

Sketch the oscilloscope waveforms when the core is turned 2 turns in (+2) from the neutral position on the graticule provided.

Waveform Sketch 6.1

6.2c

a CH.1

The waveform sketch, for perfectly coupled coils, would look most like: CH.2

b

CH.1

c CH.2

CH.1

d

CH.1

CH.2

CH.2



Switch OFF the power supply and reset the timebase fine control to the calibrated position.

Notes: ................................................................................................................................................................. ................................................................................................................................................................. .................................................................................................................................................................

105

Linear Position or Force Applications Chapter 6

6.3

IT02 Curriculum Manual

The Linear Variable Capacitor Any capacitor consists of two conducting plates separated by an insulator which is referred to as the dielectric. The capacitance of the device is directly proportional to the cross sectional area that the plates overlap and is inversely proportional to the separation distance between the plates. A variable capacitor can therefore be constructed by varying either the area of plates overlapping or the separation distance.

Plated Brass Sleeve (fixed plate)

I/P

O/P

length l

Metal Slug (moving plate)

10kΩ

Spring

0V Contacts

(a)

(b)

Fig 6.6

Fig 6.6(a) shows the construction of the capacitor fitted in the DIGIAC 1750 unit, being fitted at the end of the coil former of the LVDT. This uses the magnetic slug core as the moving plate of the capacitor. The fixed plate consists of a brass sleeve fitted around the coil former. The capacitance magnitude depends on the length ( l) of the slug enclosed within the brass sleeve, the capacitance increasing with increase of length l. Fig 6.6(b) shows the circuit arrangement in the DIGIAC 1750 unit. The main characteristics of the unit are: Capacitance (minimum)

25pF

Capacitance (maximum)

50pF

Mechanical travel

15mm

Table 6.2

106

IT02 Curriculum Manual

6.4

Linear Position or Force Applications Chapter 6

Practical Exercise Characteristics of a Variable Capacitor Transducer

LVDT

A.C. AMPLIFIER O/P

VARIABLE CAPACITOR

I/P

40kHz FILTER

FULL WAVE RECTIFIER

O/P I/P

O/P I/P VIN

I/P

O /P

I /P

10 1000 100

O /P GAIN WHEATSTONE BRIDGE D

40kHz OSCILLATOR

B

DIFFERENTIAL AMPLIFIER O/P A-B

O/P

0V

+

A

12k

C A 3

OUT

B

IN 1V

+5V

0V

V

AMPLIFIER #1 I/ P

O/ P

Rx -

.5

+

.4 1 100 10

OFFSET

GAIN COARSE

.6 .7 .8

.3

.9

.2 .1

1.0

GAIN FINE

Fig 6.7

The purpose of the Differential Amplifier is to provide a reference to give zero output voltage at any desired value of input voltage. The reference voltage is adjusted by the setting of the 10-turn potentiometer. 

Connect the circuit as shown in Fig 6.7 with the digital multimeter on the 20V DC range connected to the output of Amplifier #1.



Set the capacitor moving plate fully out to the minimum capacitance position, and then turn it back in until the marker on the operating control is first at the top. Now the device is near to its minimum capacitance position.



Set the AC Amp gain to 1000.



Switch ON the power supply and set the GAIN COARSE control of Amplifier #1 to 100 and GAIN FINE control to 0.4. Check that the OFFSET control is set for zero output with zero input and adjust if necessary.



Adjust the 10-turn potentiometer on the Wheatstone Bridge panel to give zero (as near as possible) output from Amplifier #1 (as close to 0V as possible) as indicated on the digital multimeter.



Turn the operating screw inwards in steps of 1 turn clockwise to increase the capacitance and at each step note the output voltage and enter the value in Table 6.3.

107

Linear Position or Force Applications Chapter 6

Approximate Capacitance Turns of screw

IT02 Curriculum Manual

25pF ←Screw full out, minimum 0

1

2

3

4

50pF

Screw full in, maximum→ 5

6

7

8

9

10

0

Output Voltage

V

V

V

V

V

V

V

V

V

V

V

Table 6.3 

Plot the graph of output voltage against core positions above on the axes provided: 3.6 3.4 3.2 3.0 2.8 2.6 2.4 Output 2.2 Voltage (volts) 2.0

1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0

2

3

45678

9 10 Core Position (turns in )

Graph 6.2

6.4a

Enter the output voltage when the core is in position 4 above in V.

6.4b

Is the characteristic linear?

Yes

108

or

No

IT02 Curriculum Manual

6.5

Linear Position or Force Applications Chapter 6

The Strain Gauge Transducer Fine Resistance Wire

Fine Resistance Wire

(b) Sensitive Axis

(c) (a) Fig 6.8

Fig 6.8 shows the construction of a strain gauge, consisting of a grid of fine wire or semiconductor material bonded to a backing material. When in use, the unit is glued to the beam under test and is arranged so that the variation in length under loaded conditions is along the gauge sensitive axis (Fig 6.8(a)). Loading the beam increases the length of the gauge wire and also reduces its cross-sectional area (Fig 6.8(c)). Both of these effects will increase the resistance of the wire. +5V

Load Platform

Strain Gauges

O/P

+ Beam 0V

(a)

(b)

Fig 6.9

The layout and circuit arrangement for the DIGIAC 1750 unit is shown in Fig 6.9. Resistors are electro-deposited on a substrate on a contact block at the right-hand end of the assembly.

109

Linear Position or Force Applications Chapter 6

IT02 Curriculum Manual

The gauge is normally connected in a Wheatstone Bridge arrangement with the bridge balanced under no load conditions. Any change of resistance due to loading unbalances the bridge and this is indicated by the detector (Galvanometer).

Standard

Strain

Dummy

Active

Resistance

Gauge

Gauge

Gauge

DC Supply

D Standard Resistance

Dummy

D Standard Resistance

Standard

(a)

Active D

Standard

Active

(b)

Dummy

(c)

Fig 6.10

Fig 6.10(a) shows the basic Wheatstone Bridge arrangement with one strain gauge transducer. This circuit is liable to give inaccurate results due to thermal changes. A variation of temperature will also produce a change of resistance of the gauge and this will be interpreted as a change of loading. To correct for this an identical gauge is used and connected in circuit as shown in Fig 6.10(b). This gauge is placed near to the other gauge but is arranged so that it is not subjected to any loading. Any variation of temperature now affects both gauges equally and there will be no thermal effect on the bridge conditions. The gauge subjected to loading is referred to as the active gauge and the other is called the dummy gauge. The output from the circuit is small and to increase this, four gauges are normally used with two active gauges and two dummies as shown in Fig 6.10(c). The DIGIAC 1750 uses two active gauges formed along the axis of the beam and two dummies formed at right angles to these. The main characteristics of the device are: Load capacity

Non-linearity

0.10%

Maximum deflection

0.5mm

Hysteresis

0.03%

Sensitivity

25µV/g

Creep

0.05%

Table 6.4

110

100g

IT02 Curriculum Manual

6.6

Linear Position or Force Applications Chapter 6

Practical Exercise Characteristics of a Strain Gauge Transducer

STRAIN GAUGE

INSTRUMENTATION AMPLIFIER O/P

O/P

B

-

+

A

+

LOAD

x100 AMPLIFIER O/P I/P

A-B

+100VIN

MOVING COIL METER 5

AMPLIFIER #1 I/ P

-10

O/ P

0

5 +10

+ -

.5

+

.4 1 100 10

OFFSET

GAIN COARSE

.6 .7

.8

.3

.9

.2 .1

1.0

0V

L J

GAIN FINE

Fig 6.11

You will need ten similar weights, such as ten equal value coins, to increase the loading in regular steps. 

Connect the circuit as shown in Fig 6.11 and set Amplifier #1 GAIN COARSE control to 100.



Switch ON the power supply and with no load on the strain gauge platform, adjust the offset control of Amplifier #1 so that the output voltage is zero.



Place all ten of your weights on the load platform and adjust the GAIN FINE control to give an output voltage of 7.0V as indicated on the moving coil meter. Note that this value of output voltage should cover all ranges of coins within the setting of the GAIN FINE control.



Place one weight (coin) on the load platform and note the output voltage. Record the value in Table 6.5 overleaf.



Repeat the process, adding further weights one at a time, noting the output voltage at each step and recording the values in Table 6.5.

111

Linear Position or Force Applications Chapter 6

Number of coins

0

1

2

3

4

IT02 Curriculum Manual

5

6

7

8

9

10

0

Output Voltage

V

V

V

V

V

V

V

V

V

V

V

Table 6.5 

Plot the graph of output voltage against number of coins on the axes provided:

Output 7 Voltage 6.5 (volts) 6 5.5 5 4.5 4 3.5 3 2.5 2

1.5 1 0.5 0 0

1

2

3

45678

9 10 Number of coins

Graph 6.3

a

112

6.6a

Enter the output voltage obtained with four coins on the platform.

6.6b

Your characteristic sketch is most similar to:

b

c

d

IT02 Curriculum Manual

Linear Position or Force Applications Chapter 6

Student Assessment 6 1.

A Linear Variable Differential Transformer (LVDT) has two secondary coils A and B. When the core is moved to be nearest to coil A the voltages induced will be:

a c 2.

250mV aiding

b

500mV opposing

c

500mV aiding

d

1.0V opposing

A Linear Variable Differential Transformer (LVDT) has its core centralized under the primary and the voltage induced in one of the secondary coils is 500mV. The secondary output voltage will be:

a 4.

b greater in coil A than in coil B d reduced to zero

A Linear Variable Differential Transformer (LVDT) has its core centralized under the primary and the voltage induced in one of the secondary coils is 500mV. The voltage induced in the other coil will be:

a 3.

the same in both coils greater in coil B than in coil A

0V

b

250mV

c

500mV

d 1.0V

The output voltage of a Linear Variable Differential Transformer (LVDT) is taken to a full-wave rectifier. As the core is moved through the LVDT from one end to the other the output voltage of the full-wave rectifier will:

a

reverse polarity as the core passes through the central (neutral) position

b

remain constant at all positions of the core

c

gradually reduce from a maximum positive to zero

d

reduce to minimum and then return to maximum positive again

Continued ...

113

Linear Position or Force Applications Chapter 6

IT02 Curriculum Manual

Student Assessment 6 Continued ... 5.

The Linear Variable Capacitor mounted on the DIGIAC 1750 Trainer varies in capacitance by changing:

6.

a b

the nature of the dielectric varying the distance between the plates

c

varying the effective cross-sectional-area of the plates

d

the reactance of the capacitor due to a change in frequency

From the information given in the notes about the main characteristics of the Linear Variable Capacitor, as the slug is moved the capacitance varies by about:

a 7.

25pF/mm

b

15pF/mm

c

1.67pF/mm

d 0.6pF/mm

When using a circuit similar to that investigated in Practical Exercise 6.4, as the slug of a Variable Capacitor is moved in towards the middle of the sleeve the output voltage:

a 8.

remains the same

b

increases

c

changes polarity

d reduces

The main disadvantage of using a single Strain Gauge in a Wheatstone Bridge sensing circuit is that it:

9.

a

has poor sensitivity

b

gives a non-linear response

c

is affected by temperature

d

cannot be null-balanced

The strain gauge bridge used in the DIGIAC 1750 Trainer has:

a c

one active and one dummy strain gauges four active strain gauges

b d

two active and two dummy strain gauges two active and four dummy strain gauges

10. The difference between an active and a dummy strain gauge is that:

114

a

they are made of different materials

b

an active gauge is longer than a dummy

c

an active gauge is shorter than a dummy

d

they are mounted in different directions

IT02 Curriculum Manual

Environmental Measurements Chapter 7

Chapter 7 Environmental Measurements

Objectives of this Chapter

Equipment Required for this Chapter

Having studied this Chapter you will be able to: 

Describe the construction and characteristics of an air flow transducer.



Describe the construction and characteristics of an air pressure transducer.



Describe the construction and characteristics of a humidity transducer.

• • •

DIGIAC 1750 Transducer and Instrumentation Trainer. 4mm Connecting Leads. Digital Multimeter.

115

Environmental Measurements Chapter 7

7.1

IT02 Curriculum Manual

The Air Flow Transducer

RTD and Heater

RTD Unheated

Fig 7.1

Fig 7.1 shows the construction of an Air Flow Transducer, consisting of two RTD's (Resistance Temperature Dependent) mounted in a plastic case. One of the devices has an integral heating element incorporated with it and the other is unheated. The operation of the device uses the principle that when air flows over the RTD's, the temperature of the heated unit will fall more than that of the unheated unit. The temperature difference will be related to the air flow rate which will in turn affect the resistance of the RTD's. With the DIGIAC 1750 Trainer, the transducers are enclosed in a clear plastic container and provision is made for air to be pumped over the device. Fig 7.2 shows the electrical circuit arrangement and main characteristics of the device in the DIGIAC 1750 Trainer.

+5V

Heater power

-

+ RTD

RTD

Heater

Fig 7.2

116

1.7k Ω

Output voltage (-). Pump OFF

2.1V

Output voltage (+). Pump OFF

1.7V

Voltage change (airflow)

0.06V

Table 7.1

0V

1W

Output impedance

IT02 Curriculum Manual

7.2

Environmental Measurements Chapter 7

Practical Exercise Characteristics of an Air Flow Transducer

AIR FLOW SENSOR

PRESSURE

FLOW

AIR PRESSURE SENSOR

-

-

O/P

O/P

PUMP OFF

+

ON

V

MOVING COIL METER

+

5

AMPLIFIER #1 0V

INSTRUMENTATION AMPLIFIER O/P B

I /P

5 +10

+ -

.5

+

.4 1 100 10

A-B

A

0

-10

O/P

+ OFFSET

GAIN COARSE

.6 .7

.8

.3

.9

.2 .1

1.0

0V

L J

GAIN FINE

Fig 7.3 

Connect the circuit as shown in Fig 7.3 and set the GAIN COARSE control of Amplifier #1 to 10 and GAIN FINE control to 1.0. Check that the pump control is set to OFF.



Set the digital multimeter to the 20V range.



Switch ON the power supply and allow the temperature to stabilize.









Adjust the OFFSET control of Amplifier #1 for zero output continuously during this time, setting the GAIN COARSE control to 100 when stabilized conditions are approached. Set the Flow/Pressure control to FLOW. Check that the OFFSET control is set for zero output voltage. Use the digital multimeter to note the voltages at the - and + outputs from the transducer, then note the Amplifier #1 output voltage displayed on the Moving Coil Meter. Record the values in Table 7.2 overleaf.

117

Environmental Measurements Chapter 7



IT02 Curriculum Manual

Switch the pump ON and note the voltages again when conditions have stabilized, recording the values in Table 7.2 Pump OFF

Transducer - Output Voltage Transducer + Output Voltage Amplifier #1 Output Voltage

Pump ON

V

V

V

V

0

V

Table 7.2

The RTD's have a positive temperature coefficient.

7.2a

Which output is connected to the heated RTD, - a or + b ?



Switch OFF the power supply and the pump.

Notes: ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ 118

IT02 Curriculum Manual

7.3

Environmental Measurements Chapter 7

The Air Pressure Transducer Fig 7.4 shows the construction of an air pressure transducer and also shows the electric circuit arrangement of the DIGIAC 1750 unit. The device consists of an outer plastic case which is open to the atmosphere via two ports. Within this case is an inner container from which the air has been evacuated and a strain gauge Wheatstone bridge circuit is fitted on the surface. +5V Backing Plate -

Strain Gauge

O/P

Ports

Contacts

+

Vacuum Cavity 0V

Construction

Electrical Circuit

Fig 7.4

The air pressure in the outer container will produce an output from the bridge and variation of the pressure will produce a variation of this output. The transducer output can be calibrated and may be called an absolute pressure transducer. Provision is made for air to be fed to the unit from the pump. The main characteristics of the device are: Type

SPX200AN

Sensitivity (typical)

300µV/kPa

Temperature coefficient

1350ppm/°C

Output Voltage Pump OFF (-) Output Voltage (+) Pump ON

2.48V

Voltage difference Pump OFF Voltage difference Pump ON Output impedance

35mV 39mV 1.6kΩ

2.51V

Table 7.3

119

Environmental Measurements Chapter 7

7.4

IT02 Curriculum Manual

Practical Exercise Characteristics of an Air Pressure Transducer

AIR FLOW SENSOR

PRESSURE

FLOW

AIR PRESSURE SENSOR

-

INSTRUMENTATION AMPLIFIER O/P

-

O/P

O/P

PUMP OFF

+

ON

+

B

-

x100 AMPLIFIER O/P I/P

A-B

A

+100VIN

+

MOVING COIL METER AMPLIFIER #1

5

I/P

-

O/P .5

+

.4 1 100 10

OFFSET

.6

GAIN COARSE

0

5 +10

+

.7

.3

.8

.2

.9 .1

-10

1.0

GAIN FINE

0V

L J

Fig 7.5 

Connect the circuit as shown in Fig 7.5 and set the Amplifier #1 GAIN COARSE control to 10 and GAIN FINE control to 0.3. Ensure that the pump

switch is set OFF. 

Switch ON the power supply and adjust the OFFSET control of Amplifier #1 for zero output voltage. The unit is now calibrated zero for the current value of the atmospheric pressure.



Set the Flow/Pressure control to PRESSURE and then switch the pump ON. The output voltage from the Amplifier #1 will increase. Note the value of this voltage.

Output voltage (Pump ON) = 7.4a

V

Enter your value of output voltage with the Pump ON in V.

Note that a large amplification is required due to the low magnitude of the device output. 

120

Switch OFF the power supply and the pump.

IT02 Curriculum Manual

7.5

Environmental Measurements Chapter 7

The Humidity Transducer Fig 7.6(a) shows the construction of a humidity transducer, consisting of a thin disc of a material whose properties vary with humidity. Each side of the disc is metalized to form a capacitor.

