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BASIC ELECTRICITY AND ELECTRONICS
STUDENT’S MANUAL
Chapter 3
Assignments using
Workboard 12-200-B
3 Assignments using Workboard 12-200-B
3.1
The Semiconductor Diode Assignment
3.1.1 Objectives
 To recognise diodes in various physical forms.
 To determine the diode polarity and to understand the need for correct connection.
 To obtain knowledge of the forward voltage/current characteristic and the
conduction voltage for a silicon diode.
3.1.2 Prerequisite Assignments

Resistance

Resistor Networks

Resistors in Series and Parallel

Superposition Theorem

Thévenin's Theorem

Power
3.1.3 Knowledge Level
Before working this assignment you should:

Know the operation of series dc circuits.
3.1.4 Equipment Required
Qty
12-200S
Apparatus
1
Basic Electricity and Electronics Module 12-200-B
1
Power Supply Unit, 0 to 20 V variable dc, regulated
(eg, Feedback Teknikit Console 92-300).
3-1
BASIC ELECTRICITY AND ELECTRONICS
STUDENT’S MANUAL
2
Chapter 3
Assignments using
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Multimeters
OR
Feedback Virtual Instrumentation may be used in place of one
of the multimeters
1
3-2
Function Generator, 100 Hz – 5 kHz 20 V pk-pk sine
(eg, Feedback FG601)
12-200S
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3.1.5 Background
A Semiconductor Junction Diode (or just Diode) is made from a piece of P-type and a
piece of N-type semiconductor joined together. See fig 1.
Fig 1 Junction Diode
If a voltage (potential difference) is applied across the two terminals, the Diode will
conduct electricity.
The amount of current that flows depends upon the magnitude and polarity of the applied
voltage.
A diode will conduct electricity if connected one way round in a circuit and will not conduct
if connected the other way round.
The Diode is represented in circuits by the symbol shown in fig 2.
Fig 2 Diode Symbol
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BASIC ELECTRICITY AND ELECTRONICS
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3.1.5.1 Forward/Reverse Biased Connections
When a diode is connected so as to conduct it is said to be Forward Biased.
When a diode is connected so as NOT to conduct, it is Reverse Biased.
Fig 3 shows the two methods of connecting diodes.
Fig 3 Diode Bias
A knowledge of the conduction characteristic when a diode is forward biased is very
important .
Fig 4 Typical Silicon Diode - Forward Characteristic
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3.1.6 Practical 1
Firstly in this Practical you will identify two physical forms of semiconductor diode that are
provided in the equipment.
Then you will investigate the circuit shown in fig 5, below.
Fig 5
Initially you will measure the voltage across the circuit and the current that flows through
the diode. You will then reverse the connections of the diode and measure the new
voltage and current values.
From these measurements you will be able to understand the basic operation of a diode.
Connect up the circuit as shown in the Patching Diagram for this Practical.
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Practical 1 Patching Diagram
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3.1.6.1 Perform Practical
The Diode is represented in circuits by the symbol shown in fig 6.
Fig 6 Diode Symbol
Two types of diode are shown in fig 7.
Fig 7 Two Types of Diode
The power diode can handle larger currents and power than the 1N4007 type. This is
because of its larger size and metal case that can be bolted to a heat-sink to dissipate
higher power.
Identify the two forms of diode provided. You will be using the 1N4007 type in this
Assignment.
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BASIC ELECTRICITY AND ELECTRONICS
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Determining Diode Polarity
Ensure that you have connected up the circuit as shown in the Patching Diagram and that
it corresponds with the circuit diagram of fig 8.
Note that the resistor limits the current to a safe value.
Fig 8 Diode Test Circuit

Switch on the power supply.

Set the power supply control to give 10 V on the meter.
Go to the Results Table section of this Assignment and copy fig 9 to tabulate your results.
Record the current measurement in the first row of the table.

Now, switch off the power supply and reverse the 1N4007 diode to give the circuit
of fig 10
Fig 10 Diode reversed
3-8

Switch on the power supply and readjust the voltage to 10 V.

Read the new value of diode current and record it in the second row of your table.
12-200S
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3.1.6.2 Questions
1. Which side of a diode should be connected to the positive voltage supply to make it
conduct current?
2. When the diode was connected the opposite way round was the current
a) slightly smaller?
b) much smaller?
or c) too small to measure?
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3.1.7 Practical 2
In this Practical you will examine in more detail the current that flows with the forward
biased connection of the diode and you will plot its forward characteristic.
The circuit that you will be using is shown in figure 11.
The current that flows through the diode also flows through the 100Ω resistor and the
voltage across the resistor will be directly proportional to that current. As the resistor value
is low, it will have negligible effect on the overall working of the circuit.
Fig 11 Test Circuit
Connect up the circuit as shown in the Patching Diagram for this Practical.
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BASIC ELECTRICITY AND ELECTRONICS
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Practical 2 Patching Diagram
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Chapter 3
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BASIC ELECTRICITY AND ELECTRONICS
STUDENT’S MANUAL
3.1.7.1 Perform Practical
The Characteristics of Forward Biased Diodes
Ensure that you have connected up the circuit as shown in the Patching Diagram and that
it corresponds with the circuit diagram of fig 12.
The 2.2 kΩ potentiometer will provide fine control over the applied voltage.
Fig 12 Test Circuit
NOTE: Vd = Vs - Vr and Vr = If x 100
If 
Vr
A
100
Vr
A 1000mA
100
 I f  10Vr mA

Go to the Results Table section of this Assignment and copy fig 13 to tabulate your
results.

Turn the potentiometer to zero; fully anti-clockwise.

Switch on the power supply and adjust it to supply 20 V.

Adjust the potentiometer to give a voltage of 1 V on the voltmeter showing V s.

Now use the power supply variable control to set Vs to:
0, 0.1 V, 0.2 V, etc, up to 1.0 V.
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Note Vr for each setting and enter it in your table.

Now, with the power supply variable control set to supply 20 V, use the
potentiometer to set Vs to:
1.5 V, 2.0 V, 2.5 V and 3.0 V.
Again enter the values of Vr in your table.

Calculate Vd and If as shown in fig 13 and enter these also in the table.
Use the spreadsheet or prepare a graph like Fig 14 on which to plot your results.
Plot Vd against If on the axes of the graph.
Fig 14 Diode Forward Characteristics
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3.1.7.2 Questions
1. At what approximate value of Vd does the current If begin to rise noticeably?
2. Does Vd rise much above this value for larger values of If?
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3.1.8 Results Required
When you have performed this Assignment you should have:
 identified the diodes provided,
 determined how diode conduction is dependent on applied voltage polarity,
 measured and plotted the forward characteristic of a diode,
 determined the typical conduction voltage for a silicon diode.
Your report should contain:

the circuits that you investigated,

the results that you achieved,

the graph plotted,

conclusions on your findings in the Assignment.
To produce your report you should use a word processing package.
To achieve the calculated values you could use a spread sheet package.
12-200S
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3.1.9 Practical Considerations and Applications
Both the 1N4007 and the power diode are made of Silicon and the forward conduction
voltage of about 0.6 V is typical of silicon junctions.
Also typical of silicon diodes is the very small reverse current.
The power diode passes a greater reverse current than 1N4007. This is because it is
designed for much larger forward currents - up to 6 A average. At the low voltage used in
this experiment the reverse amount will still be very small.
Diodes can withstand high reverse voltage but will eventually break down at some voltage
and may be irreparably damaged.
Diodes have very many applications at many different power, voltage and current levels. A
very important application is the production of direct voltage from alternating voltage and
this is dealt with in the Assignments which cover Rectification.
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BASIC ELECTRICITY AND ELECTRONICS
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3.1.10 Results Tables
Circuit
Current (mA)
Fig 8
Fig 10
Fig 9
Vs
(V)
Vr
(V)
Vd = V s – Vr
(V)
If = 10 Vr
(mA)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.5
2.0
2.5
3.0
Fig 13
12-200S
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3.1.11 Further Work
Construct the circuit of fig 15 and apply the method used earlier in this Assignment to find
Vd and If.
Fig 15 Diode Power Dissipation

3-18
Calculate the power dissipation of the diode and check to see if it becomes warm to
the touch after a few minutes operation.
12-200S
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3.2
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Half-Wave Rectification Assignment
3.2.1 Objectives

To learn to recognise a half-wave rectified sinusoidal voltage.

To understand the term 'mean value' as applied to a rectified waveform.

To understand the effect of a reservoir capacitor upon the rectified waveform and
its mean value.
3.2.2 Prerequisite Assignments

The Semiconductor Diode
3.2.3 Knowledge Level
Before working this assignment you should:

Know the operation of transformers.

Know the operation of series and parallel ac circuits.

Know how to use an oscilloscope.
3.2.4 Equipment Required
Qty
Apparatus
1
Basic Electricity and Electronics Module 12-200-B
1
Power Supply Unit, ac. supply; 12 V rms; 50 or 60 Hz
(isolated from other supplies).
(eg, Feedback Teknikit Console 92-300).
1
Multimeter
OR
Feedback Virtual Instrumentation may be used in place of the
multimeter
1
12-200S
Oscilloscope
3-19
Chapter 3
Assignments using
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BASIC ELECTRICITY AND ELECTRONICS
STUDENT’S MANUAL
3.2.5 Background
In The Semiconductor Diode Assignment you found that a diode conducts current in one
direction (from anode to cathode) but not in the reverse direction.
A widely used application of this feature is the conversion of alternating voltages to direct
voltages (fig 1). This assignment studies the simplest circuits for achieving this conversion,
which is called Rectification or, in some cases, Detection.
Fig 1
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3.2.6 Practical 1
In this Practical you will see the typical waveform of a half-wave rectified sinusoidal
voltage.
You will measure the period and the peak voltage of the rectified waveform and you will
calculate the mean value of the voltage.
You will reverse the rectifying diode and observe its effect.
The circuit that you will be using is shown in fig 2.
Fig 2
Connect up the circuit as shown in the Patching Diagram for this Practical.
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Practical 1 Patching Diagram
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3.2.6.1 Perform Practical
Ensure that you have connected up the circuit as shown in the Patching Diagram and that
it corresponds with the circuit shown in fig 3.
Fig 3 Half-wave Rectification

Switch on the oscilloscope and the sinusoidal supply.

With the oscilloscope dc coupled adjust the timebase and the Y amplifier sensitivity
to obtain a steady trace of about 4 cm vertical and 5 ms/cm horizontal.
You should see a waveform as in fig 4.
Fig 4 Half-wave Rectified Waveform

Measure and record time T and peak voltage V pk.

Sketch the waveform and label it to show the periods when the diode is conducting
and those when it is not. Time T depends upon the frequency of your power supply.
For a 50Hz supply it should be 20 ms and for 60 Hz it should be 17 ms.

Confirm this. Vpk should be very nearly equal to the peak voltage of the alternating
supply.
The waveform of fig 4 goes only positive relative to zero volts. If you connect a moving coil
dc voltmeter across the output as in fig 3, the mechanical inertia of the meter will not allow
the needle to respond to the rapid voltage changes. Instead, it indicates the MEAN voltage
of the waveform.
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The mean value of a half-sinusoid can be shown by geometry to be:
2Vpk

But at every half-cycle the voltage is zero. The mean value of the waveform, therefore, is:
1  2Vpk T
T  Vpk
  0  

T 
2
2 

Note the mean voltage indicated by the voltmeter, and compare it with 0.32 Vpk.
The mean voltage you obtained is positive relative to zero.

