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Topic 1.7 – Switching Circuits.
Learning Objectives:
At the end of this topic you will be able to;
1.7.1 – Introduction.
 Understand that switching circuits are often used:
o as an interface between analogue and digital sub-systems;
o as a transducer driver to drive output devices;
1.7.2 – NPN Transistor.
 Draw a circuit diagram to show how npn transistors can be used as part of a
switching circuit;
 Identify the base, collector and emitter leads on a transistor;
 State that a small base current can be used to control a much larger load
current;
 Apply the following rules to a given transistor switching circuit:
o For VIN < 0.7V, the transistor is off
o For VIN > 0.7V, the transistor is on;
1.7.3 – N Channel MOSFET.
 Draw a circuit diagram to show how a MOSFET can be used as part of a switching
circuit.
 Identify the gate, source and drain on a MOSFET;
 State that a gate voltage can be used to control a large load current;
 Explain why a MOSFET should be used to drive heavy loads due to the
limited output capability of logic systems;
1.7.4 - Thyristor
 Identify the gate, anode and cathode on a thyristor;
 State that a small gate voltage can be used to latch a large load current;
 Draw a circuit diagram to show how a thyristor can be used as a latching
transducer driver incorporating a reset switch.
1.7.5 – Voltage Comparators.
 Know that comparators have a greater sensitivity than transistor switches;
 Use data sheets to identify pin connections on a dedicated comparator IC;
 Predict the output voltage given the input voltages in a comparator circuit;
 Appreciate the current driving limitations of comparators;
 Design comparator circuits which cause output devices to respond to
information from sensors;
 Design circuits to increase the output capabilities of a comparator circuit by
the addition of a transistor switch.
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GCSE Electronics.
Unit E1 : Discovering Electronics
Switching Circuits
1.7.1
Introduction
In our previous topics we have mentioned the need for an output switching
device, or transducer driver to interface a low power electronic circuit to
output devices that often require a larger current than the electronic circuit
can provide. This is the most common use for a switching circuit. However
there are cases where switching circuits are required to act as an interface
between analogue sensors and digital sub-systems.
In this topic we will consider four different switching devices that have very
different properties. It will be your task in an examination to select the most
appropriate switching circuit from the ones discussed here, and in some cases
you will have to give reasons for your choice so this is a very important topic
in your understanding of electronic systems.
1.7.2
NPN Transistor
The first switching device we will look at is the transistor. Transistors have
three connecting leads. They are called the emitter (e), base (b), and
collector (c). The following diagram shows the symbol used for the NPN
transistor which is the only type we will look at in this course.
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Topic 1.7 – Switching Circuits.
APPEARANCE
All transistors have three leads but their appearance can vary, depending
upon their particular application. We shall now consider some types of cases
used and how we can identify the emitter, base and collector leads.
Case: E-line
This type of package encapsulates the transistor in a plastic case. The
ZTX300 is a popular choice in this series. It is a general purpose transistor
which can be used for fast switching or as a voltage amplifier. The following
diagrams illustrate the shape of the case and how to identify the leads.
The ZTX300 can be used for switching devices which take current up to
100mA. It can withstand a voltage of 25V across its collector-emitter and
can dissipate a power of 300mW.
Case:
T018 and T039
In such packages, the transistor is enclosed in a metal can. The small tag on
the case serves as a guide for pin (lead) identification. The collector is usually
connected to the metal case.
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Unit E1 : Discovering Electronics
Another popular switching transistor, the BFY51, is enclosed in a type T039
can, as shown below.
The larger can suggests that this transistor can switch higher currents and
dissipate more power than the BC108. Check this out in a component
catalogue. Power dissipation can be increased by fitting a heat sink onto the
case. Look up heat sinks in the catalogue.
Case: T03
This type of case is used for high power applications. Such transistors,
mounted on a suitable heat sink, can switch very high currents and dissipate
power in excess of 100W.
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Topic 1.7 – Switching Circuits.
WARNING:
Transistors can be easily damaged if connected the wrong way in an electrical
circuit. The above package descriptions are only a few of the many types
available. Do not take these pin identifications for granted. Always look up
your transistor type in the supplier’s catalogue. It will tell you the case style
used and there will be a page showing the pinout for all of the different
packages as shown below.
Common transistor pinouts
Pins
NPN
Pins
NPN
BC147
BC148
BC149
BC107
BC108
BC109
BC171
BC172
BC173
BC182
BC183
BC184
BC167
BC168
BC169
BC237
BC238
BC239
BC207
BC208
BC209
BC437
BC438
BC439
BC413
BC414
BC547
BC548
BC549
BC582
BC583
BC584
BC467
BC468
BC469
2N3903
2N3904
TIP3055
9013
9014
MJE
3055T
BD267A
TIP31A
TIP41A
BD131
BD139
BD263
2N3054
2N3055
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GCSE Electronics.
Unit E1 : Discovering Electronics
Transistor Switching Action
The arrow on the emitter shows the direction that conventional current can
easily flow through the transistor. In our switching circuits the emitter is
connected to the zero volt line.
There are two routes that current can take through the transistor i.e from
collector to emitter and from base to emitter.
Current can only flow in the collector circuit if a small current flows in the
base circuit. A small base current is used to control a much larger current
in the collector circuit.
The simplest circuit that we can set up with a transistor is shown below:
+6V
Lamp
Flying Lead
Collector
Base
Emitter
0V
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Topic 1.7 – Switching Circuits.