Capacitor Plates

I/P

O/P

Dielectric Disc

47kΩ 0V Contacts

(a)

(b)

Fig 7.6

Variation of humidity of the surrounding air alters the permittivity and/or thickness of the dielectric material, changing the value of the capacitor. The unit is housed in a perforated plastic case. Fig 7.6(b) shows the electrical circuit arrangement for the DIGIAC 1750 unit. The unit is connected in series with a resistor with the output taken from the resistor. With an alternating voltage applied to the input, the output voltage will vary with humidity due to the variation of capacitance of the transducer.

121

Environmental Measurements Chapter 7

IT02 Curriculum Manual

The main characteristics of the device are: Type

90001

Capacitance (25°C, 45%R/H)

122pF ± 15%

Sensitivity

0.4pF/%RH

Humidity Range

10%-90% RH

Table 7.4

Note: R/H is Relative Humidity,

Amb ie nt H umid it y Sat urated Air

x

100%.

The device is slow to respond fully to humidity changes, taking in the order of minutes, but this will normally be of no consequence in practice since natural changes in humidity are very slow. The variation of output voltage from the circuit is only a small percentage of the output and this is difficult to detect. In the practical exercise you will use signal processing circuits which are available on the DIGIAC 1750 to convert the output to asmall DC signal, out the standing DC level andTrainer thus enable amplification of the voltagebalance changes.

Notes: ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................ ................................................................................................................................................................

122

IT02 Curriculum Manual

7.6

Environmental Measurements Chapter 7

Practical Exercise Characteristics of a Humidity Transducer

40kHz OSCILLATOR

A.C. AMPLIFIER O/P

HUMIDITY SENSOR I/P

O/P

40kHz FILTER

FULL WAVE RECTIFIER

O/P

O/P

I/P I/P

O/P

I/P

V

10 1000 100

DIFFERENTIAL AMPLIFIER O/P

GAIN SLIDE

C

2

3

4

5

6

7

8

9

10

0V

B

-

A

+

A-B

B 1

A

10k

MOVING COIL METER

5

AMPLIFIER #1 +5V

VIN

I/ P

-10

O/ P

0

5 +10

+ -

.5

+

.4 1 100 10

OFFSET

GAIN COARSE

.6 .7

.8

.3

.9

.2 .1

1.0

0V

L J

GAIN FINE

Fig 7.7 

Connect the circuit as shown in Fig 7.7, setting the AC Amplifier gain control to 10 and the Amplifier #1 GAIN COARSE control to 10 and GAIN FINE to 1.0.



Switch ON the power supply, remove the leads from the Differential Amplifier inputs and connect a short circuit between them. Adjust the OFFSET control of Amplifier #1 for zero output. Switch GAIN COARSE to 100 and make a final adjustment.



Replace the connections to the inputs of the Differential Amplifier and adjust the control of the 10k Ω carbon resistor for zero output from Amplifier #1. It may be advisable to set the coarse gain to 10 initially and then back to 100 finally during this process.

The bridge circuit is now balanced for the ambient conditions, the Differential Amplifier input from the 10kΩ variable resistor balancing that from the rectifier.

123

Environmental Measurements Chapter 7



IT02 Curriculum Manual

Note the output voltage from the rectifier circuit as indicated by the digital voltmeter. Output Voltage

Digital Meter

Ambient Conditions After Breathing

Moving Coil Meter 0 V

V

V

V

Table 7.5 

Now place your mouth near the humidity transducer and breath on it for a short time. The reading indicated by the Moving Coil Meter will change slowly.



Note the maximum value of the voltage and also the reading of the digital voltmeter.

Considering the readings obtained. Which meter do you consider gives a better indication of the voltage changes: a the Digital Multimeter, or 7.6a

b the Moving Coil Meter?

Enter your answer, a or b .

The time taken for the output voltage to return to zero after reaching the maximum voltage illustrates the slow response of the device to humidity changes. Time taken for output to return to zero =

Was the time taken for recovery less than 10 minutes a , or more b ? 7.6b

Enter your answer, a or b . 

Switch OFF the power supply.

Note: It is advisable to check the OFFSET of Amplifier #1 at regular intervals in case there has been any drift. This can be checked by just removing both of the input connections from the Differential Amplifier. The OFFSET control can then be adjusted if necessary. The ambient humidity conditions should not change during the test, but should a change occur, the bridge output will not return to zero.

124

IT02 Curriculum Manual

Environmental Measurements Chapter 7

Student Assessment 7 1.

The operating principle of the Air Flow Transducer relies on the use of:

a 2.

3.

strain gauges

b

RTD's

c

a capacitor

d a pressure pump

The Instrumentation Amplifier used in these experiments is a form of:

a

differential amplifier

b

bandpass filter

c

AC amplifier

d

summing amplifier

In Practical Exercise 7.2 (Air Flow Transducer Characteristics) the moving coil meter was balanced to zero at the start of the experiment using the:

a

10-turn variable resistor on the Wheatstone Bridge panel

b

10kΩ carbon slider potentiometer

c

balancing inputs on the Instrumentation Amplifier

d

offset control on Amplifier #1

4.

The operating principle of the Air Pressure Transducer relies on the use of: a strain gauges b RTD's c a capacitor d a pressure pump

5.

The output from an Air Pressure Transducer device is derived from a:

a 6.

series resistor

b

series capacitor

c

bridge circuit

d

x100

amplifier

At the start of the Air Pressure Transducer Characteristic experiment the output of the device is calibrated zero against:

a

relative humidity

b

ambient temperature

c

atmospheric pressure

d

ambient illumination

Continued ...

125

Environmental Measurements Chapter 7

IT02 Curriculum Manual

Student Assessment 7 Continued ... 7.

The operating principle of the Humidity Transducer relies on the use of:

a 8.

b

RTD's

c

a capacitor

d a pressure pump

The output from a humidity detector circuit varies between DC values of 3.50V and 3.52V over its full humidity range. Which of the following signal processing circuits would be necessary to provide an output range from 0 - 10V DC?

a 9.

strain gauges

AC amplifier

b

oscillator

c

DC amplifier

d 40 kHz filter

In the Humidity Transducer investigation, the DC component of the full-wave rectifier circuit was balanced out using:

a

10-turn variable resistor on the Wheatstone Bridge panel

b

10kΩ carbon slider potentiometer

c

balancing inputs on the Instrumentation Amplifier

d

offset control on Amplifier #1

10. The device investigated in this chapter with the slowest response time was the:

126

a

Air Flow Sensor

b

Air Pressure Sensor

c

Strain Gauge

d

Humidity Sensor

IT02 Curriculum Manual

Rotational Speed or Position Measurements Chapter 8

Chapter 8 Rotational Speed or Position Measurements

Objectives of this Chapter

Equipment Required for this Chapter

Having studied this Chapter you will be able to: 

Describe the construction, principles and application of Slotted Opto Transducers for counting and speed measurement.



Describe the construction, principles and application of Reflective Opto Transducers and Gray Coded Disc for position measurement.



Describe the construction, principles and application of Inductive Transducers for speed measurement.



Describe the construction, principles and application of Hall Effect Transducers to speed and positional measurement.



Describe the construction, principles and application of a Tacho-Generator to speed measurement.

• • •

DIGIAC 1750 Transducer and Instrumentation Trainer. 4mm Connecting Leads. Digital Multimeter.

127

Rotational Speed or Position Measurements Chapter 8

8.1

IT02 Curriculum Manual

The Slotted Opto-Transducer +5V

+12V

Slotted Aluminum Disc

Spindle

680Ω Infra-red LED

Photo-transistor

Case

O/P

47kΩ Contacts

470Ω

0V

(a)

(b)

Fig 8.1

Fig 8.1(a) shows the construction of a slotted opto transducer, consisting of a gallium arsenide infra-red LED and silicon phototransistor mounted on opposite sides of a gap in the case, each being enclosed in a plastic case which is transparent to infra-red radiations. The gap between them allows the infra-red beam to be broken when a solid object is inserted. The collector current of the phototransistor is low when the infra-red beam is broken and increases when the beam is admitted. Positive voltage pulses are obtained from the emitter circuit of the phototransistor each time the beam is admitted and hence the device generates pulses which are suitable for counting rotations. A slotted aluminum disc connected to the motor shaft assembly rotates in the transducer gap in the DIGIAC 1750 unit and an LED is provided to indicate when the slot position allows the beam to be admitted. Fig 8.1(b) shows the electrical circuit arrangement for the DIGIAC 1750 unit The main characteristics of the device are: Type

Output Voltage (beam broken)

0.1V

Output Voltage (beam admitted)

4.9V

Table 8.1

128

K8102

IT02 Curriculum Manual

8.2

Rotational Speed or Position Measurements Chapter 8

Practical Exercise Characteristics of a Slotted Opto Transducer

DC MOTOR

I/P

SLOTTED OPTO SENSOR

COUNTER/ TIMER I/P

O/P

TIM E

R E SET

O/P

FR E E R U N

C O U NT

1s

MOVING COIL METER WIREWOUND TRACK 6 5 C

5 POWER AMPLIFIER

7

4 3

8

2

9 1

10

B

O/P

0

5 +10

-10

V +

I/P

A

-

10k

0V +12V

L J

0V

Fig 8.2



Connect the circuit as shown in Fig 8.2 and set the 10k Ω wirewound resistor control fully counter-clockwise for zero output voltage.



Switch ON the power supply.



Rotate the shaft by hand using the large aluminum disc provided with the Hall effect device. Note and record in Table 8.2 the output voltage from the Slotted Opto Transducer output socket and also the state of the indicating LED: (a) with the beam broken by the aluminum disc, and (b) with the beam admitted through the slot in the aluminum disc. Beam Broken Output Voltage

Beam Admitted V

V

LED - ON/OFF Table 8.2

129

Rotational Speed or Position Measurements Chapter 8

Set the Timer/Counter to COUNT and FREE RUN. The display should show zero. If not, press RESET.





IT02 Curriculum Manual

Rotate the shaft backwards and forwards by hand so that the slot in the aluminum disc passes between the opto transducer.



Note the counter display, this should increment by 1 each time the slot is in line with the transducer beam. This illustrates the use of the opto transducer for counting applications.



Now adjust the 10kΩ wirewound resistor control to give a drive voltage to the motor of 2V as indicated by the Moving Coil Meter. The motor should operate and rotate the shaft. The counter value will increment once for each revolution of the shaft and can be used to measure the shaft speed:



Motor Drive Voltage (volts) Shaft Speed (rev/sec) Shaft Speed (rev/min)

Press the RESET button and hold down. With a watch, stop watch if available, release the reset button at a suitable time and note the count value after one minute. This value represents the shaft speed in revolutions per minute (rev/min). Record the value in the last row of Table 8.3. 2

3

4

5

6

7

8

9

10

Table 8.3 



Repeat with a motor drive voltage of 3V and add the result to Table 8.3. Set the COUNTER/TIMER FREE RUN/1s switch to 1s (1 second). Set the 10k Ω resistor to give a motor drive voltage of 4V. Press the RESET button of the counter. The counter now counts for one second and the count value is "frozen" at the end of this time. The count displayed represents the number of revolutions per second of the shaft. Press RESET again. The displayed value should correspond with the previous value. Record the value in Table 8.3 in the shaft speed (rev/sec) row.

130

IT02 Curriculum Manual

Rotational Speed or Position Measurements Chapter 8



Repeat the procedure with the other motor drive voltages shown in Table 8.3 and for each setting note the shaft speed in rev/sec as displayed by the counter and add to the table. Switch OFF the power supply.



Multiply each recorded rev/sec value by 60 to give the shaft speed in revolutions per minute (rev/min or rpm) and add to the last row of Table 8.3.



Plot the graph of motor speed in rev/min against drive voltage on the axes provided:

3000 2800 Motor Speed 2600 (rev/min) 2400 2200 2000 1800 1600 1400 1200 1000 800 600 400 200 0

2

3

4

5

6

7 8 9 10 Motor Drive Voltage (volts)

Graph 8.1

8.2a

From your graph deduce and enter the motor drive voltage needed to give a speed of 1800 rev/min in V. Keep the motor drive circuits connected for later experiments.

131

Rotational Speed or Position Measurements Chapter 8

8.3

IT02 Curriculum Manual

The Reflective Opto Transducer

Gray Code Disc Drive Shaft

Infra-red LED

Phototransistor

Motherboard Reflective Opto-Sensors

PlanView

SideView(elevation)

Fig 8.3

Fig 8.3 shows the construction of a reflective opto transducer, consisting of an infra-red LED and phototransistor. This is similar to the slotted opto transducer, but in this device the components are arranged so that the beam is reflected back if a reflective surface is placed at the correct distance. A non reflective surface breaks the beam. Three separate units are provided with the DIGIAC 1750 unit, being mounted in line vertically. The reflective surface is a Gray-coded disc, which is fixed approximately 4mm from the transducers. With the beam not reflected the output from the phototransistor emitter is low. When the beam is reflected the output is high. Three LED's are provided to indicate when the beam is reflected from the respective transducer unit. The output A is the least significant bit (LSB) and C is the most significant bit (MSB). The Gray code is used for the encoded disc rather than normal binary because only one digit changes state at any boundary with this code and this minimizes any possibility of error in identifying the actual position when at a segment boundary.

132

IT02 Curriculum Manual

Rotational Speed or Position Measurements Chapter 8

The arrangement of the Gray-coded disc and the respective LED outputs is shown in Fig 8.4. Gray Code Disc 4

5 A

P osi t i on

C

B

A

B

0

0

0

0

1

0

0

1

2

0

1

1

3

0

1

0

4

1

1

0

5

1

1

1

6

1

0

1

7

1

0

0

3

6 C

2

7

0

1

Fig 8.4

The dark areas break the beam and produce a low output from the associated transducer and the bright areas reflect the beam and produce a high output. The DIGIAC 1750 unit operates as a rotational angular position transducer but similar principles can be used for linear position applications. Slotted opto devices could be used with a transparent disc (transparent where the above disc is reflective).

A B C 01234567

Linear Position Fig 8.5

Fig 8.5 shows a linear Gray-coded track, the A track is the LSB and C the MSB. The resolution provided with a 3-bit code (3 opto devices) is poor but this can be improved by increasing the number of devices and tracks.

133

Rotational Speed or Position Measurements Chapter 8

IT02 Curriculum Manual

Note the Gray code pattern: START

REPEATS

LSB

A

1 unit length '0'

2 unit lengths '1'

2 unit lengths '0'

MSB

B C

2 unit length '0' 4 unit length '0'

4 unit lengths '1' 8 unit lengths '1'

4 unit lengths '0' 8 unit lengths '0'

Table 8.4

The electrical circuit arrangement for the DIGIAC 1750 unit is shown in Fig 8.6:

+12V

+5V

C

B

A

C

B

0V

Fig 8.6

The main characteristics of the device are:

Type Output Voltage (beam broken)

Output Voltage (beam admitted) Table 8.5

134

K8711 0.5V

5V

A

IT02 Curriculum Manual

8.4

Rotational Speed or Position Measurements Chapter 8

Practical Exercise Characteristics of Reflective Opto Transducers and Gray Code Disc

REFLECTIVE OPTO SENSORS

C

0V

B

A

V

Fig 8.7 

Connect the circuit as shown in Fig 8.7 with the digital multimeter on the 20V DC range.



Switch ON the power supply and rotate the drive shaft by hand to alter the LED states.



Rotate the shaft until it is in the position with all LED's OFF. Use the digital multimeter to measure the voltage at each of the outputs and record in Table 8.6.

Output A B

Output Voltage LED OFF LED ON V

V

V

V

V

V

C Table 8.6 

Turn the shaft until all LED's are ON and repeat the readings, recording the results again in Table 8.6.

135

Rotational Speed or Position Measurements Chapter 8



IT02 Curriculum Manual

With the shaft initially in the position with all LED's OFF, rotate the shaft counterclockwise, when looking at the coded side of the disc, and note the state of the LED's at each change of state. Denote an LED OFF as logic state 0 and LED ON as logic state 1.



Record the values in Table 8.7. Position

C

B

A

0 1 2 3 4 5 6 7

Table 8.7

Check the sequence against that shown in the table in Fig 8.4.

8.4a

Enter the voltage at the 'B' output when the LED is ON.

8.4b

Enter the voltage at the 'B' output when the LED is OFF.

8.4c

The code which you have recorded for step 6 in the form C B A is:

a 110 

136

b

011

Switch OFF the power supply.

c

101

d

none of these

IT02 Curriculum Manual

8.5

Rotational Speed or Position Measurements Chapter 8

The Inductive Transducer

Thick slotted disc

10kΩ O/P I/P

1mH Ferrite bobbin

Coil

Fig 8.8

Fig 8.8 shows the construction and electrical circuit arrangement for the Inductive Transducer provided with the DIGIAC 1750 unit. This consists of a 1mH inductor and a slotted aluminum disc fitted to the drive shaft which rotates above the inductor. The inductance of the unit varies with the position of the slot. With an aluminum disc the inductance increases with the slot positioned directly above the inductor. If a magnetic disc was used, the inductance would decrease for the condition when the slot was above the inductor. Note that, if unscreened, an inductor will be liable to pick up any stray interference, such as that which may be generated by the motor commutator switching. This can generate spurious short duration output pulses which may need to be suppressed by using a low pass filter. The main characteristics of the device (in circuit under the disc) are: Inductance

(under slot)

1mH

Inductance change

(under disc)

7µH

Output voltage

(under slot)

6.9mV

Output voltage change

(under disc)

2mV

Table 8.8

137

Rotational Speed or Position Measurements Chapter 8

8.6

IT02 Curriculum Manual

Practical Exercise Characteristics of an Inductive Transducer

40kHz OSCILLATOR

INDUCTIVE SENSOR

O/P

A.C. AMPLIFIER O/P

40kHz FILTER

I/P

FULL WAVE RECTIFIER

O/P

O/P

I/P

I/P VIN

I/P

10 1000 100

O/P GAIN

+5V WHEATSTONE BRIDGE D

DIFFERENTIAL AMPLIFIER O/P 12k

C

B

-

A

+

A

x100 AMPLIFIER O/P I/P

A-B OUT

B

3

+100VIN

IN 1V

Rx

0V

MOVING COIL METER

5

AMPLIFIER #1 I/ P

-10

O /P

0

5 +10

+ -

.5

+

.4 1 100 10

OFFSET

GAIN COARSE

.6 .7

-

.3

.8

.2

.9 .1

1.0

0V

L J

GAIN FINE

Fig 8.9 

Connect the circuit as shown in Fig 8.9. Set the AC Amplifier gain to 1000 and Amplifier #1 GAIN COARSE to 10 and GAIN FINE to 1.0. Set the drive shaft with the disc slot in the top vertical position.