3-24
Rearrange the circuit to give a negative voltage. Confirm this by experiment.
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3.2.6.2 Questions
1. Why is Vpk not be exactly equal to the peak voltage of the alternating supply?
2. By how much does it differ?
3. The mean voltage you obtain from this Practical is positive relative to zero. How could
you obtain a negative voltage?
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3.2.7 Practical 2
In this Practical you will see the effect of adding a capacitor across the output of a halfwave rectifier circuit. This is shown in fig 5, below.
Fig 5 Half-wave Rectifier with Reservoir Capacitor
You will see how this capacitor ‘smoothes’ the output voltage to give a more nearly
constant dc voltage. However, you will also see that there are still variations (ripple) on
this output voltage.
You will change the capacitor to one with a larger value and observe any changes in
output voltage waveform.
Connect up the circuit as shown in the Patching Diagram for this Practical.
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Practical 2 Patching Diagram
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3.2.7.1 Perform Practical
Very often when rectifying an alternating voltage, you wish to produce a steady direct
voltage free from variations of the sort observed in fig 5. One way of doing this is to
connect a capacitor in parallel with the load resistor as in fig 6.
Fig 6 Half-wave Rectifier with Reservoir Capacitor
The effect of a capacitor can be seen with reference to fig 7.
Fig 7 The Effect of a Reservoir Capacitor
The capacitor C (usually called the reservoir capacitor) becomes charged-up by the
current through the diode during the positive half-cycle. Then, when the supply voltage
starts to reduce again, the capacitor keeps the output voltage high and the diode cuts off.
Capacitor C then discharges through R until the next positive half-cycle occurs.
Ensure that you have connected up the circuit as shown in the Patching Diagram and that
it corresponds with the circuit shown in fig 6. Initially, the 2.2 µF capacitor should be
connected in circuit as capacitor C.
Observe the output waveform on the oscilloscope and note the value of the peak-to-peak
variations in voltage. Note also the new mean voltage on the voltmeter.

3-28
Now replace the 2.2 µF capacitor by a much larger value of 47 µF, making sure to
connect the + side of the capacitor to the diode cathode (the capacitor is electrolytic
and MUST be connected in the correct polarity) and answer Questions 2 and 3 for
this Practical.
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3.2.7.2 Questions
1. Is the mean voltage with the 2.2 µF capacitor added greater or less than it was before?
2. The variations on the rectified waveform are called RIPPLE. Is the ripple with the 47 µF
less than or more than it was with the lower value capacitor?
3.
Is the mean rectified voltage now greater or less?
12-200S
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3.2.8 Results Required
When you have performed this Assignment you should have:

seen how a diode circuit can convert an alternating voltage into a direct voltage,

measured the peak and calculated the mean voltage values for a half-wave rectifier
circuit,

observed the effect of adding a reservoir capacitor to the basic half-wave rectifier
and the ripple voltage resulting,

observed the effect of the capacitor value on the degree of smoothing and the
amplitude of the ripple voltage.
Your report should contain:

the circuits that you investigated,

the results that you achieved,

the waveforms drawn,

conclusions on your findings in the Assignment.
To produce your report you should use a word processing package.
To achieve the calculated values you could use a spread sheet package.
3-30
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3.2.9 Practical Considerations and Applications
When rectification is used to provide a direct voltage power supply from an alternating
source, the ripple is an undesirable feature. For a given capacitor value, a greater load
current (smaller load resistor) discharges the capacitor more and so increases the ripple
obtained.
Fig 8 shows this.
Fig 8 The Effect of Load Current
Several methods are available to reduce ripple:
1. Larger capacitors, the uses of which are limited due to cost and size, and also
because large capacitors can require very large charging currents to be supplied
through the diode.
2. Electronic stabilisation This reduces ripple as well as keeping the output voltage
steady when the load or input voltage changes.
3. Full-wave rectification. With this, the ripple is much reduced as every half-cycle of
the input, instead of every other half-cycle, contributes to the rectified output.
In fig 9 it can be seen that capacitor charging occurs at half the previous interval and the
amount of discharge for a given load current is therefore less.
Fig 9 Full-wave Rectification
The Full-Wave Rectification Assignment deals with methods of achieving full-wave
rectification.
When diodes are used for detection purposes in the reception of modulated radio signals,
quite different considerations apply. These are not discussed in detail here.
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3.2.10 Further Work
A half-wave rectifier, as in fig 10, produces a certain amplitude (peak-to-peak) of ripple.
Fig 10 Rectifier Circuit
If the load resistor is reduced to half of its original value, what increase in capacitor value
will restore the ripple to the same value as before?

Confirm your answer by practical experiment, starting with:

R = 10 kΩ and C = 47 µF.
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Full-Wave Rectification Assignment
3.3.1 Objectives

To learn to recognise a full-wave rectified sinusoidal voltage waveform.

To understand the working of a diode bridge circuit as a full-wave rectifier and its
advantage over half-wave rectification.

To understand the effect of a reservoir capacitor upon the rectified waveform and
its mean value.

To have an awareness of the two diode form of full-wave rectifier.
3.3.2 Prerequisite Assignments

Half-wave Rectification
3.3.3 Knowledge Level
Before working this assignment you should:

Know the operation of transformers.

Know the operation of series and parallel ac circuits.

Know how to use an oscilloscope.
3.3.4 Equipment Required
Qty
Apparatus
1
Basic Electricity and Electronics Module 12-200-B
1
Power Supply Unit, ac supply; 12 V rms; 50 or 60 Hz. (isolated
from other supplies).
(eg, Feedback Teknikit Console 92-300).
1
Multimeter
OR
Feedback Virtual Instrumentation may be used in place of the
multimeter
1
12-200S
Oscilloscope
3-33
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BASIC ELECTRICITY AND ELECTRONICS
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3.3.5 Background
At the end of the Half-wave Rectification Assignment ways of reducing the ripple or
voltage variation on a rectified direct voltage were discussed. One of these was to use
every half-cycle of the input voltage instead of every other half-cycle.
A circuit which allows this is shown in fig 1, and is known as the diode bridge.
Fig 1 A Diode Bridge Rectifier
During the positive half-cycle of the supply 'A' is more positive than 'B'. Diodes D1 and D2
therefore conduct while diodes D3 and D4 are reverse-biased. The current flows as shown
in fig 2.
Fig 2 Positive Half-cycle
Fig 3 Negative Half-cycle
During the negative half-cycle the current flow is as represented by fig 3.
In each case the current in the load is in the same direction.
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A typical bridge rectifier component is shown in fig 4a.
Note that, on your equipment, the bridge rectifier component may be a small cube, as
shown in fig 4a, or may be cylindrical in construction.
Fig 4b shows the circuit symbol and how the rectifier terminals are labelled.
Fig 4 Bridge Rectifier
The terminals labelled + and - are so called because these are the polarities that will exist
across the load.
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3.3.6 Practical 1
In this Practical you will see the typical waveform of a full-wave rectified sinusoidal voltage.
You will measure the period and the peak voltage of the rectified waveform and you will
calculate the mean value of the voltage.
The circuit that you will be using is shown in fig 5.
Fig 5
You will add a reservoir capacitor and observe its effect.
Connect up the circuit as shown in the Patching Diagram for this Practical.
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Practical 1 Patching Diagram
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3.3.6.1 Perform Practical
A Bridge Rectifier with Resistive Load
Ensure that you have connected up the circuit as shown in the Patching Diagram and that
it corresponds with the circuit shown in fig 6.
Note:
Prior to connecting an ac power supply to the board, ensure that the
supply is isolated from the ground.
Fig 6 Test Circuit

With the oscilloscope dc coupled, adjust the controls to obtain a steady trace of
about 4 cm vertical and 5 ms/cm horizontal. You should observe a waveform as in
fig 7. Time 'T' will be 10 ms for 50 Hz supply, and 8.5 ms for 60 Hz.
Fig 7 Full-wave Rectified Waveform
Note the value of Vpk and also the mean value of output voltage indicated on the
voltmeter. Compare these figures with those obtained in Half Wave Rectification.
2Vpk

As the mean value of a half-cycle of a sine wave is, and every half-cycle is present, this
should be the mean value measured.
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Confirm this from your readings.
The Effect of a Reservoir Capacitor

Add a 2.2 µF capacitor in parallel with the load resistor and note the new mean
value and the peak-to-peak ripple amplitude of the rectified waveform. Compare
these figures with those obtained in the Half-wave Rectifier Assignment for the
same load and capacitor values.
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3.3.7 Questions
1. Should Vpk be the same as it was for a half-wave rectifier? Does your observation
confirm your answer?
2. How does the mean value compare with that found for half wave rectification?
3. Fig 9 shows the discharge curve for a reservoir capacitor in half-wave and full-wave
rectification, for the same load and capacitor values.
Fig 9
A - Start of discharge
B - End of discharge; full wave
C - End of discharge; half wave
A capacitor discharges into a resistor in an exponential fashion, that is with a rate of
discharge that reduces as the discharge progresses.
With this in mind, would you expect the peak-to-peak ripple in full-wave to be:
(a) half that in half-wave
(b) less than half
(c) more than half
Explain your answer and confirm it by reference to measurements made in the Halfwave Rectification Assignment and this Assignment for similar load conditions.
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3.3.8 Results Required
When you have performed this Assignment you should have:
 seen how a bridge rectifier circuit can convert an alternating voltage into a direct
voltage,
 measured the peak and calculated the mean voltage values for a bridge rectifier
circuit,
 observed the effect of adding a reservoir capacitor to the circuit.
Your report should contain:

the circuits that you investigated,

the results that you achieved,

the waveforms drawn,

a comparison of the performance of half-wave and bridge rectifier circuits,

conclusions on your findings in the Assignment.
To produce your report you should use a word processing package.
To achieve the calculated values you could use a spread sheet package.
12-200S
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3.3.9 Practical Considerations and Applications
The alternating input voltage to a rectifier is usually obtained from the main supply through
a transformer, for two reasons:
1. 1.To obtain the desired voltage by choice of the transformer ratio.
2. To provide isolation from the main supply for safety reasons.
Fig 9 shows such an arrangement with a bridge rectifier.
Fig 9 Transformer-fed Bridge Rectifier
In this figure, although the load current is always in one direction, the current in the
transformer secondary is alternating.
Fig 10 shows another method of full-wave rectification, using a centre-tapped transformer
winding and two diodes.
Fig 10 Full-wave Rectifier using Two Diodes
The arrows show how current flows on alternate half-cycles. The value of the output
waveform is exactly the same as that for a bridge circuit provided each half of the
transformer windings has the same rms voltage as the whole of the winding in fig 9.
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The circuit saves two diodes, but increases the cost of the transformer. In fig 10 each halfsecondary winding must have the same voltage rating as the single secondary of fig 9.
Suppose the half-secondaries were wound with wire of half the cross-sectional area, so as
to fit the two into the same space as the one secondary of fig 9, and use the same amount
of copper. Each half-secondary would then have twice the resistance.
The current flows in each half-secondary only on alternative half-cycles, but would
generate twice the I2R loss in the active cycle.
Each half-secondary would thus develop as much heat as the single secondary of fig 9,
ie, twice as much for both. A larger transformer would therefore be required to avoid
excessive heating. Its greater cost would usually outweigh the cost of the two diodes
saved.
In full-wave rectification the basic repetition rate of the ripple is twice that of the supply
(eg, 100 Hz for a 50 Hz supply). In half-wave the frequency is the same as the supply
frequency. This is often useful as an indication that one half of a bridge or full-wave
rectifier is faulty.
12-200S
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Notes
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12-200S
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STUDENT’S MANUAL
3.4
Chapter 3
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The Zener Diode Assignment
3.4.1 Objectives

To recognise Zener diodes in various physical forms and to distinguish them from
rectifying diodes.

To understand the constant-voltage characteristic of a reverse-biased Zener diode.

To understand the use of a Zener diode in a simple voltage regulator circuit.
3.4.2 Prerequisite Assignments

The Semiconductor Diode
3.4.3 Knowledge Level
Before working this assignment you should:

Know what is meant by internal resistance and the effect it has on terminal voltage.
3.4.4 Equipment Required
Qty
Apparatus
1
Basic Electricity and Electronics Module 12-200-B
1
Power Supply Unit, 0 to 20 V variable dc, regulated.
(eg, Feedback Teknikit Console 92-300).
2
Multimeters
OR
Feedback Virtual Instrumentation may be used in place of one
of the multimeters
12-200S
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3.4.5 Background
In the Semiconductor Diode Assignment you found that a reverse-biased diode passes
negligible current. You also learnt that it will eventually suffer breakdown and damage if
the reverse voltage is made too high. See fig 1.
Fig 1 Reverse Breakdown of a Diode
Zener diodes are specially constructed to break down at controllable voltages and to do so
without damage to the device. As we shall see, this feature can be put to good use.
Two Zener diodes are contained in the 12-200. They are 10 V and 7.5 V types and are
shown in fig 2 with the standard circuit symbol.
Fig 2 Zener Diodes and Symbol
Zener diodes look very similar to rectifier diodes and the terminal names and identification
methods are the same. The larger types have greater power and current capacities.
The two types of diode can usually be distinguished only by their type numbers. For Zener
diodes these often, but not always, contain the letter Z.
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3.4.5.1 A Simple Zener Diode Voltage Regulator
The Zener diode has a region in its reverse characteristic of almost constant voltage
regardless of the current through the diode. This can be used to regulate or stabilise a
voltage source against supply or load variations.
Fig 3 shows an unregulated voltage source supplying current to a variable load.
Fig 3 An Unregulated Power Source
If either Vs or RL changes, so will the voltage across the load VL.
One way of keeping this voltage more constant is to connect across the load a Zener
diode whose breakdown voltage is the desired constant voltage.
Fig 4 shows a Practical circuit of this kind.
Fig 4 A Simple Zener Diode Regulator
12-200S
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3.4.6 Practical 1
In this Practical you will plot the typical reverse characteristic of a zener diode.
You will measure the voltage across the diode and also that across a series resistor.
You will use Ohm’s Law to calculate the current through the diode.
The circuit that you will be using is shown in fig 5.
Fig 5
You will plot the reverse characteristic and you will also calculate and plot the power
dissipation against the diode voltage.
Connect up the circuit as shown in the Patching Diagram for this Practical.
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Practical 1 Patching Diagram
12-200S
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3.4.6.1 Perform Practical
As shown in the Patching Diagram for this Practical, construct the circuit of fig 6.
Fig 6 Test Circuit
The method of obtaining the voltage-current characteristic is the similar to that of the
Semiconductor Diode Assignment but notice that the Zener diode is reverse-biased.

Using the power supply variable control, set Vs to the following values:
0 V, 2 V, 4 V, 6 V, 6.5 V, 7.0 V, 7.5 V, 8.0 V, 8.5 V, 9.0 V, 10 V, 15 V and 20 V

For each value record Vr.
Go to the Results Tables section of this Assignment and copy fig 7 to tabulate your
results.
Calculate:

Vr
Vd = Vs - Vr and Id = 1000= Vr mA

Prepare a graph like fig 8 and plot Id against Vd.
Note:
This is a graph of the reverse part of the Zener diode characteristic and would normally
be shown in the negative quadrant of the complete diode characteristic
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Fig 8 The Zener Diode Characteristic
Calculate the power dissipated in the diode for each value of Vd and Id, and enter it into
the last column of your table.

Pd = Vd x Id mW
Plot Pd against Vd on your graph.
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3.4.6.2 Questions
1. Describe the Zener diode characteristic in your own words.
2. The nominal voltage of the BZY55C7V5 is 7.5 V. Does your graph agree with this
exactly? If not, can you suggest a reason for any difference?
3. Why is the series resistor in Fig 6 necessary?
4. The maximum allowable power dissipation of type this type of Zener diode is 400 mW.
Does your maximum value of Pd approach this limit?
3-52
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3.4.7 Results Required
When you have performed this Assignment you should have:
 seen that, in its breakdown region a Zener diode has an almost constant voltage
regardless of diode current,
 understood how this feature can be used to stabilise a varying voltage,
 understood the need to keep the diode power dissipation below the allowable
maximum.
Your report should contain:

the circuit that you investigated,

the results that you achieved,

the characteristic graph drawn,

conclusions on your findings in the Assignment.
To produce your report you should use a word processing package.
To achieve the calculated values you could use a spread sheet package.
12-200S
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3.4.8 Practical Considerations and Applications
Zener diodes are widely used as voltage stabilisers and voltage references.
They are manufactured with Zener voltage ratings of between about 2.7 volts and 200
volts, usually in 'preferred' voltage steps, eg, 2.7, 3.3, 3.6, 3.9, 4.3, 4.7, 5.1 etc, just as 1%
tolerance resistors are manufactured.
The power dissipation rating of a Zener diode is an important parameter.
Zener diodes are manufactured with power ratings between 250 mW and in excess of
100 watts.
Although Zener voltages are fairly insensitive to changes in diode current, they are
however sensitive to temperature changes. Normally the Zener voltage is specified at a
temperature of 25°C, but diodes will have a temperature coefficient. Typical figures for this
range from -5.0 mV/deg C for a 2.7-volt device to +60 mV/deg C for a 75-volt device. The
zero temperature coefficient is given around 5.6 volts Zener voltage.
Zener diodes have many uses other than for providing stable or reference voltage sources
(eg, they can be used for clipping, thereby doing away with the need for a voltage source
as the clipping reference).
Zener diodes are often used for over-voltage protection, being connected across the load.
The Zener voltage is chosen such that under normal operating conditions the diode is
reverse-biased below the Zener voltage, so the device acts as an ordinary diode (i.e. nonconducting).
If however the voltage rises above the Zener voltage, the diode will break down and pass
a heavy current. The excess voltage may be dropped in a resistor, as in fig 9(a), or the
fuse will blow, as in fig 9(b).
Fig 9 A Zener Diode used as Load Protection
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BASIC ELECTRICITY AND ELECTRONICS
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3.4.9 Results Tables
Vs
(V)
Vr
(V)
Vd =Vs - Vr
(V)
Id = Vr
(mA)
Pd =Vd x Id
(mW)
0
2
4
6
6.5
7.0
7.5
8.0
8.5
9.0
10.0
15.0
20.0
Fig 7
12-200S
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Notes
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STUDENT’S MANUAL
3.5
Chapter 3
Assignments using
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Transistor Familiarisation Assignment
3.5.1 Objectives

To recognise transistors in various physical forms and to identify their terminals.

To understand the basic construction of PNP and NPN transistors.

To understand junction biasing and the direction and magnitude of current flows.
3.5.2 Prerequisite Assignments

The Semiconductor Diode
3.5.3 Knowledge Level
Before working this assignment you should:

Know the operation of parallel dc circuits.
3.5.4 Equipment Required
Qty
Apparatus
1
Basic Electricity and Electronics Module 12-200-B
1
Power Supply Unit, +5 V and -15 V variable dc, regulated.
(eg, Feedback Teknikit Console 92-300).
3
Multimeters
OR
Feedback Virtual Instrumentation may be used in place of one
of the multimeters
12-200S
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3.5.5 Theory
Transistors are three-terminal devices constructed in the form of two semiconductor
junctions, rather like two junction diodes.
Fig 1 shows the two types, NPN and PNP, governed by the physical arrangement of the Pand N-type semiconductor materials.
Fig 1 Two Types of Transistor
Each of the PN junctions in this diagram behaves individually like the simple diode you
studied in the Semiconductor Diode Assignment but, when joined together in this way, the
behaviour is very different.
In normal use the emitter-base diode is forward-biased and behaves almost exactly like
an independent diode. The collector-base diode, however, is reverse-biased and
normally you would expect it to pass no current. But if the E-B diode is conducting forward
current, this influences the reverse-biased C-B diode and causes it to pass almost as
much reverse current.
Fig 2 shows this for PNP and NPN types. The small difference current flows in the base
circuit.
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Fig 2 Transistor Current Flow
The ratio
IC
is usually called hfb.
IE
Because IC is almost as big as IE, hfb is nearly 1.

The ratio

Thus
IC
= hfb
IE
= nearly 1 (eg, 0.99)
IC
is usually called hfe
IB
IB + Ic = IE =
IC (
IC
hfb
1
– 1) = IB
h fb
and
hfe =
IC
=
IB
if
hfb
1
=
1
hfb  1
1
hfb
hfb = 0.99,
hfe
=
0.99
0.01
= 99.
It is this large ratio between IC and IB that makes the transistor a useful amplifying device
when connected so that IB is derived from an input and IC provides an output.
In the Assignment you will first identify some actual transistors and then confirm the
directions and magnitudes of currents, finding hfb and hfe in the process.
12-200S
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3.5.6 Practical 1
In this Practical you will investigate the currents that flow in a transistor.
The transistor that you will use is a general purpose NPN transistor with the type number
BC107. It is of the type called a ‘low power’ or ‘small signal’ transistor to distinguish it from
transistors that can operate at significantly higher power levels – they are called ‘power
transistors’.
The circuit that you will be using is shown in fig 3.
Fig 3
You will measure the base current of the device and the resulting collector current. You
will then change the base current and see how the collector current changes.
You will also measure the base-emitter voltage and compare it with that of a forward
biased diode.
In the circuit of fig 3, the capacitor is provided to ensure that the circuit is stable and has
no effect on your measurements.
Connect up the circuit as shown in the Patching Diagram for this Practical.
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Practical 1 Patching Diagram
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3.5.6.1 Perform Practical
Fig 4 illustrates a typical low power transistor.
Fig 4 Typical Low-power Transistor (as supplied)
Transistors are made in many other physical forms. Fig 5 shows some other types you
could have to recognise.
Fig 5 Different Transistor Styles
Make sure you can accurately identify the terminals of the transistors in the kit.
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Ensure that you have connected up the circuit as shown in the Patching Diagram and that
it corresponds with the circuit shown in fig 6.
Fig 6 Transistor Test Circuit
As stated, this uses the BC107-NPN transistor and the capacitor is provided to ensure that
the circuit is stable and has no effect on your measurements.

Turn the potentiometer to zero (anti-clockwise) and switch on both power supplies.

Slowly increase VEB by turning the potentiometer clockwise until IC just begins to
flow.

Connect the voltmeter temporarily between E and B on the transistor (3 V dc range)
and note the value of VEB in the table.
Note: if you are using virtual instrumentation to measure this voltage you will need to
measure between emitter and 0 V and then base and 0 V and subtract the readings to
obtain VEB.
Go to the Results Tables section of this Assignment and copy fig 7 to tabulate your
results.

Remove the voltmeter and then continue increasing VEB until IC approximately
equals 1 mA and record the values of IC and IB in the table.