Remove the leads from the input to the Differential Amplifier, short the inputs together and switch ON the power supply.





138

Adjust the

OFFSET control of Amplifier #1 for zero output.

Replace the leads to the input of the Differential Amplifier and adjust the control of the 10kΩ 10-turn resistor so that the meter reading is again zero. The control setting will be critical with such high overall amplifier gains.

IT02 Curriculum Manual

Rotational Speed or Position Measurements Chapter 8

Check the zero reading and then rotate the motor shaft to obtain the maximum output voltage when the slot is immediately above the Inductive Sensor. Note the value of this voltage:



Output voltage with slot over the inductor =

8.6a

V

Enter your maximum value of output voltage in V.

This indicates an application of inductive transducers to proximity detection of metallic objects. The device can also be used for counting or speed measurement applications. Switch OFF the power supply. Retain your circuit, but remove the Moving Coil Meter from the output of Amplifier #1 and then add the circuits of Fig 8.10.



LOW PASS FILTER O/P

DIFFERENTIATOR O/P

I/P

I/P

T I/P

DC MOTOR

100ms 10ms 1s

100ms 1s 10ms

TIME CONSTANT

To the output of Amplifier#1 O/P

8

2

9 1

10

COUNTER/ TIMER

B

TIME

R ESET

POWER AMPLIFIER

7

3

TIME CONSTANT

I/P

WIREWOUND TRACK 6 5 C 4

O/P

dVIN dt

C O UN T

FREE RUN

1s

MOVING COIL METER

I/P

5

A

-10

10k

0

5 +10

+ +12V

0V

0V

J L Fig 8.10

139

Rotational Speed or Position Measurements Chapter 8



IT02 Curriculum Manual

Set the motor speed to zero.



Set the TIME CONSTANT switches of the Low Pass Filter and the Differentiator to 1s and set the counter to COUNT and 1s.



Switch on the power supply.



Apply 2V input to the DC motor so that the shaft rotates slowly. Press the counter reset button several times and note the displayed value, this represents the speed in rev/sec.

Speed of the shaft recorded with the Inductive Sensor = 

Remove the lead from the o/p of the Low Pass Filter to the Differentiator and take the lead from the input of the Low Pass Filter and connect it to the Differentiator input. Press the Counter RESET button several times and observe the result. If the result is zero, then refer to the re-calibration procedure described in the next point and repeat the counts with and without the Low Pass Filter. When a reading has been observed restore the Low Pass Filter back into the circuit by moving the lead back and adding the connection between the Low Pass Filter and Differentiator.

Speed of the shaft recorded without Low Pass Filter =

140



Re-calibrate the Inductive Sensor circuit by removing the lead from the MC meter to the Power Amplifier and connecting it between the MC meter and the output of Amplifier #1. Adjust the control of the 10KΩ 10-turn resistor so that the meter reading is zero. Then reconnect the MC meter to the Power Amplifier.



Remove the Counter input lead from the Differentiator output and connect it to the output from the Slotted Opto Transducer. Press the counter reset button and note the displayed reading which also represents the shaft speed. Compare these value with the value obtained from using the Inductive Sensor.



Repeat the two measurements for the motor input voltages and complete Table 8.9 on the next page.

IT02 Curriculum Manual

Rotational Speed or Position Measurements Chapter 8

Motor Voltage Shaft Speed (rev/sec)

2V

4V

7V

10V

Inductive Transducer Slotted Opto Transducer

Table 8.9

8.6b

When the Low Pass Filter was removed from circuit the effect on the Counter readings was to:

a produce a constant count each time b

make no change

c reduce the count

increase the count

d

You will note that a considerable amount of signal conditioning has been required for the inductive transducer unit due to the small output voltage available and also the problem of the susceptibility of the counter to voltage spikes.

Notes:

........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................

141

Rotational Speed or Position Measurements Chapter 8

8.7

IT02 Curriculum Manual

The Hall Effect Transducer Magnetic Field Thick Aluminum Disc +5V Drive Shaft

Hall Effect Voltage VH -

Current 634SS2

O/P

+

N S

Hall Effect Sensor 0V

Embedded Magnet Motherboard

Fig 8.11

Fig 8.11 shows the layout and electrical circuit arrangement of the Hall Effect Transducer assembly fitted to the DIGIAC 1750 Trainer and illustrates the Hall Effect principle. Hall Effect Principle When current flows through the flat slice of semiconductor at right-angles to a magnetic field there is a force on each individual electron which tends to move it in one particular direction (the motor principle).

The current is pushed to one side of the slice. The surplus of electrons on one side of the slice means that this side is negatively charged, resulting in an EMF across the slice (the Hall voltage VH) which is at right-angles to both the current and the magnetic field. The value of this voltage is directly proportional to the strength of the magnetic field. The transducer provided on the DIGIAC 1750 Trainer also contains an active silicon semiconductor device to increase the output voltage and provide differential outputs, one going more positive and the other more negative (less positive). The main characteristics of the device are: Output voltage (+) (no field) Output voltage (-) (no field) Output voltage change Output voltage change (under magnet) Table 8.10

142

1.75-2.25V 1.60V 7.5-10.6mV/mT 380mV

IT02 Curriculum Manual

8.8

Rotational Speed or Position Measurements Chapter 8

Practical Exercise The Characteristics of a Hall Effect Transducer

DC MOTOR

I/P

HALL EFFECT SENSOR

COUNTER/ TIMER

B

I/P

TI ME

FR EE RU N

A-B

-

O/P

DIFFERENTIAL AMPLIFIER O/P -

+

A O/P

R ESET

C OU N T

1s

+

POWER AMPLIFIER

7

3

8

2

9 1

10

MOVING COIL METER

V

WIREWOUND TRACK 6 5 C 4

O/P

B

I/P

5

AMPLIFIER #1 I/P

A

0

5 +10

-10

O/P

+

10k

-

+12V

.5

+

.4 1 100 10

0V OFFSET

.6 .7

-

.3

.8

.2

.9

GAIN COARSE

.1

1.0

0V

L J

GAIN FINE

Fig 8.12 

Connect the circuit as shown in Fig 8.12. Set the Amplifier #1 GAIN COARSE control to 10, GAIN FINE to 0.8 and the motor drive voltage to zero. Switch ON the power supply.



Set the drive shaft position so that the magnet in the Hall effect disc is horizontal (to one side) so that there is no magnetic field cutting the Hall effect device.



Adjust the OFFSET control of Amplifier #1 for zero output indication on the Moving Coil Meter.



Note the output voltage from the - and + output sockets of the Hall Effect device with the digital voltmeter directly on the Hall Effect Sensor panel and also from the Moving Coil Meter. Record the results in Table 8.11. Magnetic Field None Maximum

Digital Multimeter Output Voltage (-) Output Voltage (+) V

Moving Coil Meter 0 V V

V

V

V

Table 8.11

143

Rotational Speed or Position Measurements Chapter 8

IT02 Curriculum Manual



Rotate the disc so that the magnet is directly above the Hall effect device. This position will be indicated by the maximum output voltage.



Note the voltages again and record in Table 8.11.

These readings illustrate the basic characteristics of the Hall Effect device and indicate its application measurement applications.to proximity detection. It is also suitable for speed 

With the output of Amplifier #1 connected to the Counter/Timer input set the controls for COUNT and 1s.



Transfer the digital multimeter to the output of the Power Amplifier and apply an input voltage of 2V to the motor so that the shaft rotates slowly. Press the Counter RESET button and note the displayed value, this representing the shaft speed in rev/sec. Record the result in Table 8.12



Remove the input to the counter from Amplifier #1 and connect it to the output of the Slotted Opto Transducer unit. Press the counter "reset" button and note the displayed value, this being the shaft speed for comparison with the previous reading. Add the value to Table 8.12. Motor Voltage Shaft Speed (rev/sec)

2V

4V

7V

10V

Hall Effect Transducer Slotted Opto Transducer Table 8.12 

8.8a

Repeat the procedure for the other values of motor drive voltage given in Table 8.12 for comparison. Switch OFF the power supply.

Enter your value of output voltage from the (+) O/P of the Hall Effect sensor with no magnetic field.

8.8b

Enter your value of output voltage from the (+) O/P of the Hall Effect sensor with maximum magnetic field.

Hall Effect devices are available for proximity detection, linear or angular displacement, multiplier and current or magnetic flux density measurement applications.

144

IT02 Curriculum Manual

8.9

Rotational Speed or Position Measurements Chapter 8

The DC Permanent Magnet Tacho-Generator Fig 8.13 shows the construction and electrical circuit arrangement of the DC Permanent Magnet Tacho-Generator fitted to the DIGIAC 1750 Trainer. This consists of a set of coils connected to a commutator which rotate inside a permanent magnet stator.

N

+12V

Coils

O/P

Commutator S

Brushes

Permanent Magnet Stator

M 0V

Armature

-12V

Connections

Fig 8.13

The rotating assembly is called the armature. With the coils rotating, an alternating EMF is generated in them. The commutator converts this to DC. The magnitude of the generated EMF is proportional to the rate of cutting flux and therefore to the rotational speed. The polarity depends on the direction of cutting flux and therefore on the direction of rotation. The diodes are fitted to limit any voltage spikes that may be generated by the commutation process (i.e. conversion from AC to DC) to a maximum of ±12V. The main characteristics of the device are: Open circuit voltage (12V to motor)

10.5V

Short circuit current (12V to motor)

750mA

Output impedance

39Ω

Output noise

200mV p-p

Table 8.13

145

Rotational Speed or Position Measurements Chapter 8

IT02 Curriculum Manual

8.10 Practical Exercise Characteristics of a Permanent Magnet DC Tacho-Generator

MOVING COIL METER AMPLIFIER #1

0

I/P

5

O/ P

-

.5

+

.4 1 100 10

OFFSET

GAIN COARSE

+10

+

.7 .8

.2

.9

0V

1.0

GAIN FINE

DC MOTOR

I/P

.6

.3

.1

5

-10

L J

SLOTTED OPTO SENSOR

TACHOGENERATOR

O/P

O/P

O/P

WIREWOUND TRACK 6 5 C

POWER AMPLIFIER

7

4 3

8

2

9 1

B

O/P

COUNTER/ TIMER I/P

T IM E

F R E E R UN

I/P

A

10

10k

V +12V

R ES ET

C O UNT

0V

Fig 8.14 



Connect the circuit as shown in Fig 8.14. Set the

COUNTER/TIMER controls to COUNT and 1s.

Set Amplifier #1 GAIN COARSE control to 10 and GAIN FINE to 0.1. 

146

Switch ON the power supply.

1s

IT02 Curriculum Manual



Rotational Speed or Position Measurements Chapter 8

Apply an input to the motor and set the shaft speed to 5 rev/sec (Note: Table 8.3 and Graph 8.1 may help) as indicated by the counter after pressing the RESET button. Note the output voltages indicated on the Moving Coil Meter and record the values in Table 8.14. Shaft Speed (rev/sec)

5

Output Voltage (Moving Coil Meter)

10

V

20

V

30

V

40

V

V

Table 8.14 

Repeat the procedure for the other shaft speed settings indicated in Table 8.14.



Draw the graph of output voltage against shaft speed on the axes provided.

10 Output 9 Voltage 8 (volts) 7

6 5 4 3 2 1 0

0

5

10

15

20

25 30 35 40 Shaft Speed (rev/sec)

Graph 8.2

8.10a

Is the characteristic linear?

Yes

8.10b

or

No

From your graph estimate and enter your recorded output voltage from the digital multimeter when the shaft speed is 25 rev/sec.

147

Rotational Speed or Position Measurements Chapter 8

IT02 Curriculum Manual

Calibration of the Moving Coil Meter to Indicate Speed Directly.

The scale to be used is 20V represents 2000 rev/min (100 rev/min/V).

MOVING COIL METER

1000 0

-10

500 5 -7

0

1500 5 2000 +8 +10

rev/min

+ 0V

L J

Fig 8.15 

Transfer the connection of the Moving Coil Meter from the input of Amplifier #1 to the output of Amplifier #1. Set the GAIN FINE control to just a little above 0.3.

148



Apply a low input to the motor and set the shaft speed to 5 rev/sec (300 rev/min) as shown on the Counter after pressing RESET. Adjust the OFFSET control of Amplifier #1 to set the Moving Coil Meter reading to -7V (Fig 8.15).



Change the motor drive voltage to set the shaft speed to 30 rev/sec (1800 rev/min) as shown on the Counter after pressing RESET. Adjust the GAIN FINE control of Amplifier #1 so that the Moving Coil Meter indicates +8V (Fig 8.15).



Repeat both of the above settings and adjustments as often as necessary to make both of them correct (changing one of them will have altered the other. Some anticipation may be helpful). The meter will then be calibrated as shown in Fig 8.15.

IT02 Curriculum Manual

Rotational Speed or Position Measurements Chapter 8



Use the calibrated Moving Coil Meter to set the motor speed as shown in Table 8.15.



Calculate the corresponding speed in rev/sec and then check at each setting against those obtained from the Opto Transducer and Counter. Shaft Speed (rev/min) Calculated Shaft Speed (rev/sec) Shaft Speed from Counter (rev/sec)

600

1000

1200

1600

Table 8.15

8.10c

Which was easier for setting the motor speed, the calibrated Moving Coil Meter a , or the Counter b ?

8.10d

Enter your motor speed as indicated on the Counter in rev/sec when the motor speed was set to 1000 rev/min.

8.10e

Enter your motor speed as indicated on the Counter in rev/sec when the motor speed was set to 1200 rev/min. 

Switch OFF the power supply

149

Rotational Speed or Position Measurements Chapter 8

IT02 Curriculum Manual

Student Assessment 8 1.

For a slotted opto transducer to count revolutions of a shaft it requires a:

a 2.

7.

150

non-magnetic disc d magnetic disc

500 µV

b

50mV

c

5V

d 12V

240

b

600

c

720

d 1440

0 1 1, 0 1 0

b

0 0 1, 0 1 0

c

1 1 0, 1 1 1

d 1 1 1, 1 0 1

The number of outputs generated by a 3-bit Gray code system is :

a 6.

c

Which of the following could NOT be in sequence (one after the other) for a Gray code output?

a 5.

reflective disc

A shaft speed of 24 rev/sec corresponds to a motor speed in rev/min (rpm) of:

a 4.

b

The output voltage generated by the slotted opto transducer used in your experiments was in the order of:

a 3.

slotted disc

3

b

6

c

8

d 10

Comparing a slotted magnetic disc to a slotted aluminum disc, the effect on the inductance of the slot passing over an inductive transducer is to:

a

increase the inductance in both cases

b

increase the inductance with a magnetic disc but decrease it with aluminum

c

decrease the inductance in both cases

d

decrease the inductance with a magnetic disc but increase it with aluminum

The purpose of the low pass filter used with the inductive transducer was to:

a

remove interference pulses

b

respond to low revolutions only

c

respond to high revolutions only

d

display only low revolution counts

IT02 Curriculum Manual

Rotational Speed or Position Measurements Chapter 8

Student Assessment 8 Continued ... 8.

9.

The purpose of the differential amplifier in the inductive transducer experiment was to:

a

respond to both increases or decreases in transducer output voltage

b

balance out the steady DC component of the rectifier output

c

invert the polarity of the signal from the transducer

d

sharpen up the output pulses from the transducer

An output voltage is generated by a Hall effect device when:

a

a slot in an aluminum disc passes over it

b

a magnetic field passes through it

c

it is exposed to light radiations

d

an external EMF is applied

10. The expected change in output voltage of an activated Hall effect transducer of the type fitted on the DIGIAC 1750 Trainer is of the order of:

a

±500µV

b

±500mV

c

±5V

d

±12V

11. The magnitude of the output voltage generated by a permanent magnet tacho-generator is dependent on:

a

rate of cutting flux

b

direction of rotation

c

frequency of interference pulses

d

number of commutator segments

12. The purpose of the commutator in a permanent magnet tacho-generator is to:

a

increase the magnitude of the output voltage

b

increase the frequency of the output voltage

c

stabilize the input voltage

d

convert the output from AC into DC

151

Rotational Speed or Position Measurements Chapter 8

IT02 Curriculum Manual

Notes:

........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................ ........................................................................................................................

152

IT02 Curriculum Manual

Sound Measurements Chapter 9

Chapter 9 Sound Measurements

Objectives of this Chapter

Having studied this Chapter you will be able to: 

Describe the construction and characteristics of a dynamic microphone.



Describe the construction and characteristics of an ultrasonic receiver and transmitter.



Equipment Required for this Chapter

• • • •

Compares the various methods of measuring sound signals.

DIGIAC 1750 Transducer and Instrumentation Trainer. 4mm Connecting Leads. Oscilloscope. 12 inch (30cm) ruler (not supplied).