Now increase VEB until IC approximately equals 10 mA; again record IC and IB.
Satisfy yourself that both IB and IC are flowing in the directions shown in fig 6

For each set of readings calculate hfe, IE and hfb as follows:
hfe 
12-200S
IC
I
I
 1000; IE  IC  B ; hfb  C
IB
1000
IE
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3.5.6.2 Questions
1. Did your reading of VEB confirm that the forward-biased E - B junction is acting like a
simple diode? Explain.
2. Do your results show that hfb and hfe increase, decrease or stay constant as Ic
increases?
3. Fig 8 shows an incomplete circuit of PNP transistor in common-emitter connection.
Fig 8
Complete the circuit with a suitable collector bias voltage and show the direction and
size of the collector current IC.
Also find hfb and hfe
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3.5.7 Practical Considerations and Applications
The measurements in this Assignment are made with the BC107 transistor used in a
circuit in which the E and C terminals are biased with voltages relative to the base B. For
this reason this circuit is called a common base connection.
It is also possible to bias the junctions with voltages relative to the emitter or collector,
giving common emitter and common collector connections as in fig 9.
Fig 9 Bias Arrangements for an NPN Transistor
In common-emitter, VCE must be larger than VBE to ensure that the C - B junction remains
reverse-biased.
In common-collector, VEC must be larger than VBC to ensure that the E - B junction
remains forward-biased.
These three connections have important differences in their responses to inputs.
The common-emitter and common-collector circuits are the most important connections
since the common-base is used only in special circumstances.
As with diodes, transistors can be made from Germanium instead of Silicon, but
Germanium devices are rarely used nowadays.
12-200S
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3.5.8 Results Required
When you have performed this Assignment you should have:

seen that transistors have two basic forms: the PNP and the NPN,

learnt that a transistor comprises two diode junctions, one forward and one reversebiased,

seen that the base current is much smaller than either the emitter or collector
current, which are themselves nearly equal,

learnt that there are three basic bias connections for a transistor.
Your report should contain:

the circuit that you investigated,

the calculations that you performed,

the results that you achieved,

conclusions on your findings in the Assignment.
To produce your report you should use a word processing package.
To achieve the calculated values you could use a spread sheet package.
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BASIC ELECTRICITY AND ELECTRONICS
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3.5.9 Results Table
IC
(mA)
VEB
(V)
just
measurable
IB
(mA)
-
0.99
-
10
-
hf e 
IC
IB
-
IE = IC + IB
(mA)
-
hf b 
IC
IE
-
Fig 7
12-200S
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Notes
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3.6
Chapter 3
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The Common-Emitter Transistor Assignment
3.6.1 Objectives

To become familiar with the common-emitter output (collector) characteristics.

To provide an understanding of the meaning and importance of Operating Point
and Load Line.
3.6.2 Prerequisite Assignments
Before working this assignment you should:

Transistor Familiarisation
3.6.3 Knowledge Level

Know the operation of a potential divider circuit.
3.6.4 Equipment Required
Qty
Apparatus
1
Basic Electricity and Electronics Module 12-200-B
1
Power Supply Unit, 0 to 20 V variable dc regulated and +5 V
dc regulated (eg, Feedback Teknikit Console 92-300).
3
Multimeters
OR
Feedback Virtual Instrumentation may be used in place of one
of the multimeters
12-200S
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3.6.5 Background
In the Transistor Familiarisation Assignment you learnt that the emitter and collector
currents of a transistor are nearly equal. Also, you learnt that the base current is the
difference between them and is therefore much smaller.
You learnt too that the transistor junctions can be biased in three different ways, called
'common-base', 'common-emitter' and 'common-collector'. This assignment looks more
closely at the common-emitter connection, using an NPN type of transistor.
Fig 1 is to remind you of some of the results from The Transistor Familiarisation
Assignment
Fig 1 NPN Transistor Common-emitter Connection
If IB is derived from an input signal and IC is used to generate an output, then the ratio I C/IB
represents the gain of the transistor in terms of the currents.
That is:
hfe =
OUTPUTCURR ENT
= CURRENT GAIN
INPUTCURRE NT
What is required is a graph that will tell us exactly how IC, IB and VCE are related to one
another.
The Collector Characteristic, which is found in this Assignment, is one such graph.
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3.6.6 Practical 1
In this Practical you will investigate how the collector current IC is controlled by the base
current IB and the collector-emitter voltage VCE.
The circuit that you will be using is shown in fig 2.
Fig 2
In this circuit the input is applied to the base and the output is from the collector. The
emitter is common to both input and output. This connection is therefore called the
common emitter connection.
You will set a value of collector-emitter voltage and then vary the base current of the
device and measure the resulting collector current. You will then change the collectoremitter voltage and see how the base current and collector current change.
You will then plot IC against VCE for each value of IB on your graph.
Connect up the circuit as shown in the Patching Diagram for this Practical.
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Practical 1 Patching Diagram
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3.6.6.1 Perform Practical
Ensure that you have connected up the circuit as shown in the Patching Diagram and that
it corresponds with the circuit shown in fig 3.
Fig 3 Test Circuit
Go to the Results Tables section of this Assignment and copy fig 4 to tabulate your results
and prepare a graph as shown in fig 5, for your results.
Fig 5 BC107 Collector Characteristics

Set VCE to 0.5 V, then use the potentiometer to adjust IB to each value given in the
table of fig 5.

At each setting record IC in the appropriate column. Then repeat for each other V CE
value.

Plot IC against VCE for each value of IB on your graph.
The graphs you now have are the 'output' or 'collector characteristics'.
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3.6.6.2 Questions
1. What happens to IC when VCE becomes less than 0.6 V?
What is the significance of this value?
2. What do you notice about the effect of V CE upon IC when VCE is greater than about
1.0 V?
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3.6.7 Practical 2
In this Practical you will begin to investigate what happens when a load resistor is
connected in the collector circuit.
The circuit that you will be using is shown in fig 6.
Fig 6
This circuit is the same common emitter connection, as used in Practical 1, but has a load
resistor R connected in series with the collector.
Now the supply voltage is designated VCC, to distinguish it from the collector-emitter
voltage VCE.
You will construct a load line on the output characteristic plotted in Practical 1
corresponding to the load resistor value.
This Practical is a paper-based exercise and requires no patching.
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3.6.7.1 Perform Practical
You should have concluded from the first Practical that IC is very little affected by VCE .
Instead however, it is almost entirely controlled by IB.
We say that the output circuit represents a constant current source.
If we wish to produce an output voltage, as we might in certain kinds of amplifier, this
current may be passed into a resistor to generate a voltage.
Fig 7 shows a circuit in which the collector bias is applied through a load resistor.
Fig 7 Addition of a Load Resistor
VCC is the term now used for the supply voltage to distinguish it from V CE, because when a
current IC is flowing these two will be different.
By Ohm's Law:
VCE = VCC - IC R.
Therefore,
when IC = 0,
VCE = VCC
and
when VCE = 0

IC =
VCC
R
On your graph plot these two points VCE and IC for:
IC = 0,
R = 2kΩ,
and
VCC = +10 V.

Plot two or three more points for other values of IC such as 2 and 5 mA using the
same values of R and VCC.

Join up the points by a line.
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Your line should be straight because the equation (V CE = VCC - IC R) is a linear one; there
being no square or cubic terms in it.
What you have now drawn is called a load line. It shows how VCE varies with IC and in
turn with IB.

Construct a second load line for:
VCC = 8 V and
R =1k ohms.
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3.6.8 Practical 3
In this Practical you will connect a load resistor in the collector circuit and set an operating
point.
You will construct the circuit and investigate its operation.
The circuit that you will be using is shown in fig 8.
Fig 8
This circuit is a practical implementation of the basic circuit of fig 7, but includes
components to enable the supply voltage and the base current to be adjusted and meters
to measure the resulting effects.
Connect up the circuit as shown in the Patching Diagram for this Practical.
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Practical 3 Patching Diagram
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3.6.8.1 Perform Practical
Ensure that you have connected up the circuit as shown in the Patching Diagram and that
it corresponds with the circuit shown in fig 8.
Go to the Results Tables section of this Assignment and copy fig 9 to tabulate your results
and prepare a graph as shown in fig 5, for your results.

Fill in the gaps in your table of Practical 2 by examination of your graph.

Make estimates if an exact value cannot be obtained.

Now alter your constructed circuit to include a load resistor of 1kΩ and set
VCC = 8 V. The circuit now looks like fig 10. Also see the Patching Diagram.
Fig 10 Test Circuit
Check your answers to items No. 1 and 4 in the table, by setting the given parameter and
measuring the other two.

Now change VCC and R to 10 V and 2kΩ respectively and check your answers to
items 2 and 3.
A circuit like fig 10 is widely used to amplify alternating signals. It is then necessary to set
the initial value of VCE to allow the output to vary both up and down. This is called setting
the operating point and is what you have just done.
Very often the operating point will be set to V CE which approximately equals
VCC
to allow
2
the maximum possible swing in either direction.
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Mark this point on each of the two loads lines on your graph (fig 6) and label the
points as operating points.
Estimate the extreme values of VCE for each load line if IB varies by (10 µA about the
operating point value. Draw a thick line along the section of load line included within these
limits. This represents a typical operating range.
Your graph should now look like fig 11.
Fig 11 Typical BC107 Characteristics
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3.6.8.2 Questions
1. Use the value of hfe found in the Transistor Familiarisation Assignment for the BC107
to estimate the peak-to-peak variation of VCE in the circuit of fig 12 when IB varies by
±2 µA.
Fig 12
Also what mean level of IB must you use to establish the operating point correctly?
(HINT: Variation of VCE = R x variation of IC)
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3.6.9 Results Required
When you have performed this Assignment you should have:

learnt that the collector or output characteristics for a common-emitter transistor
can be used to predict IC, given VCE and IB,

constructed a load line can on the characteristic to show the effect of a resistor in
the collector lead,

seen that the load line can be used to determine a suitable operating point, which
can be set by adjustment of the base current,

learnt that the variation in base current determines the operating range.
Your report should contain:

the circuit that you investigated,

the calculations that you performed,

the results that you achieved,

conclusions on your findings in the Assignment.
To produce your report you should use a word processing package.
To achieve the calculated values you could use a spread sheet package.
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3.6.10 Practical Considerations and Applications
Setting the operating point is an important matter in transistor amplifiers. The Practicals in
this Assignment have effectively done this by applying a voltage to the base-emitter
junction through a resistor, as in fig 13.
Fig 13 Simple Bias Circuit
Unfortunately this does not give a stable operating point in practice because temperature
changes cause a considerable change in IC for a given IB. Also individual transistors of
one type show some variation in characteristics.
Fig 14 shows a circuit which is much used to stabilise the operating point against such
variations.
Fig 14 A Stabilised Bias Circuit
R1 and R2 set a voltage at the base, VB, of say 4 V and IE is made to pass through resistor
RE. The emitter voltage, by Ohm's Law must be:
VE = IE x RE
and since VBE is about 0.6 V, then:
VB = VE + 0.6 = IE RE + 0.6
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Thus, if
VB = 4 V, VE = 3.4 V.
Now if IE increases, this causes VBE to reduce and in turn reduces IB and hence IE.
Thus the original change is counteracted. If IE reduces the result is similar. The operating
point has effectively been set by VB and RE, because:
  0.6 
IC  IE = VB
RE
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3.6.11 Results Table
IB
IC (mA) for Vce = ......... ..(V)
A
0.5
1
2
5
10
0
10
20
30
40
50
Fig 4
No.
VCC (V)
R ()
1
8
1000
2
10
2000
3
10
2000
4
8
1000
VCE (V)
IB (mA)
IC (mA)
25
4
5
5.5
Fig 9
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The Silicon Controlled Rectifier Assignment
3.7.1 Objectives

To recognise SCR's in different physical forms.

To understand the two-transistor analogy and the different ways of triggering an
SCR.

To appreciate the meaning of the terms 'Breakdown Voltage' and 'Holding Current'.

To appreciate the areas of application of SCR's.
3.7.2 Prerequisite Assignments

The Semiconductor Diode

Transistor Familiarisation
3.7.3 Knowledge Level
Before working this assignment you should:

Know the operation of series and parallel ac circuits.
3.7.4 Equipment Required
Qty
Apparatus
1
Basic Electricity and Electronics Module 12-200-B
1
Power Supply Unit,
0 to 20 V variable dc regulated and +15 V dc regulated.
ac supply; 12 V rms 50 or 60 Hz (isolated from other supplies.
(eg, Feedback Teknikit Console 92-300).
2
Multimeters
OR
Feedback Virtual Instrumentation may be used in place of one
of the multimeters
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3.7.5 Background
The Silicon Controlled Rectifier, or SCR, is one of several semiconductor devices which
are capable of acting as fast switches for large currents. The general name for these
devices is thyristor.
Fig 1 shows the SCR symbol and some of the physical forms in which it is found.
Fig 1 Types of SCR and Graphical Symbol.
The SCR resembles a rectifier diode but if the anode is held positive relative to the
cathode no current flows until a positive current is injected into the gate. The diode then
switches on and will not switch off until the anode-cathode voltage is removed. Hence the
name controlled rectifier.
Fig 2 shows the SCR as a three-junction device.
Fig 2 The Two-transistor Analogy of an SCR.
This can be regarded as two inter-connected two-junction transistors, one PNP and the
other NPN.
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If A is made positive relative to K, and G is left unconnected, no current will flow because
each transistor gets its base-emitter current from the other's collector emitter current. So,
until one of the transistors is given some base current, nothing can happen.
If now a current is injected into the base of transistor '2', the resulting collector current
flows in the base of '1'. This in turn causes a collector current in '1' which increases the
base current of '2' and so on. Very rapidly the two transistors force each other to conduct
to saturation; the current being limited only by resistance in the external circuit.
If the anode-cathode voltage is now reduced, the current also reduces until it goes below
some critical value, and the transistors switch off again, just as rapidly.
If a reverse voltage is applied to the SCR (anode negative to cathode) it behaves very
much like an ordinary diode. No current passes until at, some high voltage, it breaks down
completely.
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3.7.6 Practical 1
In this Practical you will measure the Trigger Current and the Saturation Voltage of a
forward biased SCR.
You will also measure the Holding Current for the device.
The circuit that you will be using is shown in fig 3.
Fig 3
You will set a value of supply voltage and then increase the gate current of the device until
the lamp lights.
You will also measure the anode-cathode voltage when the SCR is on (conducting).
Finally, you will find the holding current for the device.
Connect up the circuit as shown in the Patching Diagram for this Practical.
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3.7.6.1 Perform Practical
Ensure that you have connected up the circuit as shown in the Patching Diagram and that
it corresponds with the circuit shown in fig 4.
Fig 4 Test Circuit for Trigger Current.

Set the variable dc volts to 12 V. Turn the potentiometer to zero (clockwise) and
switch on the supplies.

Slowly rotate the potentiometer, observing the gate current meter continuously, until
the lamp suddenly lights. Record the gate current at which this occurs.

Switch off the supplies and return the potentiometer to zero.
Copy the results table as shown in fig 5, found in the Results Table section of this
assignment, for your results.

Repeat the measurement several times. to ensure that you have the correct value.
What you have found is the Trigger Current (IGT). Enter it into your table.

Now trigger the SCR on again and measure the voltage from anode to cathode.
This is the Saturation Voltage VAK (sat). Enter this in the table.

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Finally connect the multimeter (on range 100 mA) in series with the lamp as in fig 7.
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Fig 6 Test Circuit for Holding Current.

Trigger the SCR on. The meter reads the lamp current for a 12 V supply voltage.

Temporarily disconnect the gate connection.

Slowly reduce the supply voltage until the SCR current suddenly falls to zero.
Note the value of the current at which this occurs.

Repeat this procedure several times to ensure that you have the correct value.
What you have found is the Holding Current (IH). Enter this also into your table.
Notes:
To switch an SCR 'ON' the Gate Current must by at least IGT.
To switch an SCR 'OFF' the Anode Current must be no more than IH.
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3.7.6.2 Questions
1. Look at fig 7
.
Fig 7
Do you expect the saturation voltage to be greater or less than 0.6 V? Explain.
2. What do you think will happen in the circuit of fig 4 if you trigger the SCR on, and then
reduce the gate current to zero again?
Confirm your answer by experiment.
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3.7.7 Practical 2
In this Practical you will replace the variable dc anode supply with a 12 V ac supply and
investigate the performance of the circuit under ac conditions.
The circuit that you will be using is shown in fig 8.
Fig 8
Connect up the circuit as shown in the Patching Diagram for this Practical. Remember to
use ac meters.
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Practical 2 Patching Diagram
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3.7.7.1 Perform Practical
In Practical 1, you found that the anode current must be reduced to below IH to switch the
SCR off. This is the only way of switching off. You CANNOT do it by reducing the gate
current.
If the anode supply is an alternating voltage it will go negative every half-cycle, reducing
the anode current to zero.
Ensure that you have connected up the circuit as shown in the Patching Diagram and that
it corresponds with the circuit shown in fig 9.
Fig 9 Test Circuit with an ac Supply

Repeatedly increase and decrease the gate current and observe what happens.

Observe the brightness of the lamp and compare this with that for the 20 V dc
supply.
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3.7.7.2 Questions
1. What do you observe when you repeatedly increase and decrease the gate current?
Explain what you think is happening.
2. Why does the lamp burn less brightly with the ac supply than it did with the 12 V dc
supply?
3. The SCR in fig 10 has a maximum steady anode current capability of 10 A. Given that
VAK (sat) is 1.5 V at this current, find:
Fig 10
(a) The power dissipation in the SCR at maximum anode current.
(b) The value of R.
(c) The power dissipation in R.
(d) The efficiency of the SCR as a switch for a 10 A anode current.
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3.7.8 Results Required
When you have performed this Assignment you should have:

seen that SCR's can be regarded as two interconnected transistors,

learnt that SCR's can be triggered on by a gate current but triggered off only by
reducing the anode current,

investigated the operation of an SCR for both dc and ac anode supply voltages,

learnt that SCR's can be triggered unintentionally by high temperature, over voltage
or a rapidly rising anode voltage.
Your report should contain:

the circuits that you investigated,

the results that you achieved,

conclusions on your findings in the Assignment.
To produce your report you should use a word processing package.
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3.7.9 Practical Considerations and Applications
SCR's can be triggered unintentionally in several ways and you have to be aware of these
as they could be the cause of wrong operation. Fig 11 illustrates this.
Fig 11 False Triggering Mechanisms
In (a) a high temperature increases the leakage current of the two 'transistors' in the SCR.
This is the small collector-emitter current that flows when there is zero base current. If it
becomes too great it will be enough to initiate the trigger action.
If a very high forward voltage is applied, as in (b), the 'transistors' can break down and this
too will initiate triggering.
Fig 11(c) shows a very rapidly rising anode-cathode voltage. Every transistor has some
capacitance from collector to emitter as shown in fig 12. A fast-rising anode-cathode
supply causes small currents in these capacitors and can act to cause triggering.
Fig 12 Stray Capacitance Positions.
Steps must always be taken in practice to avoid each of these possible false trigger
mechanisms.
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SCR's are available to carry currents from less than 1 A up to 1000 A or more.
They therefore find use in the switching of heavy electrical equipment, where they replace
contactors. The following advantages should be obvious:

No moving parts

No contact arcing

No bad contacts due to corrosion or dirt
In addition to simply switching currents on and off, SCR's can be made to control the
mean value of a load current without dissipating large amounts of power. In this
application they can replace bulky high wattage rheostats and save electrical energy at the
same time. A good example of this is the control of theatre lighting.
The introduction to the Trigger Devices Assignment explains how this can be done.
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3.7.10 Results Table
TRIGGER CURRENT
(IGT)
SATURATION VOLTAGE
(VAK (sat))
HOLDING CURRENT
(IH)
mA
V
mA
Fig 5
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The TRIAC Assignment
3.8.1 Objectives

To recognise a TRIAC device.

To understand the bidirectional nature of the TRIAC and its areas of application.

To appreciate the different behaviour of the device in the four operating quadrants.
3.8.2 Prerequisite Assignments

The Silicon-Controlled Rectifier (SCR)
3.8.3 Knowledge Level
Before working this assignment you should:

Know how to use Cartesian axes involving four quadrants.
3.8.4 Equipment Required
Qty
Apparatus
1
Basic Electricity and Electronics Module 12-200-B
1
Power Supply Unit, +5 V dc regulated and +15 V dc regulated,
ac supply; 12 V rms 50 or 60 Hz isolated from other supplies
(eg, Feedback Teknikit Console 92-300).
1
Multimeter
OR
Feedback Virtual Instrumentation may be used in place of the
multimeter
12-200S
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3.8.5 Background
In the SCR Assignment you learnt that the SCR can be used to switch a unidirectional
current but it will not conduct in reverse.
An alternating supply is often necessary to ensure that the SCR will switch off. It is rather
inefficient, because it conducts only every other half-cycle (like a half-wave rectifier as
described in the Half-wave Rectification Assignment).
Fig 1 shows the basic single-SCR circuit and also one way of using four SCR's in a bridge
to achieve controlled full-wave rectification.
Fig 1 Half-wave and Bridge SCR Circuits
Note that, in a bridge circuit, gates G1/G2 are triggered together on one half-wave and
G3/G4 on the next.
A simpler and less expensive way of obtaining bidirectional conduction is to use a TRIAC.
Fig 2 shows a typical device and its graphical symbol.
Fig 2 A TRIAC and its Graphical Symbol
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A TRIAC, like an SCR, is a type of thyristor. Although it has the same basic four layers of
semi-conductor materials, its detailed construction is too involved to be described here.
The behaviour of the TRIAC is very similar to that of the SCR but it can be triggered into
conduction by gate current for either polarity of the voltage between terminals T 1 and T2.
Fig 3 shows the four modes in which a TRIAC can be operated.
Fig 3 TRIAC Triggering Modes
With reference to Quadrant I, the TRIAC is usually triggered by a positive gate current
(mode I +), but can be triggered by negative gate current, (mode I -).
Similarly in Quadrant III negative IG is usual (mode III-), but positive IG is possible (mode
III+).
Modes I- and III+ are, however, less sensitive than the usual modes, I+ and III-.
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3.8.6 Practical 1
In this Practical you will confirm that a TRIAC may be triggered in any of the four modes
referred to in the Background section.
The circuit that you will be using is shown in fig 4.
Fig 4
Initially, you will set the gate and supply voltages to +15 V and then increase the gate
current of the device until the lamp lights and you will note the gate current required to
achieve this.
You will then, in turn, set up the other three combinations of positive and negative gate
and supply voltages to provide the four quadrants of operation of the device, again noting
the gate currents required to obtain conduction.
Finally, you will find the holding current for the device.
Connect up the circuit as shown in the Patching Diagram for this Practical.
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3.8.6.1 Perform Practical
Ensure that you have connected up the circuit as shown in the Patching Diagram and that
it corresponds with the circuit shown in fig 5.
Fig 5 Test Circuit for Trigger Current
Copy the results table as shown in fig 6, found in the Results Table section of this
assignment, for your results.

Set the potentiometer to zero (anti-clockwise) and switch on the power supplies.

Slowly increase the gate current until the lamp lights, noting the value of I GT when
this occurs. Record this in your table under IGT for mode I+.
Switch off and move link 1 to apply -15 V to the gate circuit instead of +15 V.

Repeat the measurement and record the value of IGT for mode I-, reversing the
meter connections if necessary.
Now move link 2 to apply -15 V to the lamp.