153

Sound Measurements Chapter 9

9.1

IT02 Curriculum Manual

The Dynamic Microphone

Fig 9.1

The construction of the dynamic microphone is shown in Fig 9.1(a), consisting of a coil attached to a thin diaphragm, the coil being suspended in the field of a permanent magnet. The diaphragm moves in response to any vibration in the air caused by sound and moves the coil in the magnetic field. An alternating EMF is induced in the coil, the magnitude and frequency of which is proportional to the sound vibrations. The electrical circuit for the device provided with the DIGIAC 1750 unit is shown in Fig 9.1(b). A resistor is fitted to provide a load matched to the output impedance of the microphone. The main characteristics of the device are: Output impedance

Typically 200 - 500Ω

Frequency response (-3dB)

100Hz - 10kHz

Output voltage

5mV (normal maximum)

Table 9.1

154

IT02 Curriculum Manual

9.2

Sound Measurements Chapter 9

Practical Exercise Characteristics of a Dynamic Microphone It is most unlikely that your laboratory will include a broadband constant output audio generator system/loudspeaker amongst its facilities. Even if it did, a full acoustic booth would be required, and the noises generated would be unacceptable for other laboratory users. We are therefore not able to test the full dynamic range of a microphone, either for frequency or amplitude. It is therefore necessary for us to limit the investigation to a review of the measurement techniques that can be adopted, and it is these which will be examined, rather than the microphone itself.

A.C. AMPLIFIER O/P

MICROPHONE

I/P

FULL WAVE RECTIFIER O/P I/P

O/P

VIN 10 1000 100

L.E.D. BARGRAPH DISPLAY

GAIN I/P AMPLIFIER #1 I/P

-

O/P .5

+

OFFSET

.6

GAIN COARSE

MOVING COIL METER

.7

.4 1 100 10

DRIVER I.C.

.8

.3

5

.9

.2

-10

1.0

.1

0

5 +10

GAIN FINE

+ AMPLIFIER #2

-

I/P

O/P

0V -

.5

+

.4 1 100 10

OFFSET

GAIN COARSE

.6

L J

.7 .8

.3

.9

.2 .1

TP1

0V

1.0

GAIN FINE

Oscilloscope CH.1

Fig 9.2

155

Sound Measurements Chapter 9

IT02 Curriculum Manual

In this exercise three different forms of monitoring device will be investigated. The response time of digital multimeters is too slow to make any record of the signals at all, due to the transient nature of sound. 

Connect the circuit as shown in Fig 9.2. Set the AC Amplifier gain control to 1000 and the Amplifier #1 GAIN COARSE to 1 and GAIN FINE to 0.4.

The LED bargraph display has an excellent response time and requires 0.5V for each bar, 5V to light the whole display. This type of device is often used on HI-FI systems. 

Switch ON the power supply. Check the OFFSET control of Amplifier#1 for zero with the Moving Coil Meter temporarily connected to its output. Note the display on the Bargraph when the bench is tapped with the finger.



Tap the case of the 1750 unit and observe the effect on the Bargraph display.



Change the GAIN COARSE of Amplifier #1 to 10 and the FINE GAIN to 1.0 then talk, cough, sing or whistle near the unit. You will find that the bargraph will respond to any sound made, but needs more gain for speech or whistling.

A Moving Coil Meter is frequently used by sound (audio) engineers to indicate peak power (PPM, peak power meter), but requires a rectifier and amplifier since the moving coil meter only responds to DC, and its movement is slow to respond due to inertia and damping. 

The Moving Coil Meter is connected to the AC Amplifier output via the Full Wave Rectifier and Amplifier #2. Set the GAIN COARSE to 10 and GAIN FINE to 1.0, and zero the indication of the meter using the OFFSET control. Tap the baseboard so that all LED's of the bargraph are lit and note the maximum reading of the Moving Coil Meter. Maximum voltage indication given by the Bargraph is 5V. Maximum voltage output (Moving Coil Meter) =

9.2a

156

V

Enter your maximum voltage indicated by the Moving Coil Meter in V.

IT02 Curriculum Manual

Sound Measurements Chapter 9

Without any doubt, the oscilloscope is the most versatile device for monitoring sound, since it is able to give an indication of frequency, waveform and magnitude of signals and is very sensitive, even to small signals. 

Set the timebase of the oscilloscope to 2ms/div and the CH.1 Y1 amplifier to 1V/div.



Generate various types of sound and observe the display on the oscilloscope. Note that sound engineers, to save their embarrassment, will often count say from one through ten and back again - to test a microphone circuit. It may be necessary to vary the Y amplifier setting to obtain the most satisfactory displayed waveform.



9.2b

Change the timebase setting to 0.5ms/div. and try whistling two different notes, one low pitch and the other high, and observe the effect on the number of cycles (frequency) of the displayed waveform.

The most commonly observed waveform for all types of sound is a:

a square wave

9.2c

sinewave

c

triangular wave d

irregular wave

The steadiest, purest waveform was produced by:

a tapping

9.2d

b

b

talking

c

coughing

d

whistling

The effect of a high-pitched note compared to a low note was to produce:

a the same number of cycles

b

less cycles, lower frequency

c more cycles, higher frequency

d

no difference at all

157

Sound Measurements Chapter 9

9.3

IT02 Curriculum Manual

The Ultrasonic Transmitter/Receiver O/P

Fine Wire Mesh

I/P

Case Cavity Diaphragm

Piezo Ceramic Element

Ultrasonic Receiver

Ultrasonic Transmitter 0V

0V

Contacts

(a)

(b)

Fig 9.3

The construction of both ultrasonic devices and their electrical circuit arrangements for the DIGIAC 1750 unit are shown in Fig 9.3. The receiver and transmitter are almost identical and consist of a slice of ceramic material with a small diaphragm fixed to it, inside the case of the unit. The operation of the receiver relies on the principle that certain ceramic materials produce a voltage when they are stressed. This is referred to as the piezo-electric principle. Vibration of the diaphragm stresses the ceramic material and produces an output voltage. The reciprocal applies to the transmitter. An applied alternating voltage produces stress which causes the ceramic slice to vibrate. The dimensions of the components are arranged so that there is resonance (best response) at around 40kHz. This is above the audible range (maximum 20kHz) and is therefore referred to as ultrasonic. The ceramic slice is arranged in four quarters which are connected in series for the receiver and in parallel for the transmitter. The main characteristics of the devices are: Receiver Peak resonance (typical)

40kHz

Directional angle Impedance Output amplitude Table 9.2

158

Transmitter

30° 30kΩ 5-60mV

500Ω

IT02 Curriculum Manual

9.4

Sound Measurements Chapter 9

Practical Exercise Characteristics of an Ultrasonic Transmitter/Receiver

ULTRASONIC TRANSMITTER

40kHz OSCILLATOR O/P

I/P

A.C. AMPLIFIER O/P

ULTRASONIC RECEIVER

O/P

40kHz FILTER

I/P

LOW PASS FILTER O/P

FULL WAVE RECTIFIER

O/P I/P

I/P

O/P I/P VIN

10 1000 100

100ms 10ms 1s

GAIN

TIME CONSTANT L.E.D. BARGRAPH DISPLAY I/P

AMPLIFIER #1 I/P

-

DRIVER I.C.

O/P .5

+

.4 1 100 10

OFFSET

GAIN COARSE

MOVING COIL METER

.6 .7

.3

.8

.2

.9 .1

1.0

GAIN FINE

5 -10

0

5 +10

+ 0V

L J

Fig 9.4 

Connect the circuit as shown in Fig 9.4. Set the AC Amplifier gain control to 1000 and Amplifier #1 GAIN COARSE control to 10 and GAIN FINE to 0.5. Switch the Low Pass Filter time constant to 100ms.



Switch ON the power supply and adjust Amplifier #1 OFFSET to give zero



Note the bargraph display as you move your hand or any other object over the ultrasonic devices. The display should respond, indicating the receipt of a signal of frequency 40kHz by the ultrasonic receiver.

output on the Moving Coil Meter.

159

Sound Measurements Chapter 9



9.4a

IT02 Curriculum Manual

Place a small book (approximately 6 inches (15cm) × 4 inches (10cm) or other flat object 3 feet (90cm) above the Ultrasonic Transducers. Slowly move the object closer to the transducers, watching the output reading on the bargraph display, until the object is covering the transducers.

At which of the following positions is the maximum output obtained? a Object in contact with the Ultrasonic Tranducers.

b

Object 4 inches (10cm) above the unit.

c

Object 12 inches (30cm) above the unit.

d

Object 3 feet (90cm) above the unit.

Remove any other equipment from the vicinity so that you have free access to the ultrasonic transmitter/receiver area. 

9.4b

Increase the Amplifier #1 GAIN FINE control to 1.0. Hold a thin object such as a pencil approximately 6 inches (15cm) above the Ultrasonic Transducers, move it horizontally and vertically and note the effect on the output response. This indicates how critical the direction angle is for the device.

Is the position of the reflector critical?

Yes 

9.4c

or

No

Put a sheet of paper over the Ultrasonic Transducers to intercept the path and move your hand up and down above the transducers.

Does the beam pass through a piece of paper?

Yes

or

No

In this exercise the received signal has been amplified, rectified, filtered (to remove all unwanted frequencies) and then amplified again to operate the display. Pulsed ultrasonic devices can be used for distance measurement to reflecting surfaces by measurement of the time between the transmission and return of the pulsed signal.

160

IT02 Curriculum Manual

Sound Measurements Chapter 9

Student Assessment 9 1.

2.

The dynamic microphone operates due to:

a

a change of resistance

b

the piezo electric effect

c

the Doppler effect

d

a conductor cutting magnetic flux

The output impedance of the microphone fitted on the DIGIAC 1750 Trainer is:

a 3.

6.

8.

Ω

c

200 - 500

Ω

d 1500 - 1200

Ω

digital multimeter b

oscilloscope

c

LED bargraph

d moving coil meter

rectifier

b

low pass filter

c

band pass filter

d

40kHz oscillator

Ultrasonic Receivers and Transmitters rely for their operation on:

a

a change of resistance

b

the piezo electric effect

c

the Doppler effect

d

a conductor cutting magnetic flux

The peak resonance of the ultrasonic receiver and transmitter used on the D1750 unit is at a frequency of:

a 7.

100 - 150

The Moving Coil Meter cannot respond to sound signals without a:

a 5.

b

The most versatile test instrument for monitoring sound signals is the:

a 4.

25 - 75 Ω

20kHz

b

30kHz

c

40kHz

d 60kHz

Which audio transducer on the D1750 unit has the highest impedance?

a

Ultrasonic receiver

b

Ultrasonic transmitter

c

Microphone

d

Low pass filter

The frequency response (-3dB) of the microphone used on the D1750 unit is:

a

0 - 40KHz

b

0 - 10KHz

c

100Hz - 40KHz

d

100Hz - 10KHz

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Notes: ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...........................................................................................................................................

162

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Sound Output Chapter 10

Chapter 10 Sound Output

Objectives of this Chapter

Equipment Required for this Chapter

Having studied this Chapter you will be able to: 

Describe the construction and characteristics of a moving coil loudspeaker.



Describe the construction and characteristics of a buzzer.

• • • • • •

DIGIAC 1750 Transducer and Instrumentation Trainer. 4mm Connecting Leads. BNC to 4mm Connecting Lead. Digital Multimeter. Oscilloscope. Function Generator.

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10.1 The Moving Coil Loudspeaker

Paper Cone I/P

Contacts

100Ω Loudspeaker

Frame S

Permanent Magnet

S N

Coil

0V

(a)

(b)

Fig 10.1

The construction of a moving coil loudspeaker is shown in Fig 10.1(a). It is similar to the moving coil microphone. The permanent magnet, coil and diaphragm are much the same but in this device the diaphragm is attached to a large paper cone supported by a frame. The cone is free to move with the coil. Alternating currents flowing in the coil cause it react with the magnetic field and move in and out. With applied currents at frequencies in the audible range, the cone movement will cause a variation of pressure in the surrounding air particles and produce sound waves that are audible to the human ear. If a speaker is placed in a vacuum, there are no air particules, so the movement of the cone does not produce any sound. The electrical circuit of the device fitted to the DIGIAC 1750 unit is shown in Fig 10.1(b). The 100Ω resistor is fitted to limit the maximum power dissipation to 100mW, half of the rated value for the loudspeaker. The main characteristics of the device fitted to the DIGIAC 1750 unit are: Impedance

8Ω

Power rating

200mW rms.

Frequency response (-3dB)

400-5000Hz

Table 10.1

Note that the speaker response is well below the maximum frequency detectable by the human ear (approximately 20kHz).

164

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Sound Output Chapter 10

10.2 Practical Exercise Characteristics of a Moving Coil Loudspeaker Function Generator

WIREWOUND TRACK 6 5 C

POWER AMPLIFIER

LOUDSPEAKER

7

4 3

8

2

9 10

1

B

O/P I/P

I/P

A

10k MICROPHONE

x100 AMPLIFIER

0V O/P

O/P I/P +100VIN

TP1

0V

CH.1

TP2

Oscilloscope

0V

CH.2

Fig 10.2



Connect the circuit as shown in Fig 10.2 and switch ON the power supply.



Set the 10kΩ wirewound variable resistor to position 5 on its scale (see Fig10.2).



Set the oscilloscope timebase initially to 1ms/div, CH.1 Y amplifier to 5V/div and CH.2 Y amplifier to 0.2V/div.



Set the function generator to 200Hz sinewave output and adjust the amplitude control to maximum and then adjust the 10kΩ wirewound resistor to give a signal input of 10Vp-p (2 div.) as seen on CH.1 of the oscilloscope. The signal input level of 10Vp-p is to be carefully maintained for tests at all frequencies.

The microphone and its amplifier will pick up all of the background sounds and interference the laboratory. Try to ignore these in taking your readings at so lower signal levels.inYou will be contributing to other peoples background noises, try to keep yours to a minimum.

165

Sound Output Chapter 10



Frequency (Hz)

200

IT02 Curriculum Manual

Take readings at each of the frequencies given in Table 10.2, ensuring that the input signal remains constant at 10Vp-p. 300

400

500

600

700

800

900

1k

2k

3k

Output Voltage Vpeak-to-peak Table 10.2 

One of your readings should have been much greater than any of the rest. Return to this frequency and use the fine frequency control on the function generator to peak the signal to maximum. Record the value in Table 10.3.



Ensure that the timebase controls are in the calibrated settings and measure the number of divisions taken for one complete cycle. Record in Table 10.3.



Measure the frequency as follows, adding the results to Table 10.3:

One cycle 6.7 div.

Peak Signal Amplitude Vp-p

Number of divisions

Time for one cycle (Τ) ms

Frequency f= 1 Τ

Hz

Table 10.3

The time for one cycle is calculated by multiplying by the timebase setting, for example 6.7 x 0.2ms = 1.34ms. 1 The reciprocal of this gives the frequency: 13 . 4 × 10 − 3 = 746Hz. Note that this example has been chosen to be different from the result which you should get.

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Sound Output Chapter 10

The frequency which you calculate is the natural resonant frequency of the loudspeaker. The response curve of the loudspeaker has a very pronounced peak at this frequency. It is caused by the dimensions of the loudspeaker cone, largely the cone diameter. 

Plot the response of the loudspeaker on the axes provided. A logarithmic scale is used for frequency because this matches the response of the ear. 0.5 Output Voltage (Vp-p)

0.4

0.3

0.2

0.1

100

200

500 700 1k

2k

5k 7k 10k frequency (Hz)

Graph 10.1

If this type of loudspeaker was used for music output then the response of the electronic driving circuit would need to be shaped to compensate for the response. This would be done by boosting both the lower and higher frequencies. If used as an alarm generator then it would be best to choose the resonant frequency for greatest efficiency, to generate the loudest sound output from a given power input. 10.2a

Enter your calculated loudspeaker resonant frequency in kHz. 

Switch OFF the power supply.

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10.3 The Buzzer The construction of the buzzer used in the DIGIAC 1750 unit is shown in Fig 10.3(a).

I/P

Diaphragm Magnet

Spring

Iron core

Circuit board

Coil 0V Contacts

(a)

(b)

Fig 10.3

A small transistorized oscillator circuit feeds an alternating EMF to an iron cored coil. The alternating magnetic field produced by the coil attracts and repels a small permanent magnet attached to a spring. This magnet vibrates against a diaphragm and creates a loud noise. In control system applications the device is used as an alarm indication. The electrical circuit of the device is shown in Fig 10.3(b). The diode is fitted to prevent damage to the transistorized circuit if the supply is connected with incorrect polarity. The polarity of the input supply should be positive. The rated voltage is 12V. The main characteristics of the device fitted to the DIGIAC 1750 unit are: Supply voltage

8V

12V

16V (max.)

Supply current

15mA

-

30mA

400Hz

-

Output frequency Output sound level Table 10.4

168

-

70dBA at 7.87" (20cm)

IT02 Curriculum Manual

Sound Output Chapter 10

10.4 Practical Exercise Characteristics of a Buzzer

WIREWOUND TRACK 6 5 C

POWER AMPLIFIER

MICROPHONE

BUZZER

7

4 3

8

2

9

A

I/P

O/P

I/P

A

10

1

O/P

B

10k 0V

COUNTER/ TIMER

+12V A.C. AMPLIFIER O/P I/P

DIFFERENTIATOR O/P I/P

T

I/P

TIME

dVIN dt R E SE T

10 1000 100

GAIN

F RE E R U N

100ms 1s 10ms

COUNT

1s

MOVING COIL METER

TIME CONSTANT

5 -10

0

5 +10

+ 0V

L J

Fig 10.4



Connect the circuit as shown in Fig 10.4. Set the control of the 10k Ω resistor for zero output voltage (fully counter-clockwise). Connect the digital multimeter as an ammeter on the 20/200mA range between the output of the power amplifier and the buzzer to monitor the buzzer current. Set the A.C. Amplifier to 1000 and the Differentiator to 1s.

Note: When you first switch on, there may be readings on the counter immediately, due to background noise being picked up by the microphone and processed by the Counter. The readings should be ignored as they will not affect the experiment results.

169

Sound Output Chapter 10



IT02 Curriculum Manual

Switch ON the power supply and adjust the 10kΩ resistor to increase the voltage applied to the buzzer. Note the voltage on the Moving Coil Meter at which the buzzer begins to operate. Press RESET on the Counter to read the buzzer frequency.

The buzzer begins to operate at 

V at a frequency of

Hz

Alter the setting of the 10k Ω resistor to increase the voltage applied to the buzzer to 4V, 6V, 8V and then 10V as given in Table 10.5. Record the current and frequency at each step. Voltage Current Frequency

4V

6V

8V

10V

12V

mA

mA

mA

mA

mA

Hz

Hz

Hz

Hz

Hz

Table 10.5 

Transfer the positive lead of the digital multimeter from the output of the Power Amplifier to the +12V socket to bypass the 10k Ω resistor and Power Amplifier and apply the full 12V directly to the buzzer. Record the current and frequency again in Table 10.5.



170

Switch OFF the power supply.

10.4a

Enter your minimum voltage for buzzer to operate.

10.4b

Enter your current reading in mA when the applied voltage is 10V.

10.4c

Enter your frequency reading in Hz when the applied voltage is 12V.

IT02 Curriculum Manual

Sound Output Chapter 10

Student Assessment 10 1.

From this chapter or a previous one, identify a device that emits a sound wave of constant frequency in the audio range when a DC voltage is applied:

a c 2.

3.

4.

b d

ultrasonic transmitter buzzer

From this chapter or a previous one, identify a device that emits a pressure wave at higher than audio frequencies:

a

microphone

b

ultrasonic transmitter

c

loudspeaker

d

buzzer

From this chapter or a previous one, identify a device that emits sound waves over a wide range of audio frequencies:

a

microphone

b

ultrasonic transmitter

c

loudspeaker

d

buzzer

The maximum sound wave frequency which is detectable by a human ear is approximately:

a 5.

microphone loudspeaker

20Hz

b

5kHz

c

20kHz

d 40kHz

The reason for the diode fitted in series with the buzzer in the DIGIAC 1750 Trainer is to:

a

rectify the applied AC

b

protect the electronic circuit against wrong polarity

c

convert the output to DC

d

prevent interference spikes being generated by the buzzer

Continued ...

171

Sound Output Chapter 10

IT02 Curriculum Manual

Student Assessment 10 Continued ... 6.

The resonant frequency of the loudspeaker fitted in the DIGIAC 1750 Trainer is nearest to:

a 7.

172

0-200Hz

b

300Hz-950Hz

c

1kHz-1.5kHz

d 2kHz-3kHz

A loudspeaker is fed with a 1kHz signal and placed in a vacuum. The effect would be:

a

no sound because a vacuum has no air particles

b

very reduced sound because of the box enclosing the vacuum

c

increased frequency (pitch) of the note produced by sound waves in a vacuum

d

an impure tone due to the signal waveform producing the sound

IT02 Curriculum Manual

Linear or Rotational Motion Chapter 11

Chapter 11 Linear or Rotational Motion

Objectives of this Chapter

Equipment Required for this Chapter

Having studied this Chapter you will be able to: 

Describe the construction and characteristics of a DC solenoid.



Describe the construction and characteristics of a DC relay.



Describe the construction and characteristics of a DC solenoid air valve.



Describe the construction and characteristics of a DC permanent magnet motor

• • •

DIGIAC 1750 Transducer and Instrumentation Trainer. 4mm Connecting Leads. Digital Multimeter.

173

Linear or Rotational Motion Chapter 11

IT02 Curriculum Manual

11.1 The DC Solenoid Coil

+12V

Soft iron core/Actuator shaft I/P

Return spring

End stop

0V

-12V

Case

(a)

(b)

Fig 11.1

The construction of a DC solenoid is shown in Fig 11.1(a), consisting of a soft iron core and actuator shaft which is free to move inside a coil. When the coil is energized, the soft iron core is attracted inside the coil and is held in position. When the coil is de-energized, the core returns to its neutral position under the action of a return spring. The voltage required to attract the core into the coil will be less than the rated value and will depend on the load applied to the actuator shaft. The voltage at which the core is pulled in by the coil is referred to as the pull-in voltage. With the coil energized and the core attracted, if the coil voltage is reduced gradually, when the voltage has fallen sufficiently the core will return to its neutral position under the action of the spring. This voltage is referred to as the drop out or release voltage. The release voltage will be much less than the pull-in voltage. Fig 11.1(b) shows the electrical circuit arrangement of the device fitted to the DIGIAC 1750 Trainer. When the coil is de-energized a large EMF can be induced in the coil, the magnitude depending on the inductance and the rate of change of current. Diodes are provided to limit the induced voltage to a maximum of ±12V. The main characteristics of the coil fitted to the DIGIAC 1750 Trainer are: Resistance

50Ω

Pull-in voltage

6V

Coil rating

12V/3W

Release voltage

1V

Table 11.1

174

IT02 Curriculum Manual

Linear or Rotational Motion Chapter 11

11.2 Practical Exercise Characteristics of a DC Solenoid

SOLENOID WIREWOUND TRACK 5

4

6

C

7

3

8

2

9 10

1

POWER AMPLIFIER

B

O/P

A

I/P

I/P

A

10k MOVING COIL METER 0V

+1 2V

5 -10

0

5 +10

+ 0V

L J

Fig 11.2



Connect the circuit as shown in Fig 11.2 and set the 10k Ω resistor for zero output voltage (control fully counter clockwise). Connect the digital multimeter as an ammeter on the 200mA range in between the Power Amplifier and the Solenoid.



Switch ON the power supply and rotate the 10kΩ resistor control to gradually increase the voltage applied to the solenoid coil. Note the voltage at which the iron core of the solenoid is attracted fully into the coil. This value is the pull-in voltage. Record this voltage and the current in Table 11.2 overleaf.

Note: The core will start to move at a lower value than the pull-in voltage, the actual pull-in voltage will be the value when you hear the click, as the core aligns itself inside the coil. In this position you will find a distinct resistance to pushing the actuator back towards its neutral position.

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Linear or Rotational Motion Chapter 11

IT02 Curriculum Manual

Unloaded Readings Pull In Drop-out (Release)

Voltage

Loaded

Current

Voltage

Current

V

mA

V

mA

V

mA

V

mA

Table 11.2 



11.2a

With the coil energized and the core in its pulled in position, slowly reduce the coil applied voltage and note the value at which the core returns to its neutral position, the drop-out or release voltage. Record voltage and current again in Table 11.2. Repeat the process with your finger against the actuator shaft to exert a little load and note the voltage and current required for pull in and release.

With the actuator shaft loaded the effect on the voltage and current required for pull-in was:

11.2b

a voltage increased, current reduced

b

both voltage and current reduced

c voltage reduced, current increased

d

both voltage and current increased

With the actuator shaft loaded the effect on the voltage and current required for drop-out (release) was:

a voltage increased, current reduced

b

both voltage and current reduced

c voltage reduced, current increased

d

both voltage and current increased



176

Switch OFF the power supply.

IT02 Curriculum Manual

Linear or Rotational Motion Chapter 11

11.3 The DC Relay +12V N.O.

I/P (COM.) N.C.

0V Common Coil Contacts

N.O.

N.C.

-12V

(a)

(b)

Fig 11.3

The construction of a DC relay is shown in Fig 11.3(a). It consists of a coil with an iron core which has a soft iron armature attached to a spring which holds it just above the core. Changeover contacts are attached to the spring and with the armature in its rest position it makes contact with one of the terminals. This is referred to as the normally closed (N.C.) contact. With the coil energized, the core will be magnetized and attract the soft iron armature. The spring is moved, which breaks the connection to the N.C. terminal and makes the contact to the other terminal. This terminal is referred to as the normally open (N.O.) contact. With this construction, the contacts will bounce for a short period each time they close or open (make or break) and this can cause problems with some circuits. The problem can be overcome by using an electronic debounce circuit or a time delay prior to checking the contact state after operation. Fig 11.3(b) shows the electrical circuit arrangement of the device fitted to the DIGIAC 1750 Trainer. The diodes limit any induced voltages to a maximum of approximately ±12V, as for the solenoid device. The main characteristics of the device fitted to the DIGIAC 1750 Trainer are: Coil rated voltage

12V

Coil resistance Coil operating voltage Coil release voltage

1.8V

Operate/release time

5ms

320Ω

Contact rating

12V, 1A

7.5V

Lifetime cycles

5x106

Table 11.3

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Linear or Rotational Motion Chapter 11

IT02 Curriculum Manual

11.4 Practical Exercise Characteristics of a DC Relay

P . I . N . P H O TO D I O D E

P HO TO V O LT A I C CE LL

O/P

O/P

+

+

O/ P

O/ P MOVING COIL METER

P HO TO CON D UCTIV E CELL +12V

LAMP FILAMENT

WIREWOUND TRACK 6 5 C 3

8

2

9 1

10

O/P B A

10k

5

I/P

-10

RELAY

POWER AMPLIFIER

7

4

P HOTO TRA NSISTO R

A

N.O.

5 +10

+ -

I/P I/P

0

0V

L J

N.C.

0V

Fig 11.4



178

Connect the circuit as shown in Fig 11.4 and set the 10k Ω resistor control for zero output voltage.



Switch ON the power supply. The relay will be in its de-energized state. Note the state of the Lamp. Lamp ON means that the contacts are closed. Lamp OFF means that the circuit is broken because the contacts are open.



The relay coil will have pull-in and release voltage characteristics similar to those for a solenoid.



Determine the pull-in and release voltages and currents for this device by gradually increasing and decreasing the applied voltage. Record the results in Table 11.4 opposite.



Note when a change of state of the Lamp connected to the N.O. contact occurs.



Move the lamp connection to the N.C. terminal and observe the effect on the lamp switching. Add to Table 11.4.

IT02 Curriculum Manual

Linear or Rotational Motion Chapter 11

Lamp state ON/OFF when connected to: Voltage Pull In Drop-out (Release)

Current

V

mA

V

mA

N.O. contact

N.C. contact

Table 11.4

11.4a

Enter the pull-in voltage for your relay in V.

11.4b

Enter the drop-out current for your relay in mA. 

Switch OFF the power supply.

Notes: ................................................................................................................................................................. ................................................................................................................................................................. ................................................................................................................................................................. ................................................................................................................................................................. ................................................................................................................................................................. ................................................................................................................................................................. ................................................................................................................................................................. ................................................................................................................................................................. ................................................................................................................................................................. ................................................................................................................................................................. ................................................................................................................................................................. ................................................................................................................................................................. .................................................................................................................................................................

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Linear or Rotational Motion Chapter 11

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11.5 The Air Valve Inlet port

Coil Return spring +12V I/P

Inlet valve

0V Exhaust valve

Cylinder port

(a)

-12V

Exhaust port

(b)

Fig 11.5

Fig 11.5(a) shows the construction of the device fitted to the DIGIAC 1750 Trainer. It is similar to the solenoid considered previously, but the soft iron core now operates on two valves, the inlet and the exhaust valves. With the coil de-energized the core is held, by the return spring, in the position with the inlet valve closed and the exhaust valve open. In this position the cylinder port is connected to the exhaust port outlet. When the coil is energized, the core is attracted and held in the position with the exhaust valve closed and the inlet valve open. In this position the inlet port is connected to the cylinder port In the DIGIAC 1750 Trainer, the inlet port is connected to the pump and the cylinder port is connected to a pneumatic actuator. With the pump ON, the pneumatic actuator will be operated when the coil is energized and illustrates the principle of electrical control of pneumatic devices. The electrical circuit arrangement of the device fitted to the DIGIAC 1750 Trainer is shown in Fig 11.5(b). The main characteristics of the device are: Rated voltage

12V

Coil resistance Coil pull-in voltage

140Ω 8.3V

Coil release voltage

1.7V

Table 11.5

180

IT02 Curriculum Manual

Linear or Rotational Motion Chapter 11

11.6 Practical Exercise Characteristics of an Air Valve

AIR FLOW SENSOR

PRESSURE

FLOW

AIR PRESSURE SENSOR

O/P

ON

+

A

WIREWOUND TRACK 6 5 C

MOVING COIL METER POWER AMPLIFIER

7

5

O/P

3

8

2

9

B A

10

1

O/P

PUMP OFF

+

4

AIR VALVE I/P

-

-10

I/P

0

5 +10

+

10k

0V

+ 12V

0V

L J

Fig 11.6



Connect the circuit as shown in Fig 11.6. Set the 10k Ω resistor control for zero output voltage (fully counter clockwise) and set the pump control (Air Pressure/Flow Sensor panel) to PRESSURE.



Switch ON the power supply and then switch the pump ON. The coil is deenergized in this state, the inlet valve is closed, and the pneumatic actuator will not operate.



Adjust the resistor control to apply 10V to the solenoid coil. The coil will be energized, the inlet valve will open and the exhaust valve will close. The pump pressure will be applied to the pneumatic actuator. Observe the effect on the actuator.



Reduce the voltage and observe the effect on the pneumatic actuator



Switch the pump OFF. Observe the effect on the operation of the pneumatic actuator with no air pressure when the solenoid voltage is raised and lowered.

181

Linear or Rotational Motion Chapter 11

11.6a

IT02 Curriculum Manual

With the pump OFF the pneumatic actuator:

a still operates but more slowly

b

operates at a higher voltage

c does not operate at all

d

operates in reverse

The Air Valve solenoid will have pull-in and release voltages and currents as for any solenoid. To determine these values for the device: 

With the pump switched OFF, increase and decrease the applied voltage gradually and note the voltages at which switching occurs. You will hear a faint click when the device switches. Voltage Pull In Drop-out (Release)

Current

V

mA

V

mA

Table 11.6 11.6b

Enter the pull-in voltage for your Air Valve solenoid in V.

11.6c

Enter the drop-out current for your Air Valve solenoid in mA.

11.6d

Using your pull-in figures of voltage and current, calculate and enter the resistance of your Air Valve solenoid in



182

Switch OFF the power supply.

.

IT02 Curriculum Manual

Linear or Rotational Motion Chapter 11

11.7 The DC Permanent Magnet Motor

N

Coils Commutator S

Brushes

Permanent Magnet Stator

Armature Connections

Fig 11.7

The construction of a permanent magnet DC motor is shown in Fig 11.7. The unit is identical with the tacho-generator unit but for a motor, a DC supply is fed to the armature coils. Current flowing in the armature coils sets up a magnetic field which reacts with the field of the permanent magnet to produce a force causing the armature to rotate. The force acting on the armature is proportional to the current flowing. When the armature rotates, an EMF is induced in the coils, in exactly the same way as in the tacho-generator. The self-induced EMF opposes the applied voltage and is referred to as the back EMF. The armature accelerates until the speed is such as to produce a back EMF ( e) equal to the applied voltage (V) less the voltage dropped across the armature resistance rai.

V = e + rai The speed with no load on the shaft is thus roughly proportional to the applied voltage. When a load is applied to the shaft, the speed will tend to fall, reducing the back EMF. More current flows from the supply and the current self-adjusts to the value that produces a torque (turning force) just sufficient to balance the load torque. The speed will fall slightly with load due to the increase in voltage lost across the armature coils due to the higher current.

183

Linear or Rotational Motion Chapter 11

IT02 Curriculum Manual

The electrical circuit arrangement of the device fitted to the DIGIAC 1750 Trainer is shown in Fig 11.8.

+12V L2

I/P

M C2 L1 O/P

1Ω

C1 -12V

Fig 11.8

The 1Ω resistor is fitted in series with the armature to allow monitoring of the armature current by measurement of the voltage dropped across it. Since the resistor is 1Ω, voltages measured across it in mV will directly correspond to currents in mA. The diodes limit any voltage spikes to a maximum of approximately ±12V. Capacitor C1 provides some noise filtering at the output and the combination L1, L2 and C2 reduces radiation of radio frequency noise. The main characteristics of the device fitted to the DIGIAC 1750 Trainer are: DC resistance No load current (12V applied) Stall current (12V applied) Shaft speed (no load, 12V applied)

120mA 1.93A 2400 rev/min (max.)

Starting torque

7 Ncm/A

Torque constant

3.5 Ncm/A

Time constant Efficiency Table 11.7

184

6.2Ω

19.6ms 82% (max.)

IT02 Curriculum Manual

Linear or Rotational Motion Chapter 11

11.8 Practical Exercise Characteristics of a DC Permanent Magnet Motor

DC MOTOR

I/P

SLOTTED OPTO SENSOR

COUNTER/ TIMER I/P

O/P

R E SET

O/P

TIM E

F R EE R U N

C O U NT

1s

MOVING COIL METER WIREWOUND TRACK 6 5 C

5 POWER AMPLIFIER

7

4

8

3 2 1

V 10

9

B

-10

+10

+

I/P

A

0V

0V

5

O/P

10k

-12V

0

L J

+12V

Fig 11.9 

Connect the circuit as shown in Fig 11.9. Set the 10k Ω resistor control for zero output voltage, (control fully counter clockwise), and set the counter controls to COUNT and 1s.



Switch ON the power supply and set the voltage applied to the motor, as indicated by the Moving Coil Meter, to 10V. The motor should run at a high speed. Allow it to run for a short time and then note the reading of the digital voltmeter. This reading in mV represents the current in mA taken by the motor, since it is the voltage dropped across a 1Ω resistor.



Press the Counter RESET button and note the displayed Counter value. This represents the motor speed in rev/sec. Record the values in Table 11.8 overleaf.



Repeat the procedure, noting the speed and current readings for motor applied voltages of 8V, 6V, 4V and 2V and record the values in Table 11.8.