Repeat the measurement of IGT for mode III-.
Finally restore the gate supply to +15 V and measure I GT for mode III+.
You should find that Modes I+ and III- have similar values of IGT but that modes I- and III+
require comparably greater gate currents to cause triggering.
TRIACs, like SCRs, require a minimum current, called the HOLDING CURRENT, to keep
them in conduction. You can confirm this if you wish for mode I+ by the same method as
used in the SCR Assignment.
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3.8.7 Practical 2
In this Practical you will set up a simple TRIAC switch circuit and investigate its operation.
The circuit that you will be using is shown in fig 7.
Fig 7
Connect up the circuit as shown in the Patching Diagram for this Practical.
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3.8.7.1 Perform Practical
Ensure that you have connected up the circuit as shown in the Patching Diagram and that
it corresponds with the circuit shown in fig 8.
Fig.8 Simple TRIAC Switch Circuit.
Verify that:
If link A is not connected there is no gate current and the lamp does not light.
If A is connected to point B, gate current flows at every half-cycle and the lamp lights.
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3.8.7.2 Questions
1. In fig 8 what do you expect to happen if A is connected to point C? Confirm by
experiment and explain your answer.
2. Which mode or modes are in use for:
'A' connected to 'B',
'A' connected to 'C'?
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3.8.8 Results Required
When you have performed this Assignment you should have:

learnt that a TRIAC is a four-layer thyristor device similar to the SCR,

seen that a TRIAC can be triggered into conduction in either direction,

Learnt that there are four triggering modes, of which two are preferred.
Your report should contain:

the circuits that you investigated,

the results that you achieved,

conclusions on your findings in the Assignment.
To produce your report you should use a word processing package.
12-200S
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3.8.9 Practical Considerations and Applications
TRIACs have the same breakdown voltage, temperature and rate of voltage rise
limitations as SCR's. They are available in a wide range of voltage and current ratings.
Apart from the bidirectional current capability and the need to consider the triggering
modes, TRIACs are in most other respects similar to SCRs in their range of applications.
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3.8.10 Results Table
MODE
I+
I–
III–
III+
IGT
(mA)
Fig 6
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3.8.11 Further Work
Fig 9 shows a simple TRIAC switch circuit.
12V
lamp
100R
T
2
15V
rms
R
+5V
G
T
1
0V
Fig 9 Simple TRIAC Switch Circuit.
Use your measured results for IGT and assume that the voltage from G to T 1 will be about
1 V.
Suggest a value for R which, when the switch is closed, will give:
Half-wave operation of the lamp.
Full-wave operation of the lamp.
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Trigger Devices – The DIAC and UJT Assignment
3.9
3.9.1 Objectives

To be able to recognise DIAC and UJT devices and their symbols.

To understand the need for trigger devices when used with thyristors.

To appreciate the main features of the DIAC and UJT.
3.9.2 Prerequisite Assignments

The Silicon-controlled Rectifier (SCR)

The TRIAC
3.9.3 Knowledge Level
Before working this assignment you should:

Know how to use an oscilloscope.
3.9.4 Equipment Required
Qty
Apparatus
1
Basic Electricity and Electronics Module 12-200-B
1
Power Supply Unit, 0 to 20 V variable dc regulated
+15 V dc regulated
(eg, Feedback Teknikit Console 92-300).
1
2-channel oscilloscope
1
Multimeter
OR
Feedback Virtual Instrumentation may be used in place of the
oscilloscope and multimeter
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3.9.5 Background
In the SCR and TRIAC Assignments two semiconductor switches (or thyristors) were
studied; the SCR and the TRIAC. In ON-OFF switching applications they could be
triggered by simple circuits producing steady gate currents.
Fig 1 is an example of such a circuit using an SCR.
Switch open
- no gate current
- no conduction
Switch closed
- Gate current flows
- SCR Conducts on positive half-cycles
Fig 1 An SCR Circuit.
REMEMBER: A thyristor will switch off only when its anode-cathode voltage falls to zero.
If it is required to control the mean value of a load current, rather than just switch it on and
off, there is only one method available. This is illustrated in fig 2 for an SCR.
Fig 2 SCR Conduction for Different Delay Times.
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A steady gate current would allow conduction over the full period of the positive half-cycle.
If instead, a short pulse of gate current is applied at the trigger points, conduction occurs
over part of the half-cycle only. This reduces the mean current.
The mean current can be varied by changing the delay time T between the start of the
cycle and the trigger. This is known as phase control.
Fig 3 explains why.
Fig 3 Phase Control of an SCR.
The supply wave A is delayed by the phase shift to give B. When B reaches a certain
trigger level the trigger circuit generates a gate pulse C for the SCR.
To achieve phase control, then, two things are needed:
a. A variable phase shift circuit (usually passive components such as resistors
and capacitors).
b. A trigger circuit that can produce a pulse when the delayed waveform reaches
a certain level.
In this assignment you will look at two devices which serve as trigger generators, the DIAC
(DIode AC switch) and the UJT (Uni-Junction Transistor).
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3.9.6 Practical 1
In this Practical you will investigate the operation of a DIAC.
The circuit that you will be using is shown in fig 4.
Fig 4
Initially, you will set the dc voltage to zero and then you will increase this voltage until the
device conducts.
You will use the oscilloscope to observe and measure the voltages around the circuit and
you will construct an approximate characteristic for the DIAC.
Connect up the circuit as shown in the Patching Diagram for this Practical.
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Practical 1 Patching Diagram
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3.9.6.1 Perform Practical
Figure 5 shows the symbol and a typical voltage-current characteristic. An alternative
DIAC symbol is shown in fig 6.
Fig 5 The DIAC, its Graphical Symbol and Characteristic
Fig 6 An alternative DIAC symbol
Identify the DIAC on your 12-200-B workboard.
The DIAC is made like a transistor but has no base connection. When a voltage greater
than VBR is applied, breakdown occurs. In an ordinary diode the voltage would then remain
constant as the current increased. In the DIAC the transistor action causes the voltage to
reduce as the current increases. This gives the characteristic a negative resistance, as
shown in fig 5.
The DIAC is symmetrical and therefore has the same characteristic for negative voltages.
It is the negative resistance that makes the DIAC suitable as a trigger for an SCR or
TRIAC.
To test this, ensure that you have connected up the circuit as shown in the Patching
Diagram and that it corresponds with the circuit shown in fig 7.
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Fig 7 The DIAC Test Circuit

Set the variable dc supply to zero and switch on the supplies.

Slowly increase the variable dc voltage until the waveform at Y2 suddenly appears.
That is, the DIAC 'switches on'.
Notice the very rapid rise of VR, produced by the negative resistance. See fig 8.
Fig 8 DIAC Waveforms

Measure VBR and VR from the oscilloscope.
VBR is the DIAC breakover voltage.
VR is the load voltage.
VR
is the DIAC current immediately after switch-on.
1000
VBR - VR is the voltage across the DIAC immediately after switch-on.
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From these figures it is possible to construct an approximate characteristic for the
DIAC as in fig 9.
Fig 9 The DIAC Characteristic and Load Line
Fig 9 represents the conditions just before switch-on, and Q the conditions just after. The
values shown are not necessarily the correct ones.
Prepare a graph like fig 10. on which to plot your results and use your own measurements
to draw a graph like fig 9.
Fig 10 The DIAC Characteristic
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3.9.7 Practical 2
In this Practical you will investigate the operation of a UJT.
The circuit that you will be using is shown in fig 11.
Fig 11
Again, you will set the variable dc voltage to zero and then you will increase this voltage
until the device switches on.
You will construct a characteristic for the UJT.
Connect up the circuit as shown in the Patching Diagram for this Practical.
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Practical 2 Patching Diagram
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3.9.7.1 Perform Practical
Fig 12 shows the appearance of a Unijunction Transistor (UJT) (like an ordinary
transistor), its graphical symbol, and gives an indication of its construction.
Fig 12 Unijunction Transistor Details
Identify the Unijunction Transistor (UJT) on your 12-200-B workboard.
Base B2 is biased positive relative to B1. This sets up a reverse bias at the diode PN
junction. This bias is overcome when a sufficient positive voltage is applied to E. Then
emitter current flows and the effect of the current already flowing from B 2 to B1 is to give
the E B1 diode a negative resistance, similar to that in the DIAC.
Ensure that you have connected up the circuit as shown in the Patching Diagram and that
it corresponds with the circuit shown in fig 13.
Fig 13 A UJT Test Circuit and Characteristic

Set the variable dc voltage to zero and switch on the power supply. Slowly increase
the variable dc until the emitter current suddenly increases. That is the UJT
'switches on'.
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Copy the results table as shown in fig 14, found in the Results Table section of this
assignment, for your results.
Record the value of VEB1 just before switch-on (point P), also the values of VEB1 and IE just
after switch-on (point Q).

Slowly reduce the variable dc voltage until the emitter current suddenly switches off
again.
Record, in your table the values of IE and VEB1 just before switch-off (point R) and the
value of VEB1 just after switch-off (point S).
You can now construct the characteristic for your UJT similar to the example shown in
fig 15.
Fig 15 Graph Layout for UJT Characteristic.

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Layout a graph like fig 16 on which to plot your results.
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Fig 16 The UJT Characteristic

Enter points P, Q, R and S and the load lines on the axes of your graph of using
your measured values from your table. Sketch in the UJT characteristic.
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3.9.7.2 Questions
1. What do you think will happen if VEB1 is made negative?
2. Study your graphs for the DIAC and UJT and determine what are the main differences
between the characteristics of the two trigger devices?
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3.9.8 Results Required
When you have performed this Assignment you should have:

learnt that SCRs and TRIACs used to control the mean value of a load current
need phase-delayed trigger pulses,

learnt that the DIAC and UJT are suitable devices to produce these pulses for the
TRIAC and SCR respectively,

learnt that the DIAC and UJT both possess negative-resistance characteristics
which allow them to switch on rapidly once a certain applied voltage level is
reached.
Your report should contain:

the circuits that you investigated,

the results that you achieved,

conclusions on your findings in the Assignment.
To produce your report you should use a word processing package.
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3.9.9 Practical Considerations and Applications
The DIAC, because of its bidirectional operation, is primarily used to trigger TRIACS; the
UJT, being unidirectional, is suitable only for use with SCR's.
Both devices are normally used in a circuit such as fig 17.
Fig 17 A Relaxation Oscillator used as a Trigger Generator
This circuit forms a Relaxation Oscillator and operates as follows:
R and C form a variable delaying circuit to obtain the necessary phase control as
described in the introduction. When C is sufficiently charged (ie, up to VBR for the DIAC or
VP for the UJT), the device triggers and C discharges into RL forming a short pulse. When
C is discharged sufficiently (e.g. down to point R on the UJT characteristic) the device
turns off and C starts to recharge.
All the time the supply voltage is large enough, the circuit goes on oscillating in this way.
Fig 18 shows a typical output for a UJT driven by a full-wave rectified input.
Fig 18 Waveforms of a Relaxation Oscillator.
A chain of trigger pulses is often better than just one pulse as it gives a greater certainty of
triggering the SCR or TRIAC.
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3.9.10 Results Table
CONDITION
IE (mA)
Just before switch-on
0
VEB1 (V)
POINT
P
Just after switch-on
Q
Just before switch-off
R
Just after switch-off
0
S
Fig 14 UJT Measurements
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3.9.11 Further Work
Fig 19 shows a practical relaxation oscillator circuit using a UJT.
Fig 19 UJT Relaxation Oscillator Circuit
Construct the circuit and observe the output on an oscilloscope. Sketch what you see.
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3.10 The Field Effect Transistor Assignment
3.10.1 Objectives

To understand the difference between bipolar and field-effect transistors

To distinguish between JFET and MOSFET types and between N and P channel
construction.

To be able to recognise the basic characteristics of a JFET.