185

Linear or Rotational Motion Chapter 11

Motor Applied Voltage

IT02 Curriculum Manual

10V

Armature Current

8V mA

6V mA

4V mA

2V mA

mA

Speed (rev/sec.) Speed (rev/min.) Table 11.8 

Multiply the speed in rev/sec by 60 to convert to rev/min and add the results to Table 11.8.



Slowly reduce the applied voltage until the motor just stops turning and observe the effect on the voltage and the current. Stopped voltage =

V

Stopped current =

mA

Construct the graph of speed in rev/min. against applied voltage and armature current on the axes provided:



Applied Voltage (volts) 3000

0

1

2

3

4

5

6

7

8

9

101

11

2

10

20

30

40

50

60

70

80

90

100 110 120

2800

Motor Speed (rev/min)

2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600 400 200 0

130 140 150 160 170 180 190 200

Armature Current (mA) Graph 11.1

186

IT02 Curriculum Manual

11.8a

Linear or Rotational Motion Chapter 11

From your graph deduce and enter the armature current needed to give a speed of 1500 rev/min in mA.

11.8b

Examine the two graphs. Which of the following statements is most true?

a they are identical b shaft speed is directly proportional to the applied voltage c shaft speed is directly proportional to the armature current d both characteristics are non-linear

11.8c

As the applied voltage is very slowly reduced, the moment the motor stops the effect on the armature current is to:

a reduce it directly with the applied voltage b show a sharp increase due to loss of back EMF c fall immediately to zero as torque stops d increase slightly due to reduction in armature resistance 

Set the applied voltage to 7V and note the armature current taken and the shaft speed when the motor is unloaded. Record in Table 11.9. Applied Voltage = 7V Armature current

Unloaded

mA

Loaded

400mA

Shaft speed (rev/sec) Table 11.9 

Now place your left hand near the Hall effect disc with the finger nails down and touching the baseboard of the DIGIAC 1750 Trainer. Move your fingers

gently so that comes between the Hall effect disc and theforward baseboard and your exertsmiddle a smallfinger load on the motor. 

Vary the pressure of the load so that the current is approximately 400mA (0.4V reading on the digital voltmeter) and then note the shaft speed by pressing the Counter RESET button. Record in Table 11.9.

187

Linear or Rotational Motion Chapter 11

11.8d

IT02 Curriculum Manual

Enter your speed in rev/sec with the motor loaded and the armature drawing a current of about 400mA.

Set the control of the 10k Ω resistor to the zero output voltage position. Disconnect socket C of the 10kΩ resistor from the +12V supply and re-



connect it to the -12V supply. 11.8e

The direction of rotation is:

a

11.8f

the same as before

b

reversed

Is the same speed range possible?

Yes

or

No

The characteristics are typical for this size of machine, larger machines would not have such a large drop in speed with load. 

188

Switch OFF the power supply.

IT02 Curriculum Manual

Linear or Rotational Motion Chapter 11

Student Assessment 11 1.

2.

3.

For any solenoid in which a soft iron core is drawn inside a coil, the pull-in current:

a

and release current will be the same

b

will be greater than the release current

c

will be less than the release current

d

will depend on the applied voltage

A pneumatic actuator, which is used to indicate gas pressure, is controlled by an electrically operated air valve in a similar configuration to that used on the D1750 unit. The actuator will operate:

a

if there is both an electrical input and adequate gas pressure.

b

if there is an electrical input but there is no gas pressure.

c

if there is adequate gas pressure but there is no electrical input.

d

if neither the electrical input nor adequate gas pressure is present.

A relay has changeover contacts marked N.C. and N.O. When the relay is de-energized a lamp will operate if it is connected to a supply via:

a 4.

5.

neither of them

b

N.C.

c

N.O.

d either of them

The term N.C. applied to the contacts of a relay means:

a

Not Contacted

b

Not Changeover

c

Normally Closed

d

Neither Connection

A solenoid has a coil rated for operation at 10V. You might expect the pull-in and dropout voltages to be:

a

pull-in = 12V, drop-out = 5V

b

pull-in = 5V, drop-out = 8V

c

pull-in = 10V, drop-out = 3V

d

pull-in = 5V, drop-out = 2V

Continued ...

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Student Assessment 11 Continued ... 6.

For the Air Valve investigated in this Chapter, if the coil is de-energized the air valve will:

a c 7.

8.

b d

rise immediately fall back slowly

The DC Permanent Magnet Motor investigated in this Chapter is:

a

completely unlike the tacho-generator

b

identical to the tacho-generator

c

similar to the tacho-generator but with the commutator reversed

d

similar to the tacho-generator but with more coils

A DC Permanent Magnet Motor with an armature resistance of 5 , runs at 1200 rev/min. when the applied voltage is 6V. The speed for 12V applied is likely to be:

a 9.

not be affected drop down immediately

600 rev/min

b

1200 rev/min

c

1800 rev/min

d

2400 rev/min

The voltage applied to the DC Motor in question 8 above is slowly reduced, the motor stops when the applied voltage is 2V. The armature current will then be:

a

20mA

b

50mA

c

100mA

d 400mA

10. If the DC motor in question 8 is loaded without any increase of applied voltage, the effect is likely to be:

190

a

reduced speed and increased armature current

b

reduced speed and armature current

c

increased speed and reduced armature current

d

increased speed and armature current

IT02 Curriculum Manual

Display Devices Chapter 12

Chapter 12 Display Devices

Objectives of this Chapter

Equipment Required for this Chapter

Having studied this Chapter you will be able to: 

Describe the characteristics and application of the Counter/Timer.



Describe the characteristics and application of the LED Bargraph Display.



Describe the characteristics and application of the Moving Coil Meter.



State and calculate the requirement to extend the voltage range of a Moving Coil Meter.



Select a suitable device for a particular voltage measurement.

• • • •

DIGIAC 1750 Transducer and Instrumentation Trainer. 4mm Connecting Leads. Digital Multimeter. Stopwatch (not supplied).

191

Display Devices Chapter 12

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12.1 The Timer/Counter

Reset

1 second Delay

FREE RUN COUNT

100Hz Oscillator

TIME

I/P

Decade Counter

Decoder/ Driver

Decade Counter

Decoder/ Driver

Decade Counter

Decoder/ Driver

1s

+5V

&

&

Fig 12.1

A system logic diagram of the Counter/Timer facility provided with the DIGIAC 1750 unit is shown in Fig 12.1. The output display uses three 7-segment LED's. The unit can be used in three ways: 1.

Time measurement, with the controls set to TIME and FREE RUN.

2.

Counting (pulses), with the controls set to COUNT and FREE RUN.

3.

Frequency (pulses/sec), with the controls set to COUNT and 1s.

In addition, with some signal conditioning, it can be used for voltage measurement. The main characteristics of the unit are: Input impedance Input voltage levels (TTL) Timing intervals Timing accuracy Table 12.1

192

1MΩ +5V max. 10ms 5%

IT02 Curriculum Manual

Time

TIME

Display Devices Chapter 12

and FREE RUN

With the input at TTL logic level "1", (+5V), the display increments at 10ms intervals, or every 1 second. With the input at logic level "0" (0V), the 100 displayed value is held. The unit will therefore display the time in hundredths of a second that the input is held at logic level "1". Note that with a 3-digit display, the maximum count is 999 and hence one complete cycle from 0-999 will represent 1000 x 10ms = 10s.

Counting

COUNT

and FREE RUN

The count increments by 1 each time the input voltage level changes from TTL logic level "0" to level "1", i.e. on receipt of a positive edge of a pulse of amplitude 5V. Set in this way the Counter counts input pulses and displays the total. With the 3-digit display the maximum count will be 999.

Frequency

COUNT

and 1s

The unit counts the number of positive pulses at TTL logic level "1" that are received at the input in a period of one second, following a RESET of the Counter, thus giving the count rate in pulses per second, or the frequency in Hz.

Note that you have already used the Counter/Timer to count the number of pulses received in one minute and to measure frequency in pulses/sec.

193

Display Devices Chapter 12

IT02 Curriculum Manual

12.2 Practical Exercise Time Measurement and Counting

COUNTER/ TIMER I/P

T IM E

F R EE R UN

AMPLIFIER #2 I/ P

-

R ES ET

O/ P .5

+

.4 1 100 10

OFFSET

GAIN COARSE

.6 .7

.3

.8

.2

.9 .1

C O U NT

1s

MOVING COIL METER

1.0

GAIN FINE

5 -10

0

5 +10

+ 0V

L J

Fig 12.2

Time Measurement 

Connect the circuit as shown in Fig 12.2 and switch ON the power supply. With the Amplifier #2 GAIN COARSE control set to 100 and GAIN FINE to 1.0, adjust the OFFSET control for +5V output. Switch the GAIN COARSE control to 1. The output voltage will drop to nearly zero.



Set the Counter/Timer controls to TIME and FREE RUN and press the RESET button. The display should show zero.



Switch the Amplifier #2 coarse gain control to 100. The counter display should increment at 10ms ( 1 sec.) intervals. Return the GAIN COARSE 100 control to 1, the display will be held. This illustrates the application of the unit to time measurement, the display indicating the number of 10ms intervals (or the time in hundredths of a second) that the input is held at



With Amplifier #2 GAIN COARSE set to 1, RESET the Counter display to zero. Switch Amplifier #2 GAIN COARSE to 100 and note the time taken for the count to complete one cycle from 0 to 999 and back to 0.

+5V.

194

IT02 Curriculum Manual

12.2a

Display Devices Chapter 12

The time taken for a complete cycle of counting is:

a 10ms

b

100ms

c

1s

d

10s

Use the timer facility to time some operations and obtain practice in its use, such as the time taken for you to verbally count from zero through to 250, or to write down a long word.



Counting Pulses

With the circuit still as shown in Fig 12.2, set the Counter/Timer controls to COUNT and FREE RUN and RESET the display to zero.





12.2b

Switch Amplifier #2 GAIN COARSE control from 1 to 100 and back to 1.

The count increments by:

a 1

b

10

c

100

d

1000



Repeat the process, you will find that the count increments for each change of the gain from 1 to 100, or on the application of a +5V pulse to the counter



Remove the Counter input lead from the output of Amplifier #2 and touch it on the +5V supply socket.



Return the Counter input lead back to the output of Amplifier #2 and, with the GAIN COARSE set to 100, alter the OFFSET control to give zero output. Slowly raise the setting again and watch the Counter display for a response.



Note the threshold level on the Counter input from the indication on the Moving Coil Meter.

input.

Threshold voltage level =

12.2c

V

Enter the threshold level for an increment of the count in V. 

Switch OFF the power supply.

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Display Devices Chapter 12

IT02 Curriculum Manual

12.3 Practical Exercise Frequency Measurement

WHEATSTONE BRIDGE D

DIFFERENTIATOR O/P V/F CONVERTER O/P

12k

C A

OUT

B

3

I/P

I/P

T

dVIN dt

IN

V

1V

0V

COMPARATOR

+5V

OFF ON HYSTERESIS

B

-

A

+

100ms 1s 10ms

Rx

O/P

TIME CONSTANT

COUNTER/ TIMER I/P

TI M E

RES ET

C O UNT

F R E E R UN

1s

Fig 12.3

The connection of the +5V supply places the 12k Ω fixed resistor in series with the 10kΩ 10-turn resistor to make low voltage settings easier. Switch the unknown resistor Rx OUT. A Voltage to Frequency (V/F) Converter is available in the signal conditioning circuits. This unit converts a DC voltage input to a pulsed output of frequency 1kHz/volt of input. For example, an input of 0.6V will produce an output frequency of 0.6kHz or 600Hz. The pulses from the V/F Converter are unsuitable to be fed directly to the input of the Counter/Timer. The Differentiator and Comparator are used to shape the pulses from the V/F Converter, so that they may be detected by the Counter/Timer.

196



Connect the circuit as shown in Fig 12.3 and switch ON the power supply. Set the Counter controls to COUNT and 1s. Set the Differentiator TIME CONSTANT to 1s and switch OFF the Comparator HYSTERESIS.



Set the 10k Ω 10-turn resistor output voltage to 0.1V, press the Counter RESET button and note the displayed reading. Enter the value in Table 12.2.

IT02 Curriculum Manual

Input Voltage to V/F Converter Counter Display (Hz)

0.1

Display Devices Chapter 12

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Table 12.2 

Repeat the procedure for the other voltage settings shown in Table 12.2 and record the displayed values that are obtained following the pressing of the reset button. The accuracy in the calibration of the V/F converter will affect the readings as will your accuracy in setting the voltages and also the accuracy of the 1s delay in the Counter/Timer.

12.3a

Enter your Counter Display reading for an input voltage of 0.5V.

In this exercise the V/F converter was used purely as a means of obtaining a variable frequency. However, the method used also illustrates the application of the unit to voltage measurement. The displayed Counter readings represent the voltage in mV, as can be seen from Table 12.2. The maximum voltage range is limited by the frequency capability of the counter and the number of digits in the display. The voltage range can be extended by attenuating the input to the V/F converter using the additional circuits shown in Fig 12.4. Also, carefully note the change of the voltage feed to the 10kΩ 10-turn resistor.

WHEATSTONE BRIDGE D V/F CONVERTER 12k

C

O/P

A OUT

B

3

I/P

IN

V

1V

Rx

0V

BUFFER #1

SLIDE

C

O/P

B

I/P +VIN 1

2

3

4

5

6

+12V

7

8

9

10

10k

A

0V

Fig 12.4

197

Display Devices Chapter 12

IT02 Curriculum Manual

The Buffer Amplifier is used to reduce the loading on the 10k Ω 10-turn resistor. The circuit will be calibrated so that a counter display of 600 represents a voltage of 6V. 

Connect the additional circuitry shown in Fig 12.4, to the V/F converter input. The V/F Converter, Differentiator, Comparator and Counter/Timer remain connected as shown in Fig 12.3. Set the output control of the 10kΩ slide resistor for zero output (to the left).



Set the output voltage from the 10-turn resistor to 6V as indicated by the digital voltmeter and then slowly adjust the 10kΩ slide resistor until the Counter display indicates 600 after the RESET button is pressed. You will find that the setting of the resistor control is very sensitive, it is possible to set accurately but if it is too difficult, set the value as near as you can. The unit is now calibrated.



Input Voltage

Set the 10-turn resistor control in steps to each of the other voltage values indicated in Table 12.3. Note the Counter displayed value after pressing the reset button. Record the values in Table 12.3. 1

2

3

4

Counter Display

5

6

7

8

9

9.5

600

Table 12.3

12.3b

Enter your displayed reading for an input voltage of 3.5V.

Using this principle, the counter could be calibrated for any desired voltage range. AC measurements are possible if the Full-Wave Rectifier circuit is added and the unit re-calibrated for RMS values. 

198

Switch OFF the power supply.

IT02 Curriculum Manual

Display Devices Chapter 12

12.4 The LED Bargraph Display

Light pipe

LED Chip

Connections

Fig 12.5

The construction of the Bargraph device is shown in Fig 12.5, consisting of 10 separate light emitting diodes (LED's) fitted in a 20-pin package. The light from each diode is collected by a light pipe and appears at the top surface as a red bar. A dedicated IC driver chip controls the device and provision is made for adjusting the voltage levels required for adjacent LED's to light. With the device as fitted to the DIGIAC 1750 unit the voltage level between adjacent LED's is 0.5V and hence the minimum voltage for all LED's to light is 5.0V. The device has a high input impedance, a low time constant, and is suitable for indication of an approximate and rapidly varying voltage level, but the resolution is low. The main characteristics of the device are: Input impedance

1MΩ

Input voltage range

±35V

Accuracy Segment overlap

2% 1mV

Table 12.4

The unit is adjusted so that an input of +5V just lights the last LED.

199

Display Devices Chapter 12

IT02 Curriculum Manual

12.5 Practical Exercise Characteristics of an LED Bargraph Display

WIREWOUND TRACK 6 5 C

L.E.D. BARGRAPH DISPLAY

7

4 3

8

2

9 1

I/P

B A

10

DRIVER I.C.

V

10k

+12V

0V

Fig 12.6 

Connect the circuit as shown in Fig 12.6. Set the 10k Ω Wirewound resistor control for zero output voltage (fully counter clockwise).



Switch ON the power supply. Adjust the resistor control to increase the voltage applied to the bargraph unit gradually and note the voltage values at which each LED lights. Record the values in Table 12.5.

LED number

1

Input Voltage

2 V

3 V

4 V

5 V

6 V

7 V

8 V

9 V

10 V

V

Table 12.5

12.5a

Enter the input voltage required just to light LED number 7 in V. 

12.5b

Vary the voltage rapidly by rotating the control quickly in both directions and note how the display follows. Repeat the procedure, this time noting the display on the digital meter. Switch OFF the power supply.

When the voltage was varied rapidly:

a both displays responded immediately to the variations b only the LED Bargraph was able to follow the variations precisely c only the digital multimeter was able to follo w the variations precisely d neither display responded at all

200

IT02 Curriculum Manual

Display Devices Chapter 12

12.6 The Moving Coil Meter 0

5 -10

Scale

5 +10

Pointer Radial magnetic field

Soft-iron core Permanent Magnet

Hairspring

N

S

Pivot

+

V

0V

Coil

Connections

Fig 12.7

The construction and electrical circuit arrangement of the moving coil meter fitted to the DIGIAC 1750 unit are shown in Fig 12.7. Using the connections + and -, the voltage difference between any two points in a circuit can be measured. By connecting the - socket to 0V, the voltage of any point with respect to 0V (ground) can be measured using the + connection. The moving coil meter consists of a coil suspended between the poles of a permanent magnet with a pointer attached to the coil which moves over the meter scale. The coil is held in its center position by two hairsprings. A set zero screw is attached to one of the hairsprings for adjustment of the pointer position to zero with no voltage applied to the meter. When current is fed to the coil via the hairsprings, a force is produced by interaction between the current in the coil and the permanent magnetic field, and the coil rotates. The direction of rotation depends on the direction of the current through the coil (Fleming's Rule), and the amount of rotation depends on the magnitude of the current flowing. The coil rotates until the force produced by the current is balanced by the force exerted by the hairsprings. The coil is wound on an aluminum former. When the coil rotates, an EMF is induced in this former, similar to the back EMF induced in the armature coils of a DC motor. This produces a current and a force opposing the motion of the coil (Lenz's Law).