To know the principal advantages of FETs and some applications.
3.10.2 Prerequisite Assignments

Transistor Familiarisation

The Common-Emitter Transistor
3.10.3 Knowledge Level
The Common-Emitter Transistor

Know how to use an oscilloscope.
3.10.4 Equipment Required
Qty
Apparatus
1
Basic Electricity and Electronics Module 12-200-B
1
Power Supply Unit, 0 to 20 V variable dc regulated
+15V dc regulated
(eg, Feedback Teknikit Console 92-300).
1
2-channel oscilloscope
1
Function Generator, 2 V pk-pk at 1 kHz
3
Multimeters
OR
Feedback Virtual Instrumentation may be used in place of the
oscilloscope and two multimeters
12-200S
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3.10.5 Background
Field-Effect Transistors (FETs) are made in various forms. One type, the Junction FET
(JFET) has a construction quite similar to the UJT (Trigger devices Assignment) but works
in a different way.
Fig 1 shows the construction, graphical symbol and physical appearance of a typical
JFET.
Fig 1 JFETs - Construction, Symbols, and Appearance.
As can be seen, the JFET has two forms; the N-channel and P-channel which are
analogous to PNP and NPN in ordinary transistors. You will look more closely at the Nchannel type.
Fig 2 shows an N-channel JFET and its bias voltages.
Fig 2 The Bias Arrangement for an N-channel JFET.
The channel is a resistive path through which voltage VDS can drive a current ID.
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A voltage gradient is thus formed down the length of the channel, the voltage becoming
less positive as you go from drain to source. The PN junction thus has a high reverse
bias at D and a lower reverse bias at S. This bias causes a 'DEPLETION LAYER', whose
width increases with the bias.
Depletion means a reduction of available electrons to carry current. If V GS is made more
negative, the depletion layer increases in width at all points. The values of V DS and VGS
both influence the width of the depletion layer. This alters the effective channel resistance
and hence ID. Fig 3 shows this.
Fig 3 The Depletion Effect.
As VGS increases negatively the channel is 'squeezed', reducing the current ID. But the
GATE-CHANNEL junction is like a reverse-biased junction diode and thus carries only a
very small current. ID is controlled by VGS through a 'field effect'.
Hence the name FET.
This Assignment investigates how VDS and VGS affect ID.
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3.10.6 Practical 1
In this Practical you will investigate the bias requirements of an N-channel Junction FET.
The circuit that you will be using is shown in fig 4.
Fig 4
You will measure the drain current and the gate-source voltage for a number of set drainsource voltages and plot the output (drain) characteristics for the FET.
Connect up the circuit as shown in the Patching Diagram for this Practical.
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3.10.6.1 Perform Practical
Ensure that you have connected up the circuit as shown in the Patching Diagram and that
it corresponds with the circuit shown in fig 5.
Fig 5 Test Circuit for a Typical N-channel JFET

Set the potentiometer anti-clockwise and the variable dc voltage to zero. Switch on
the power supply.
Go to the Results Tables section of this Assignment and copy fig 6 to tabulate your
results.

Now set VDS to the first value in the table and then read ID for each value of VGS.

Repeat for all the values of VDS in the table, recording the corresponding ID values.
Prepare a graph like fig 7.
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Fig 7 JFET Drain Characteristics.
Plot the results from your table onto your graph, drawing one curve of ID against VDS for
each value of VGS.
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3.10.6.2 Questions
1. Study your graphs and answer the following questions:
a) Above which values of VDS is ID almost unaffected by VDS when VGS = 0?
b) For a given value of VDS, (say 10 V), do equal changes of VGS cause equal changes
of ID?
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3.10.7 Practical 2
In this Practical you will endeavour to measure the gate current of the JFET.
The circuit that you will be using is shown in fig 8.
Fig 8
Connect up the circuit as shown in the Patching Diagram for this Practical.
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3.10.7.1 Perform Practical
Ensure that you have connected up the circuit as shown in the Patching Diagram.
On your circuit set VDS to 10 V and VGS to -1.0 V. Then alter the circuit to place the
ammeter in place of the link in the gate lead as in fig 9. and try to measure I G.
Fig 9 Measuring Gate Current.
Record if you can measure IG and, if so, its value.
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3.10.7.2 Questions
1. Can you measure IG or is it too small?
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3.10.8 Practical 3
In this Practical you will connect up the JFET in common-source connection as an
amplifier.
The circuit that you will be using is shown in fig 10.
Fig 10
You will apply a sinusoidal input voltage to the amplifier at 1 kHz and you will observe the
output voltage on an oscilloscope.
You will calculate the voltage gain of the circuit.
You will also estimate the input resistance of the amplifier.
Connect up the circuit as shown in the Patching Diagram for this Practical.
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3.10.8.1 Perform Practical
An FET can be used to amplify signals in a manner similar to a transistor in commonemitter connection. In this case it is called common-source.
To obtain an output voltage you insert a load resistance in the drain lead, the effects of
this being represented on the characteristic by a load line.
Fig 11 shows a Practical amplifier circuit with a typical characteristic and load line.
Ensure that you have connected up the circuit as shown in the Patching Diagram and that
it corresponds with the circuit shown in fig 11.
Fig 11 Test Circuit and Characteristic
On your graph draw a load-line from VDD = 15 V at an angle suitable for the 1kohm load
and select an operating point at about VDS = +10 V.

Switch on the supplies and adjust VGS to give this operating point.

Now apply an input of 2 V peak-to-peak at 1000 Hz from the generator and observe
the output on the oscilloscope.

Measure the peak-to-peak output voltage and calculate the voltage gain
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.
Vi
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Now change the resistor RG, as indicated in the Patching Diagram. Find the one that
makes the output signal about half its original size. This value is equal to the input
resistance of the amplifier as fig 12 shows.
Fig 12 Input Resistance Measurement
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3.10.8.2 Questions
1. Is the output from your amplifier a good sine-wave?
2. The input resistance of fig 11 can not be greater than the bias resistor RG. Is it ,
however, much less than this? If not, what does this indicate?
3. An important parameter of an FET used as an amplifier is its transconductance. This is
defined by:
Transconductance (gS ) =
change in Id 
change in VGS 
mA/V (common source)
Study your graph, fig 11 and estimate the change in Id for a 0.5 V change in VGS when
VDS = 10 V and VGS = 1.0 V.
Then find gs.
The voltage gain (A) for a load resistor R is given by:
g R
A = S , where R is in ohms (
10 3
)
Use this expression to verify the voltage gain
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measured in the Assignment.
Vi
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3.10.9 Results Required
When you have performed this Assignment you should:

Understand the difference between bipolar and field-effect transistors,

Know how to bias a JFET for correct operation,

Have plotted and be able to recognise the basic characteristics of a JFET,

Know the principal advantages of FETs.
Your report should contain:

the circuits that you investigated,

the results that you achieved,

conclusions on your findings in the Assignment.
To produce your report you should use a word processing package.
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3.10.10 Practical Considerations and Applications
So far, you have looked at an N-channel JFET. A P-channel JFET is very similar in
operation but uses reversed-bias voltages as in fig 13.
Fig 13 A P-channel JFET and Characteristic
Another form of FET exists whose gate is insulated from the channel (insulated gate FET).
The most common insulation method is a metal-oxide layer and the type is called
MOSFET. The gate on a JFET must not be biased in such a way as to forward-bias the
PN junction however, in a MOSFET, no such limitation applies.
It is therefore possible to bias the gate in either polarity.
Fig 14 shows the usual characteristics of two types of N-Channel MOSFET.
Fig 14 N-channel MOSFET Characteristics and Graphical Symbols
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The depletion-mode type is like a JFET with an extended gate bias range but the
enhancement-mode is quite different since at VGS = 0, no current flows at all.
This is often a useful feature.
Two matching types of MOSFET exist with P-channel and have reverse polarities of bias.
Their symbols are shown in fig 15.
Fig 15 P-channel MOSFET Symbols
You can put all these types together in a family tree as in fig 16.
Fig 16 An FET Family Tree.
MOSFETs have even higher input impedances than JFETs
All FETs are useful for amplification with minimum load on the source.
Enhancement-type MOSFETs are specially valuable as electronic switches because with
no bias they are normally non-conducting and the high gate resistance means that very
little control current is needed. MOSFETs, due to their very high gate resistance, can
easily accumulate large static charges and can become damaged unless carefully
handled. Some types are fitted internally with protective Zener diodes to prevent damage
during handling.
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3.10.11
Results Table
ID (mA) for VDS =
VGS(V)
0
0.5
1
2
(V)
5
10
15
0
–0.5
–1.0
–1.5
–2.0
–2.5
Fig 6 JFET measurements
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Notes
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Appendix A
BASIC ELECTRICITY AND ELECTRONICS
STUDENT’S MANUAL
Appendix A
SI Units
SI Units
The units used throughout this manual are those of the Systeme International d'Unites
(SI).
There are seven basic units in the International System (SI), these are:
Table 1 Basic SI Units
Quantity
Name of unit
Symbol
length
metre
m
mass
kilogram
kg
time
second
s
electric current
ampere
A
kelvin
K
candela
cd
mole
mol
thermodynamic temperature
luminous intensity
amount of substance
From these seven basic units all other units may be derived; for example:
The derived unit for force is the newton (N), which is that force which, when applied to a
body of one kilogram, gives it an acceleration of one metre per second squared.
Force (newton, N) = mass (kilogrammes, kg) x accn (metres per second squared, m/s 2).
Tables of some of these derived SI units are to be found in this appendix and the
definitions of those SI units with special names are to be found in Appendix B.
Appendix C gives the relationships between some UK units and SI units.
Some of the units are inconveniently large or small in practical circumstances. Hence
standard multiples or sub-multiples of the basic unit are commonly used. These are shown
in Table 2 reproduced overleaf.
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Appendix A
BASIC ELECTRICITY AND ELECTRONICS
STUDENT’S MANUAL
SI Units
The names of the multiples and sub-multiples of the units are formed by means of the
following prefixes:
Table 2 Multiples and Submultiples
Factor by which the unit is multiplied
Prefix
Symbol
1,000,000,000,000 = 1012
tera
T
1,000,000,000 = 109
giga
6
mega
M
3
kilo
k
2
hecto
h
1
deca
da
–1
deci
d
–2
centi
c
–3
milli
m
–6
micro

–9
nano
n
–12
pico
p
–15
femto
f
–18
atto
a
1,000,000 =10
1000 = 10
100 = 10
10 = 10
0.1 = 10
0.01 = 10
0.001 = 10
0.000 0001 = 10
0.000 000 000 = 10
0.000 000 000 000 = 10
0.000 000.000 000 001 = 10
0.000 000 000 000 000 001 = 10
In electronics, the ampere is frequently too large a unit for normal usage. Current is thus
measured often in milliamperes. There are one thousand milliamperes in one ampere.
ie, 1000 mA = 1A
1 mA = 0.001 A = 10–3 A
The unit of the ohm is often too small a unit of resistance for resistors used in electronic
circuits. Resistance may then be in kilohms or megohms.
ie, 1000 = 1k
A-2
1,000,000 = 1M = 106
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Appendix A
BASIC ELECTRICITY AND ELECTRONICS
STUDENT’S MANUAL
SI Units
Table 3 Some derived SI Units having special names
Physical quantity
SI unit
Unit symbol
force
newton
N = kg m/S2
work, energy, quantity of heat
joule
J=Nm
power
watt
W = J/s
electric charge
coulomb
C = As
electrical potential
volt
V = W/A
electric capacitance
farad
F = As/V
electric resistance
ohm
 = V/A
frequency
hertz
Hz = s-1
magnetic flux
weber
Wb = Vs
magnetic flux density
tesla
T = Wb/m2
inductance
henry
H = V s/A
Table 4 Some derived SI units with complex names
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Physical quantity
SI unit
Unit symbol
area
square metre
m2
volume
cubic metre
m3
density (mass density)
kilogram per cubic metre
kg/m3
velocity
metre per second
m/s
angular velocity
radian per second
rad/s
acceleration
metre per second
squared
m/s2
angular acceleration
radian per second
squared
rad/s2
pressure
newton per square metre
N/m2
surface tension
newton per metre
N/m
thermal conductivity
watt per metre kelvin
W/(mK)
electric field strength
volt per metre
V/m
magnetic field strength
ampere per metre
A/m
A-3
Appendix A
BASIC ELECTRICITY AND ELECTRONICS
STUDENT’S MANUAL
SI Units
Notes
A-4
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BASIC ELECTRICITY AND ELECTRONICS
STUDENT’S MANUAL
Appendix B
Appendix B
Definition of Derived
Units having Special Names
Definition of Derived Units having Special Names
FORCE
The unit of force called the newton is that force which, when
applied to a body having a mass of one kilogram, gives it an
acceleration of one metre per second squared.
ENERGY
The unit of energy called the joule is the work done when the
point of application of a force of one newton is displaced
through a distance of one metre in the direction of the force.
POWER
The unit of power called the watt is equal to one joule per
second.
ELECTRIC
CHARGE
ELECTRIC
POTENTIAL
ELECTRIC
CAPACITANCE
ELECTRIC
RESISTANCE
FREQUENCY
12-200S
The unit of electric charge called the coulomb is the quantity of
electricity transported in one second by a current of one
ampere.
The unit of electric potential called the volt is the difference of
potential between two points of a conducting wire carrying a
constant current of one ampere, when the power dissipated
between these points is equal to one watt.
The unit of electric capacitance called the farad is the
capacitance of a capacitor between the plates of which there
appears a difference of potential of one volt when it is charged
by a quantity of electricity equal to one coulomb.
The unit of electric resistance called the ohm is such that a
constant difference of potential of one volt, applied across a
conductor of one ohm resistance, produces in this conductor a
current of one ampere, this conductor not being the source of
any electro-motive force.
The unit of frequency called the hertz is the frequency of a
periodic phenomenon of which the periodic time is one second.
B-1
BASIC ELECTRICITY AND ELECTRONICS
STUDENT’S MANUAL
MAGNETIC FLUX
MAGNETIC
FLUX DENSITY
ELECTRIC
INDUCTANCE
TEMPERATURE
Appendix B
Definition of Derived
Units having Special Names
The unit of magnetic flux called the weber is the flux which,
linking a circuit of one turn produces in it an electromotive force
of one volt as it is reduced to zero at a uniform rate in one
second.
The unit of magnetic flux density called the tesla is the density
of one weber of magnetic flux per square metre.
The unit of electric inductance called the henry is the
inductance of a closed circuit in which an electromotive force of
one volt is produced when the electric current in the circuit
varies uniformly at the rate of one ampere per second.
The units of Kelvin and Celsius temperature interval are
identical. A temperature expressed in degrees Celsius is equal
to the temperature expressed in kelvin less 273.15 k
kThis is true for the thermodynamic scale and for the
international practical scale of 1948. There are, however, slight
differences between thermodynamic scale and practical scale.
B-2
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Appendix C
Values of some UK Units
in terms of SI Units
BASIC ELECTRICITY AND ELECTRONICS
STUDENT’S MANUAL
Appendix C
Values of some UK Units in terms of SI Units
1 yd
0.9144 m
1ft
0.3048 m
1 in
0.0254 m
1 mile
1609.344 m
1 in2
6.4516 x 10-4 m2
1 ft2
0.092 903 0 m2
1 yd2
0.836 127 m2
1 mile2
22.589 99 x 106 m2
1 in3
1.638 71 x 10-5 m3
1 ft3
0.028 316 8 m3
1 UK gal
0.4 546 092 m3
1 ft/s
0.3048 m/s
1 mile/h
0.447 04 m/s
Mass
1 lb
0.453 592 37 kg
Density
1 lb/in3
2.767 99 x 104 kg/m3
1 lb/ft3
16.0185 kg/m3
1 lb/UK gal
99.7764 kg/m3
Force
1 lbf
4.448 22 N
Pressure
1 lbf/in2
6894.76 N/m2
Energy
1 ft lbf
1.355 82 J
1 cal
4.1868 J
1Btu
1055.06 J
Power
1 hp
745.700 W
Temperature
1 Rankine unit
5 of Kelvin unit
9
(= 1 Fahrenheit
unit)
(= 5
Length
Area
Volume
Velocity
12-200S
9
of Celsius unit)
C-1
BASIC ELECTRICITY AND ELECTRONICS
STUDENT’S MANUAL
Appendix C
Values of some UK Units
in terms of SI Units
Notes
C-2
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Appendix D
BASIC ELECTRICITY AND ELECTRONICS
STUDENT’S MANUAL
Appendix D
D.1
Semiconductors
Semiconductors
History
Right from the early days of radio, semiconductors have been used; the early 'cat's
whisker' diode was an example. At first the action of semiconductors was not at all
thoroughly understood, and it was not until recently that these principles were further
researched.
This research led to the invention of the transistor in 1948. The transistor is a device that
can perform most of the functions of a thermionic tube, but generally with greater
efficiency, and with a great saving in space. The transistor is much smaller than a
thermionic tube, does not require any heater power, operates at much lower potentials,
and its development has made possible the reduction in power consumption and the
miniaturisation of electronic equipment desirable in modern apparatus.
D.2
Materials
A good conductor of electricity is a material that has a large number of electrons that are
free to move about the material.
An insulator is a material that has almost no free electrons, thus the movement of
electrons about the material is virtually nil. No material known is a perfect insulator, though
to all intents and purposes those shown in fig D-1 have such high resistivities that they
may be regarded as good insulators.
Fig D-1
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D-1
Appendix D
BASIC ELECTRICITY AND ELECTRONICS
STUDENT’S MANUAL
Semiconductors
Between the conductors and insulators there fall materials which are neither good
conductors nor good insulators. These are termed semiconductors. Semiconductor
materials are used in transistors, diodes, integrated circuits, etc, and the most commonly
used are Germanium and Silicon.
Pure Germanium and Silicon are not good conductors of electricity (they are insulators at
absolute zero degrees temperature, but have resistivities of about 60 and 60,000
ohms/cm3 respectively at room temperatures). Impure Silicon and Germanium have much
lower resistivities of around 0.3 ohm/cm3 dependent on the type and amount of the
impurities present.
D.3
Crystals
Most solids are crystalline in structure. This means that the atoms that make up the
material are arranged in a regular fixed relationship with each other, the specific
arrangement of atoms depending on the number of atoms present, their size, and the
electric and inter-atomic forces between them. In both Silicon and Germanium the crystal
structure is as shown in fig D-2.
Fig D-2
The atom X is the centre of the small cube and is bound to its four nearest neighbours, A,
B, C, D, which are at the alternate corners of the cube. This structure is repeated
throughout the crystal as shown.
Both Silicon and Germanium have four electrons in their outer shell (valence electrons). It
is these electrons that are shared with neighbouring atoms to achieve the bonding
between them. This is shown schematically in fig D-3.
D-2
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Appendix D
BASIC ELECTRICITY AND ELECTRONICS
STUDENT’S MANUAL
Semiconductors
Fig D-3
Note that each nucleus/inner electron core is bonded to its neighbour's effectively by a
pair of electrons. These bonds are called co-valent bonds.
D.4
Intrinsic Conduction
As the valence electrons serve to bind one atom to another in the lattice, they are also
bound tightly into the structure. Hence, in spite of there being four valence electrons
available for conduction, the crystals has a low conductivity.
At absolute zero the ideal structure shown in fig D-3 will be achieved and the crystal will be
an insulator, since no free carriers of electricity (electrons) are available.
At room temperatures, however, some electrons will gain sufficient energy from thermal
sources to enable some of the covalent bonds to be broken and make conduction
possible. In giving an electron sufficient energy to allow it to break away from the rigid
format shown in fig D3, and allowing it to wander freely through the crystal, a gap in the
valence electron pattern is formed. This is represented in fig D-4.
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D-3
Appendix D
BASIC ELECTRICITY AND ELECTRONICS
STUDENT’S MANUAL
Semiconductors
Fig D-4
The covalent gap is called a Hole. It is just the absence of an electron. Holes can move
through the lattice. This may appear strange, but an explanation follows:
When a covalent bond is broken a hole is formed. Once this happens it is relatively easy
for a valence electron from a neighbouring atom to break its covalent bond and take up
the position of the hole, thus making a new covalent bond there. It will, of course, leave a
hole behind at its old position. Thus the electron will have moved from, say, position A to
position B, and a hole moved in the opposite direction, from B to A. This process is
continued through the crystal giving what appears to be a conduction of holes in the
opposite direction to the conduction of electrons. Hole conduction can be regarded as a
way of conducting electricity without free electrons being present. Holes may be
considered as physical entities, just as electrons are, but with a positive charge instead of
a negative one.
In a pure semiconductor the number of holes is equal to the number of electrons, with
thermal energy producing hole-electron pairs, with other hole-electron pairs disappearing
due to recombination.
Hole conduction in a pure semiconductor is termed Intrinsic Conduction.
D-4
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Appendix D
BASIC ELECTRICITY AND ELECTRONICS
STUDENT’S MANUAL
D-5
Semiconductors
impurities
Impurities are atoms of elements, other than the normal crystal element, which may be
present in the lattice structure. Impurities may be found in the natural state of the crystal,
or may be added intentionally during processing.
There are two types of impurities of interest; these are called Donor and Acceptor
impurities.
Donor impurities are atoms with five electrons in the outer shell (a valency of five).
Acceptor impurities are atoms with three electrons in the outer shell (a valency of three).
D-6
Extrinsic Conduction
Let us consider what happens when a donor impurity (5-valent) occurs in the
semiconductor lattice, this is represented in fig D-5.
Fig D-5
The donor impurity has five electrons in its outer shell. Four of these valence electronics
will make the covalent bonds required for the lattice, the fifth will be nominally unbounded
and available for conduction through the crystal.
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D-5
Appendix D
BASIC ELECTRICITY AND ELECTRONICS
STUDENT’S MANUAL
Semiconductors
Suitable impurities to be used as donors include antimony, arsenic and phosphorous, and
as donors give an extra electron to the system producing an excess of negative carriers,
the semiconductor with donor impurities is known as an n-type semiconductor.
Introducing impurities into a pure semiconductor is called doping. If a semiconductor is
doped with an n-type material, the number of electrons increases, and the number of
holes decreases below that which would be available in an intrinsic semiconductor. This is
due to more recombination using the extra electrons.
If an intrinsic semiconductor is doped with acceptor impurities with three outer shell
electrons (trivalent), the situation of fig D-6 develops.
Fig D-6
Only three of the four associated covalent bonds can be made by electrons associated
with the acceptor atom, and a hole results. Conduction of electricity in an acceptor doped
semiconductor is by means of the holes, in a similar way as described earlier for intrinsic
conduction of holes.
When an intrinsic semiconductor is doped with an acceptor impurity the resultant doped
semiconductor has an excess of positive carriers (holes), and is thus known as a p-type
semiconductor.
Refer back to fig D4 and you can now appreciate the differences in conductivity for
intrinsic (pure) and doped semiconductor materials.
D-6
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Appendix D
BASIC ELECTRICITY AND ELECTRONICS
STUDENT’S MANUAL
Semiconductors
Always remember that the impurity atoms must form part of the lattice structure of the
crystal. They cannot just fit in anywhere, but must take the exact place of where a
semiconductor atom would be, and thus continue the structure of the lattice without
interruption. An impurity atom lodged as shown in fig D-7 would not produce the same
bonding, and would not give the required conductivity results.
Fig D-7
For correct operation the impurity must actually substitute itself for a germanium atom, or
be a Substitutional Impurity rather than just fitting in the gaps between atoms (the
interstices), and Interstitial impurity, as shown in fig D-7.
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D-7
Appendix D
BASIC ELECTRICITY AND ELECTRONICS
STUDENT’S MANUAL
D-7
Semiconductors
Semiconductor Devices
Semiconductor materials are used in many modern electronic devices. Normally their
action depends on there being a junction present between the two types of doped
material. This junction is known as a P-N junction, because of the types of semiconductor
material used.
Devices using semiconductors include diodes, transistors and integrated circuits, and it
has been the development of these devices, and other similar devices that has led to the
vast expansion in electronics and space-age technology that has occurred in recent years.
D-8
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