201

Display Devices Chapter 12

IT02 Curriculum Manual

The coil movement is thus damped and allows the pointer to take up its final position, after a step change of current, with the minimum of oscillation (or hunting) occurring. The meter movement is a damped control system and this effect together with the inertia of the coil system limits the response speed of the pointer. The hairsprings are fine to allow a large angular movement and high sensitivity. The amount of coil current needed for full-scale deflection (f.s.d.) will be determined by the tension of the hairsprings. The current flow in the meter circuit must be limited to this value of current. When used as a voltmeter, a series resistor (called a multiplier) is fitted to limit the current to the value required to produce full-scale deflection. For instance, if the f.s.d. current for a particular meter is 1mA, then the value of the multiplier 1 (series resistor) must be or 1kΩ for each volt (1kΩ/V) to be represented 1 × 10 − 3 by full-scale deflection. This figure (1kΩ/V) is known as the sensitivity of the meter. From this figure it is possible to calculate the loading resistance of a meter when it is operated on any voltage range. A 10V voltmeter using a 1mA f.s.d. meter would require a multiplier of 10 x 1k Ω = 10kΩ. Many analog multimeters are based on a 50 µA meter movement (50µA f.s.d.). 12.6a

Calculate and enter the sensitivity of a 50 A meter movement in k /V.

The main characteristics of the meter fitted to the DIGIAC 1750 unit are: Full-scale current

±1mA

Sensitivity

1kΩ/V

Total voltmeter resistance

20kΩ

Accuracy Table 12.6

202

± 1-2%

IT02 Curriculum Manual

Display Devices Chapter 12

12.7 Practical Exercise Characteristics of a Moving Coil Meter MOVING COIL METER WIREWOUND TRACK 6 5 C 4 3

8

2

9 1

0

5

7

5

-10

B

+10

+ A

10

Set Zero -

10k

L J

V 0V +12V

-12V

Fig 12.8 

Connect the circuit as shown in Fig 12.8. Set the resistor control to its central position and check that the Moving Coil Meter pointer is at zero. Adjust the Set Zero screw (Fig 12.8) if necessary to set the pointer to zero. Use only the correct small screwdriver for this task.



Switch ON the power supply. Set the resistor output voltage to 0V as indicated by the digital multimeter and note the voltage indicated by the Moving Coil Meter. Enter the value in Table 12.7.

Digital Meter

V

Moving Coil Meter

-10

V

-8

V

-6

V

-4

V

0

+2

-2

V

+4 V

+6 V

+8 V

+10 V

V

Table 12.7 

Repeat the procedure for all positive values of voltage listed in Table 12.7.



Repeat the procedure for the negative values of voltage indicated in Table 12.7, but setting up with the Moving Coil Meter and reading the digital multimeter. Record the results in Table 12.7. Switch OFF the power supply.

12.7a

Which meter gives the greatest resolution (indicates the smallest change)?

12.7b

Which meter is the easiest to use to set up a selected voltage?

a Digital Multimeter

a Digital Multimeter

b

b

Moving Coil Meter

Moving Coil Meter

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Display Devices Chapter 12

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12.8 Practical Exercise Extending the Voltage Range of a Moving Coil Meter MOVING COIL METER CARBON TRACK 6 5 C 4

WIREWOUND TRACK 6 5 C

7

4

3

8

2

9 1

10

B

3

8

A

2

9 1

100k

5

7

B A

10

V

5 +10

+ -

10k

0V +12V

0

-10

-12V

L J

Fig 12.8

The voltage range of a moving coil meter can be increased by adding a resistor in series with it to extend the existing multiplier. 

Connect the 100k Ω variable resistor in series with the Moving Coil Meter as shown in Fig 12.8. Note that the ±12V supplies are being used together as a single-ended +24V supply.



Switch ON the power supply and use the 10k Ω variable resistor to set the voltage to 10V as indicated on the digital multimeter.



Adjust the 100k Ω variable resistor so that the Moving Coil Meter reads +5V.



Keep re-adjusting both settings until they are correct.

When completed, the Moving Coil Meter is calibrated for a voltage range of ± 20V. 

Check this by setting the voltage to 20V (digital multimeter) and note the Moving Coil Meter scale reading. Switch OFF the power supply.

Moving Coil Meter scale reading with 20V applied = 

12.8a

204

V

Isolate the 100k Ω Carbon Track Resistor from the circuit and use your digital multimeter on an Ohms (Resistance) range to measure the resistance of the part of the 100k Ω variable resistor which was connected into circuit.

Enter your measured value of the 100k

variable resistor in k .

IT02 Curriculum Manual

Display Devices Chapter 12

12.9 Practical Exercise Comparison of Voltage Display Devices

L.E.D. BARGRAPH DISPLAY I/P

DRIVER I.C.

+12V

MOVING COIL METER

WIREWOUND TRACK 6 5 C

5

7

4

8

3

9

2 1

0

-10

10

10k

5 +10

B

+ A

V

-12V

0V

L J

Fig 12.9



Connect the circuit as shown in Fig 12.9. All three voltage display devices are connected in circuit for comparison of their characteristics.



Switch ON the power supply.



12.9a

Vary the output voltage slowly over the range 0V through +5V and back to 0V and note the meter indications.

From your observations, which of the following is true?

a the Bargraph is too slow to follow the variations b the Digital Multimeter responds to the changes accurately and immediately c both the Moving Coil Meter and the Ba rgraph follow the variations well d the Moving Coil and Digital Meters give the best response

205

Display Devices Chapter 12



12.9b

IT02 Curriculum Manual

Vary the output voltage over the same range rapidly and note the readings of the Moving Coil Meter and Bargraph.

The fastest response came from the:

a Moving Coil Meter 

b

Bargraph

Increase the input voltage from 0V to +3V, with the 3V indicating LED of the Bargraph just on, and note the readings of all the meters. Record the results in Table 12.8 Voltage indications

Bargraph

All three devices in circuit Moving Coil Meter removed

3V (sixth bar)

Digital Multimeter

Moving Coil Meter V

V

V

Table 12.8

12.9c

Enter your reading from the Moving Coil Meter in V. 

12.9d

Enter your reading from the Digital Multimeter in V when the Moving Coil Meter is removed from circuit.

12.9e

The loading caused by the Moving Coil Meter can be improved by:

a only using it on low voltage ranges b

only using it on AC

c using a full-wave rectifier for DC

using a buffer amplifier



206

Remove the lead to the + connection of the Moving Coil Meter thus disconnecting it from the circuit. Note and record in Table 12.8, the revised readings of the Digital Multimeter and Bargraph.

Switch OFF the power supply.

d

IT02 Curriculum Manual

Display Devices Chapter 12

Student Assessment 12 The following questions all relate to devices fitted to, or recommended for use with, the DIGIAC 1750 Trainer. 1.

For the Counter/Timer unit to measure a period of time, the TIME/COUNT and switches are set to:

FREE RUN/1s

2.

3.

a

TIME & FREE RUN

b

COUNT & FREE RUN

c

TIME & 1s

d

COUNT & 1s

For the Counter/Timer unit to count input pulses, the TIME/COUNT and FREE RUN/1s switches are set to:

a

TIME & FREE RUN

b

COUNT & FREE RUN

c

TIME & 1s

d

COUNT & 1s

For the Counter/Timer unit to measure frequency, the TIME/COUNT and FREE RUN/1s switches are set to:

a c 4.

b d

TIME & 1s

COUNT & FREE RUN COUNT & 1s

The Counter/Timer display reads 254 when the unit is used to measure time. The amount of time elapsed is:

a 5.

TIME & FREE RUN

254 µs

b

254ms

c

2.54s

d 25.4s

The LED Bargraph is displaying six LED's ON. The input voltage is in the range:

a

2.50-3.45V

b

3.0-3.45V

c

2.75-3.25V

d

6.00-6.45V

Continued ...

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Student Assessment 12 Continued ... 6.

7.

The device with the fastest response time for measurement of voltage is the:

a

Moving Coil Meter

b

Digital Multimeter

c

Counter/Timer

d

Bargraph

The f.s.d. current of the Moving Coil Meter is:

a 8.

b

±500µA

c

±1mA

d

±10mA

The Moving Coil Meter has a multiplier (series resistor) to range it for 10V. The value of additional multiplier for a range of 30V is:

a 9.

±50µA

10k Ω

b

20k

Ω

c

30k

Ω

d 50k

Ω

The best device for monitoring a rapidly, randomly varying voltage would be:

a

digital multimeter

b

moving coil meter

c

bargraph

d

oscilloscope

10. The best device for monitoring a slowly varying, precise voltage measurement (such as that found when adjusting a voltage setting to, say, 3.7V) would be:

a

digital multimeter

b

moving coil meter

c

bargraph

d

oscilloscope

11. The best device for monitoring instantaneous repetitive variations of voltage at very high frequencies would be:

a

digital multimeter

b

moving coil meter

c

bargraph

d

oscilloscope

12. The best device for voltage measurements in a high impedance circuit:

208

a

digital multimeter

b

moving coil meter

c

bargraph

d

oscilloscope

IT02 Curriculum Manual

Signal Conditioning Amplifiers Chapter 13

Chapter 13 Signal Conditioning Amplifiers

Objectives of this Chapter

Having studied this Chapter you will be able to: 















Equipment Required for this Chapter

Describe the characteristics and application of DC amplifiers. Explain the term "Offset" and the need for offset control. Describe the characteristics and application of an AC amplifier. Describe the characteristics and application of a power amplifier. Describe the characteristics and application of a current amplifier. Describe the characteristics and application of a buffer amplifier. Describe the characteristics and application of an inverter amplifier. Describe the characteristics and application of a differential amplifier.

• • •

DIGIAC 1750 Transducer and Instrumentation Trainer. 4mm Connecting Leads. Digital Multimeter.

• • •

Oscilloscope. Function Generator. BNC to 4mm connecting lead.

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13.1 DC Amplifiers

I/P

O/P

Fig 13.1

The symbol used for a DC amplifier is shown in Fig 13.1. The device consists of directly coupled amplifiers (without coupling capacitors) which are therefore capable of amplifying both DC and AC signals. There may be many active devices (transistors) in a DC amplifier such as the types of Integrated Circuit (IC) Operational Amplifier (Op Amp) chosen for the DIGIAC 1750 Trainer. The ratio of the output signal voltage to the input signal voltage is referred to as the voltage gain of the circuit (Av). With the input to these amplifiers at zero, the output should be zero, but there could be a small value of voltage. This is more of a problem with high gain circuits and an offset control may be provided to counteract the effect. This control is adjusted with zero input, to set the output voltage to zero. Given data for an amplifier normally specifies the input offset voltage for the device. This represents the difference in voltage at two input connections that may be required to produce zero output voltage. The second input connection is not accessible for the DC amplifiers provided with the DIGIAC 1750 Trainer although an offset control is provided for Amplifier #1/2 connected internally. Various DC amplifier circuits are provided with the DIGIAC 1750 Trainer, but only three are specifically designed for amplification applications, these being:

210

1.

Amplifier #1 having a variable preset gain over the range of 0.1 to 100 approximately. This amplifier is provided with an "offset" control.

2.

Amplifier #2 which is identical to Amplifier #1.

3.

X100 Amplifier

which has a fixed gain of 100 and has no offset control.

IT02 Curriculum Manual

Signal Conditioning Amplifiers Chapter 13

The requirements for an ideal amplifier are:

High input impedance to prevent loading the signal source. Low output impedance to ensure good transfer of signal to any succeeding stage and prevent loss of signal. High gain to reduce the number of amplifier stages required. Broad bandwidth to ensure that all required signals for a given band of frequencies are passed without attenuation. Low distortion so that only the amplitude of the signal is altered (high fidelity). Low noise factor to reduce the introduction of unwanted signals or interference. Stability. No tendency to self- (spurious) oscillation. These requirements apply to any type of amplifier, not just to DC amplifiers.

Amplifiers can be connected in cascade (one after another), to increase the overall gain, if required. Note The output voltage that can be provided by a DC amplifier cannot exceed the value of its supply voltage. In the case of the DIGIAC 1750 Trainer the output voltage is limited to a maximum of approximately ±10V.

The main characteristics of these devices are: Amplifier #1/2 Input signal voltage (max.)

X100

Amplifier

12V

12V

0.1 - 100

100

Voltage gain error (max.)

± 30%

± 4%

Output noise voltage (typ.)

10mV

10mV

fully adjustable

± 30mV

100kΩ

101kΩ

Voltage gain (nominal)

Output offset voltage (max.) Input impedance Table 13.1

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13.2 Practical Exercise Characteristics of DC Amplifiers

SLIDE

BUFFER #1

C

O/P

B I/P 1

2

3

4

5

6

7

8

9

10

A

10k

+VIN

MOVING COIL METER

5

AMPLIFIER #1 +5V

I/ P

-10

O/ P

0

5 +10

+ 0V

V

-

.5

+

.4 1 100 10

-5V OFFSET

GAIN COARSE

.6 .7

.8

.3

.9

.2 .1

1.0

0V

L J

GAIN FINE

Fig 13.2 

Connect the circuit as shown in Fig 13.2 with the Amplifier #1 in circuit. Set the GAIN COARSE control to 100 and GAIN FINE to 1.0 for both amplifiers, Amplifier #1 and Amplifier #2. Note that Buffer #1 is needed so that the OFFSET adjustment does not affect the input voltage.

212



Switch ON the power supply. Set the 10k Ω variable resistor mid-way for exactly zero volts output as indicated by the digital multimeter. Adjust the OFFSET control of Amplifier #1 so that the output voltage is zero (or as near as it is possible to get to zero).



Increase the input voltage positively and note the output voltage. This increases to saturation quickly and then remains at this maximum value for further increase of input voltage. Record the value of this saturation voltage at the Moving Coil Meter in Table 13.2.



Repeat for the negative saturation voltage, recording again in Table 13.2.



Set the input voltage so that the output voltage is between +7 and +8V (Moving Coil Meter) and use the digital multimeter to note the values of the Amplifier #1 input and output voltages. Record the results in Table 13.2.

IT02 Curriculum Manual



Signal Conditioning Amplifiers Chapter 13

Outp ut volta ge

), this representing the maximum gain Inp ut vo ltage with positive polarity possible for the amplifier. Add this to Table 13.2.

Calculate the gain (

Gain (Av) set to x = 100 1001.0 Saturation voltage Input voltage Output voltage

Positive

Amplifier #1 Negative V mV V

Positive V

Vm

Amplifier #2 Negative V

Vm V

V mV

V

V

Voltage gain (Av) Table 13.2 

13.2a

Repeat with the 10k Ω variable resistor adjusted to give between -7 and -8V, to determine the gain of the amplifier for negative polarity input signals.

Enter your value of positive polarity gain for Amplifier #1 with maximum gain settings.

13.2b

Enter your value of negative polarity gain for Amplifier #1 with maximum gain settings.

This dual-polarity operation signifies that the amplifier is capable of amplifying AC signals as well as DC voltages. 

13.2c

Replace Amplifier #1 in the circuit with Amplifier #2 and repeat the procedures to adjust the OFFSET and to determine its maximum positive and negative gain values.

Enter your value of positive polarity gain for Amplifier #2 with maximum gain settings.

13.2d

Enter your value of negative polarity gain for Amplifier #2 with maximum gain settings.

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

Reset both Amplifier #1 and Amplifier #2 GAIN COARSE control to 1 and GAIN FINE to 0.1 for minimum amplifier gain.



With Amplifier #1 in circuit and an input voltage of +4V approximately, note and record the values of the input and output voltages in Table 13.3. Gain (Av) set to x = 0.1 10.1

Positive

Amplifier #1 Negative

Input voltage V V

V

V

Output voltage V V

V

V

Positive

Amplifier #2 Negative

Voltage gain (Av) Table 13.3

13.2e



Reset the input to -4V and repeat the readings, recording the results in Table 13.3.



Change to Amplifier #2 and repeat the readings for both polarities.



Using the values of input and output voltages given in Table 13.3 calculate the gain for each of the four conditions and add the results to the table.

Enter your value of positive polarity gain for Amplifier #1 with minimum gain settings.

13.2f

Enter your value of negative polarity gain for Amplifier #2 with minimum gain settings. 

Replace Amplifier #2 with the X100 Amplifier. Temporarily ground the input and note the output voltage with zero input voltage (the output offset voltage) using the digital multimeter.Use the same 0V patch panel as you use for the digital multimeter. Note that there is no offset control with this amplifier. The offset is adjusted to an acceptably low figure during production.

214

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Signal Conditioning Amplifiers Chapter 13

Nominal Gain (Av) = 100

X100

Positive

Saturation voltage Input voltage Output voltage

Amplifier Negative

V mV V

V Vm V

Voltage gain (Av) Table 13.4

13.2g



Repeat the procedures to measure the saturation voltages and the input and output voltages with the output set to a value between±(7-8)V. Record the values in Table 13.4.



Calculate the gain for both polarities and add these to Table 13.4.

Enter your calculated value for the gain of the X100 Amplifier with positive polarity.

13.2h

Enter your calculated value for the gain of the X100 Amplifier with negative polarity. 

Compare the results with the amplifier specifications given earlier.



Switch OFF the power supply.

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13.3 The AC Amplifier The symbol for an AC amplifier is the same as for a DC amplifier.

I/P

O/P

Fig 13.3

The AC amplifier provided with the DIGIAC 1750 Trainer is a two-stage IC amplifier which has three fixed gain settings, 10, 100 and 1000. The mimic diagram on the DIGIAC 1750 Trainer shows the capacitors in the input and output circuits. These capacitors remove any DC level and hence there is no offset problem with an AC amplifier. Two of the main aspects of amplifiers are in conflict with each other, gain and bandwidth. As the gain of an amplifier is increased its bandwidth will be reduced. It is common to specify a gain bandwidth product for an amplifier. For instance, an amplifier with a gain bandwidth product of 106 could have a gain of 100 with a bandwidth of 104 or 10kHz, or a gain of 1000 with a bandwidth of 1kHz. This is why the amplifier on the DIGIAC 1750 Trainer is a 2-stage circuit; to get a bandwidth of 16kHz (covering the full audio band) and a gain of up to 1000. When the gain is switched to 100 (or 10) the bandwidth will be increased. The main characteristics of the device are: Input voltage (max.) Bandwidth (-6dB, gain = 1000) Maximum gain at 40kHz Output noise voltage (gain = 1000)

±12V

10Hz - 16kHz 225 100mV

Table 13.5

A high proportion of the output noise will be found to be stray pick-up of the output of the 40kHz oscillator which is adjacent to the AC Amplifier.

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13.4 Practical Exercise Characteristics of the AC Amplifier From Function Generator A.C. AMPLIFIER O/P

SLIDE C

I/P

B 1

2

3

4

5

6

7

8

9

10

A

10k

10 1000 100

GAIN 0V

TP1

0V

TP2

0V

Oscilloscope

C H. 1

CH.2

Fig 13.4



Construct the circuit of Fig 13.4. Set the slider of the 10kΩ variable resistor to mid-way. This is to operate as a fine amplitude control on the input signal.



Switch the output of the Function Generator to a 1kHz sinewave. Switch the oscilloscope timebase to 0.5ms/div, Y1 amplifier (CH.1) to 10mV/div and the Y2 amplifier (CH.2) to 5V/div.



Switch ON the power supply and adjust the Function Generator output amplitude control to obtain 20Vp-p output from the AC Amplifier as Ω slider variable resistor indicated on CH.2 of the oscilloscope. Use the 10k for the final adjustment if necessary. Measure the input amplitude (CH.1) and record in Table 13.6.

Switch the AC Amplifier to maximum gain, 1000.

Gain setting Output voltage Input voltage

1000

100

10

20Vp-p

20Vp-p

20Vp-p

mVp-p

mVp-p

Vp-p

Amplifier gain Table 13.6

217

Signal Conditioning Amplifiers Chapter 13

13.4a

IT02 Curriculum Manual



Switch the AC Amplifier gain to 100 and repeat the setting of the output voltage to 20Vp-p and again measure the input signal amplitude, changing the Y1 amplifier setting as required. Record the result in Table 13.6.



Switch the AC Amplifier gain to 10 and repeat the setting and measurement.



Calculate the amplifier gain ( Outp ut volta ge ) for each setting of the gain Inp ut voltage switch and add the results to Table 13.6.

Enter your value of gain at 1kHz with the AC Amplifier set to x100. 

Change the Function Generator frequency to 40kHz and the oscilloscope timebase setting to 5µs/div, switch the AC Amplifier gain to 1000 and repeat the setting and measurement. Input voltage for 20Vp-p output =



mVp-p

Calculate the amplifier gain at 40kHz. Amplifier gain at 40kHz =

13.4b

Enter your value of gain at 40kHz with the AC Amplifier set to x1000. 

218

Switch OFF the power supply.

IT02 Curriculum Manual

Signal Conditioning Amplifiers Chapter 13

13.5 The Power Amplifier The symbol for a power amplifier is again the same as that for any DC amplifier.

I/P

O/P

Fig 13.5

The main characteristic of a power amplifier is the capability of a large power output. In order to do this the output impedance of the amplifier must be very low in order to provide a heavy current to a load without loss of output voltage across the output impedance. The components used must also be capable of dissipating the heat generated in high current circuits. The device provided with the DIGIAC 1750 Trainer has unity gain and a maximum output current of the order of 1.5A. The main characteristics of the device are as follows: Input voltage (max.)

±12V

Input impedance

100kΩ

Output current Output power (limited by power supply) Upper -3dB frequency

1.5A 9W 10.6kHz

Table 13.7

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13.6 Practical Exercise Application of a Power Amplifier

POWER AMPLIFIER O/P I/P

From Function Generator SLIDE

A.C. AMPLIFIER O/P

C

LOUDSPEAKER

I/P

B I/P 1

2

3

4

5

6

7

8

9

10

10k

A

10 1000 100

GAIN 0V

TP1

0V

TP2

0V

Oscilloscope

CH.1

CH.2

Fig 13.6

220



Connect the circuit of Fig 13.6.



Switch ON the power supply and adjust the Function Generator to give a sinewave input at 1kHz to the AC Amplifier. Increase the amplitude to give maximum undistorted output from the amplifier.



Connect the Loudspeaker directly to the output of the AC Amplifier and observe the effect on the output waveform.

IT02 Curriculum Manual

13.6a

Signal Conditioning Amplifiers Chapter 13

With the Loudspeaker connected directly to the output of the AC Amplifier the waveform looks like:

a

b



13.6b

c

d

Transfer the output of the AC Amplifier to the input of the Power Amplifier. Transfer the oscilloscope CH.2 connection to the output of the Power Amplifier. Finally connect the output of the Power Amplifier to the Loudspeaker.

With the Loudspeaker connected via the Power Amplifier the waveform looks like:

a

b



c

d

Switch OFF the power supply.

Note that you have already used the Power Amplifier for DC applications when driving the lamp for opto-electronic experiments and for driving the motor for rotating motion investigations.

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13.7 The Current Amplifier and Buffer Amplifier

I/P

O/P

Fig 13.7

The symbol for a current amplifier is once more the same as for any DC amplifier. The amplifier converts an input current to an output voltage. The device provided with the DIGIAC 1750 Trainer is intended for use with the P.I.N. photodiode, giving an output voltage 10,000 times the input current. An input current of 1mA (max.) will provide 10V (max.) at the output. The main characteristics of the Current Amplifier are shown in Table 13.8 below. The symbol for a buffer amplifier is again as shown in Fig 13.7. These amplifiers have a high input impedance and a low output impedance and are inserted in the circuit between a device having a high output impedance and one having a low input impedance to prevent loading, as shown in Fig 13.8. Device 1 (High output impedance)

Buffer

Device 2 (Low input impedance)

Fig 13.8

The characteristics are similar to those of the Power Amplifier but they have a much lower output current capability, (of the order of 20mA maximum for the devices provided with the DIGIAC 1750 Trainer). Two buffer amplifiers are provided with the DIGIAC 1750 Trainer, Buffer #1 and Buffer #2 and their main characteristics are shown in Table 13.9.

Input current (max) Transfer ratio Table 13.8

222

1mA 10,000V/A

Input voltage (max.)

±12V

Input impedance

100kΩ

Input offset voltage

300µV

Voltage gain Table 13.9

1.0

IT02 Curriculum Manual

Signal Conditioning Amplifiers Chapter 13

13.8 Practical Exercise Characteristics and Applications of Current and Buffer Amplifiers

SLIDE

CURRENT AMPLIFIER

C

O/P B 1

2

3

4

5

6

7

8

9

10

I/P 104 I IN

A

10k

MOVING COIL METER +5V

BUFFER #1

WIREWOUND TRACK 6 5 C 7

4 3

8

2

9

B

10k

0

5 +10

-10

I/P +VIN

+ -

A

10

1

5 O/P

V

0V

L J

0V

Fig 13.9 

Connect the circuit as shown in Fig 13.9 with the Buffer Amplifier out of circuit initially. Set the 10kΩ wirewound resistor for zero output (control fully counter clockwise) and the 10kΩ slider resistor for maximum resistance (slider to right).



Switch ON the power supply and set the output voltage from the 10k Ω wirewound resistor to 1V as indicated by the digital voltmeter.



Vary the slider resistor control from maximum resistance to minimum and note the reading of the digital voltmeter. You will note that it falls due to the increased current loading. Note the lowest value. 10k

slider resistance minimum voltage =

V

The current has varied from 0.1mA to 1.0mA approximately but this has been sufficient to produce the voltage drop above. The Buffer Amplifier can be used to reduce this loading effect.

223

Signal Conditioning Amplifiers Chapter 13





IT02 Curriculum Manual

Disconnect the lead between socket B of the Wirewound Track potentiometer and socket A of the Slide potentiometer. Connect socket B of the Wirewound Track to the input socket of Buffer #1. Connect the output socket of Buffer #1 to socket A of the Slide potentiometer. Buffer #1 is now connected between the Wirewound Track potentiometer and the Slide potentiometer.



13.8a

With the 10k Ω slider control at maximum (slider to right) set the voltage as indicated by the digital voltmeter to 1.0V. Vary the 10k Ω slider control over its full range and note the reading of the digital voltmeter.

The variation of voltage with the Buffer Amplifier in circuit was:

a >0.5V

b

0.3V

c

0.1V

d

virt ually nil



Check that the output from the 10k Ω wirewound resistor is still 1.0V and then remove the digital multimeter from the circuit, switch to a 2mA range and reconnect it as an ammeter into the circuit between the 10k Ω slider resistor and the Current Amplifier to monitor the input current.



Set the 10k Ω slider resistor control to each of the settings indicated in Table 13.10 and for each setting note the input current and the output voltage for the Current Amplifier.

Resistor setting

10

Input current Output voltageV

8 mA

V

V

6 mA

V

V

mA

4 mA

2 mA

1 mA V

Table 13.10 

224

Plot the graph of Output voltage against Input current for the Current Amplifier.

IT02 Curriculum Manual

Signal Conditioning Amplifiers Chapter 13

10 9 Output Voltage 8 (volts)

7 6 5 4 3 2 1 0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7 0.8 0.9 1.0 Input current (mA)

Graph 13.1

13.8b

Is the graph linear?

Yes

13.8c

or

No

For each 0.1mA of input current the output voltage of the Current Amplifier changes by approximately:

a 0.1V

b

0.5V

c

1.0V

d

2V

This exercise has illustrated the characteristics of the current amplifier and the application of a buffer amplifier for circuits requiring a low output current. 

Switch OFF the power supply

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13.9 The Inverter Yet again the symbol is the same as for any amplifier.

I/P

I

O/P

Fig 13.10

The inverter amplifier, as the name implies, reverses the polarity of the voltage applied to the input, either DC or AC. The device provided with the DIGIAC 1750 Trainer has a voltage gain of unity. One aspect of all IC amplifiers which has not been mentioned before is the slew rate. This imposes a limitation on alternating signals on the rate at which the output voltage can change with respect to time. You can have either a small signal voltage at a high frequency or a larger signal voltage at a lower frequency. This is not quite the same thing as the gain/bandwidth product which was introduced earlier, as you will see from the experiment which follows. The main characteristics of the device are: Input voltage (max.) Voltage gain

-1.0

Input impedance

100kΩ

Input offset voltage

300µV

Slew rate (typ.) Table 13.11

226

±12V

0.15V/µs

IT02 Curriculum Manual

Signal Conditioning Amplifiers Chapter 13

13.10 Practical Exercise Characteristics of an Inverter

INVERTER O/P I/P -VIN

V -5V

0V

+5V

Fig 13.11 

Connect the circuit as shown in Fig 13.11.



Switch ON the power supply. With the Inverter input connected to the +5V supply note the value of the output voltage in Table 13.12. Inverter input

+5V

Inverter output

-5V V

V

Table 13.12 

13.10a

Transfer the Inverter input to the -5V supply and again note the value of the output voltages.

The polarity of the output voltage is:

a the same as the input 

b

opposite to the input

Switch OFF the power supply.

The output voltage magnitude may not be identical with the input due to the offset voltage. No facility for adjusting this has been provided.

227

Signal Conditioning Amplifiers Chapter 13

40kHz OSCILLATOR

IT02 Curriculum Manual

SLIDE

INVERTER

C

O/P B 1

2

3

4

5

6

0V

7

8

9

10

A

10k

TP1

0V

O/P I/P -VIN

TP2

0V

Oscilloscope CH.1

CH.2

Fig 13.12 

Connect the circuit as shown in Fig 13.12. Switch ON the power supply.



Set the oscilloscope timebase to 5 µs/div. and both Y amplifiers (CH.1 & CH.2) to 0.5V/div.



Adjust the control of the 10k Ω slider resistor to give an input voltage of 1Vp-p.



Sketch the input and output (Output 1) waveforms on the graticule provided:

Input

Output 1

Output 2

Waveform Sketch 13.1

228

IT02 Curriculum Manual

13.10b



Change the Y2 (CH.2) amplifier to 1V/div and increase the setting of the 10kΩ slider resistor until the full effect of the slew rate is observed.



Add a sketch of the output (Output 2) waveform.

Your sketched waveforms are most similar to:

a

c

b



13.10c

Signal Conditioning Amplifiers Chapter 13

Check the slew rate (

voltage ti me ( µs)

d

) against the specification given earlier.

Enter the value of peak-to-peak output voltage at which the slewing first occurred with an input signal of 40kHz. 



13.10d

Replace the input to inverter with a 5kHz sinewave output from the Function Generator. Increase the amplitude of the signal until slewing again begins to occur. Note the maximum peak-to-peak value of the undistorted output signal.

Enter the value of peak-to-peak output voltage at which the slewing first occurred with an input signal of 5kHz.

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13.11 The Differential Amplifier

B

-

A

+

I/P's

O/P

Fig 13.13

The symbol for a differential amplifier is shown in Fig 13.13. The amplifier has two inputs which can be driven by separate signals. It is called differential because the output voltage depends on the difference in voltages applied to the two inputs. If the two inputs are driven by the same signal in phase then theoretically there should be no output. There will, however, be a small output the amount being determined by the common mode gain, which is designed to be as near to zero as possible. For the device provided on the DIGIAC 1750 Trainer, the output voltage is given by (VA - VB). Two differential Amplifier". amplifier circuits are provided, the basic secondfunctions being labeled "Instrumentation This carries out the same as the differential amplifier but has an improved (reduced) common mode gain. The main characteristics of the devices are: Differential Amplifier Input voltage (max.)

±12V

Differential gain

1.0

Common mode gain (max.)

0.02

0.006

Input impedance (input A)

200kΩ

100kΩ

Input impedance (input B) Table 13.13

230

Instrumentation Amplifier

100kΩ

IT02 Curriculum Manual

Signal Conditioning Amplifiers Chapter 13

13.12 Practical Exercise Characteristics of a Differential Amplifier

SLIDE

C B

1

2

3

4

5

6

7

8

9

10

WIREWOUND TRACK 6 5 C

+5V

7

4

-5V

3

8

2

9 1

DIFFERENTIAL AMPLIFIER O/P

A

10k

B

-

A

+

A-B

B

V

A

10

10k

0V

Fig 13.14



Connect the circuit as shown in Fig 13.14 and switch ON the power supply.



Moving the digital voltmeter lead as necessary, set the voltage at input A of the Differential Amplifier to -3V and input B also to -3V and note the resulting output voltage. Record the value in Table 13.14.

Step

1

2

3

4

5

6

7

8

Input B voltage

-3V

+1V

+4V

+2V

0V

+4.5V

+2V

-2.7V

Input A voltage

-3V

+1V

+4V

+4V

+3V

+2.2V

-3V

+3.6V

Output voltage V V

VV

V

V

V

V

Table 13.14 

Repeat the procedure for each of the other pairs of inputs in Table 13.14 and record the output voltage again.

231

Signal Conditioning Amplifiers Chapter 13

13.12a

Enter your output voltage reading for Step 1 in V.

13.12b

Enter your output voltage reading for Step 3 in V.

13.12c

Enter your output voltage reading for Step 5 in V.

13.12d

Enter your output voltage reading for Step 6 in V.

13.12e

Enter your output voltage reading for Step 7 in V.

13.12f

Enter your output voltage reading for Step 8 in V.

13.12g

In Steps 1, 2 & 3 the amplifier was working in:

a differential mode

b

inverting mode

c common mode

d

opposing mode



232

IT02 Curriculum Manual

Switch OFF the power supply.

IT02 Curriculum Manual

Signal Conditioning Amplifiers Chapter 13

Student Assessment 13 1.

2.

3.

4.

5.

The term "input offset voltage" applied to a DC amplifier means the voltage:

a

at the input with no signal applied

b

at the output with no signal applied

c

needed at the input for zero output voltage

d

needed at the input with signal applied

The main difference between an AC amplifier and a DC amplifier is that the:

a

AC amplifier does not suffer from offset

b

only the AC amplifier can amplify AC signals

c

DC amplifier will not pass low frequency signals

d

gain of an AC amplifier will be greater

The type of amplifier which will pass AC signals is:

a

the DC amplifier only

b

the AC amplifier only

c

either the DC or the AC amplifier

d

neither the DC nor the AC amplifier

The purpose of a buffer amplifier is to:

a

remove DC signals

from AC signals

b

increase the signal level at low frequencies

c

convert positive polarity signals into negative polarity signals

d

reduce loading on a high impedance source

Comparing a buffer amplifier to a power amplifier, the buffer amplifier will:

a

only operate on DC

b

deliver less current

c

only operate on AC

d

restrict the bandwidth

Continued ...

233

Signal Conditioning Amplifiers Chapter 13

IT02 Curriculum Manual

Student Assessment 13 Continued ... 6.

A signal source has an open-circuit EMF of 10V and an output impedance of 100k . If this was connected directly to a 10k load the voltage delivered to the load would be:

a 7.

5V

c

1.1V

d