There are a couple of things to note about the way in which the transistor is
connected, (i) the emitter terminal is connected directly to the 0V line, (ii) a
resistor has been added to the base terminal, this is to limit the current
flowing in the base circuit as only a small current is needed to switch the
transistor on, (iii) the load (a lamp in this case) is connected into the collector
circuit.
The flying lead shown in the circuit diagram can now either be connected to
0V or to +6V to demonstrate the switching action of the transistor.
Case 1 : Flying lead connected to 0V.
+6V
Lamp
Collector
Base
Flying Lead
VIN
Emitter
0V
In this case with the flying lead connected to 0V, there is no difference in
voltage between the base and the emitter terminal, VIN = 0V, therefore no
current flows, the transistor is switched off, and the lamp does not light.
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Unit E1 : Discovering Electronics
Case 2: Flying lead connected to +6V.
+6V
Flying Lead
Lamp
Collector
Base
VIN
Emitter
0V
In this case with the flying lead connected to the +6V line, there is a voltage
difference between the base and emitter terminals which causes current to
flow from the base to the emitter, VIN = 6V. This switches the transistor on
which allows a larger current to flow through the collector and emitter, and
the lamp lights. Note: both base and collector currents come out of the
emitter.
This simple circuit provides a very good demonstration of the switching action
of the transistor. The flying lead can be connected to the output of a digital
processing system, e.g. a logic gates, and depending on whether the output of
the digital system is at logic 0 or logic 1, this will determine the state of the
lamp.
There are two basic rules we have to remember about the transistor.
i. if the voltage between the base and emitter, usually referred to as VIN
< 0.7V then the transistor will be off;
ii. if the voltage between the base and emitter, usually referred to as VIN
> 0.7V then the transistor will be on.
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Topic 1.7 – Switching Circuits.
Practical Applications for Transistor Switching Circuits.
(a)
LIGHT ACTIVATED SWITCH
(i)
Lamp on in darkness.
In bright light the resistance of the LDR (R2) will be low. Most of the
current flowing down the voltage dividing chain flows through the LDR to the
0V line. Point A is near to 0V (VIN < 0.7V) and the transistor will be off as
shown below.
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Unit E1 : Discovering Electronics
In darkness the resistance of the LDR will be very high. The voltage at point
A is now high (VIN > 0.7V), current flowing through the resistor R takes the
easier path to 0V i.e through the base-emitter junction. The transistor is now
turned on and the lamp lights up.
(ii)
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Lamp on in bright light.
Topic 1.7 – Switching Circuits.
In darkness the resistance of the LDR will be very high. The voltage at point
A will be near to 0V, too low to turn the transistor on (VIN < 0.7V), so the
lamp will be off, as shown below.
In bright light the resistance of the LDR decreases. The voltage at point A
increases and the transistor turns on (VIN > 0.7V), also turning on the lamp.
In both cases; replacing R1 with a variable resistor would provide means of
adjusting the sensitivity of the systems.
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Unit E1 : Discovering Electronics
(b)
TEMPERATURE ACTIVATED SWITCH
(i)
Low temperature indicator.
At low temperature the
resistance of the thermistor
is high. Point A will be higher
than 0.7 volt and current
flows through the base
circuit. The transistor is
switched on and the indicator
lights up, as shown opposite.
At high temperature the
resistance of the thermistor
is low. Point A will be near to
0V (lower than 0.7 volt) and
the transistor is off. The
indicator will be off as shown
opposite.
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Topic 1.7 – Switching Circuits.
(ii)
High temperature indicator.
At low temperature the
resistance of the thermistor
is high. Point A will be near to
0V (<0.7 volt). The transistor
is off and the indicator will be
off as shown opposite.
At high temperature the
resistance of the thermistor
is low. Point A will be above
0.7 volt and the transistor is
on. The indicator is on as
shown opposite.
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Unit E1 : Discovering Electronics
INVESTIGATING TRANSISTOR CIRCUITS.
Activity 1:
1a.
Set up the following circuit. The lamps are 6V,0.06A type.
1b.
By connecting point A to the 0V line then the +6V line determine which
condition switches the transistor on. Report on your findings and explain
why this is so.
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How do you know that a small base current is controlling a much larger
current in the collector circuit ?
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Topic 1.7 – Switching Circuits.
Activity 2:
We shall now use some of the input signal sensors, covered in Topic 1.5 to
switch the transistor.
2a.
Set up the following arrangement.
2b.
Cover the window (or move the slider if using a simulator) of the light
dependent resistor and note what happens. Explain why the system
behaves in this way.
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2c.
Replace the 1kΩ resistor with a 10kΩ variable resistor. Experiment with
the setting of the variable resistor and comment on your findings.
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GCSE Electronics.
Unit E1 : Discovering Electronics
2d.
Draw a circuit diagram showing a suitable arrangement for switching on
the lamp when bright light falls on the LDR.
2e.
Set up the system and try it out.
DESIGN PROBLEM 1:
Design a system, using a transistor switch that switches on an
electric motor when the temperature is near 36°C (body
temperature).
Circuit Diagram:
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Topic 1.7 – Switching Circuits.
After designing the system, set it up and try it out. Give a full report on
the performance of your system.
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GCSE Electronics.
Unit E1 : Discovering Electronics
Student Homework 1:
Answer all questions in the spaces provided, continue on a separate piece of
paper if required.
1.
i)
ii)
What type of transistor is shown above ? .....................................
[1]
The three leads of this device have special names, what are they ?
.....................................
iii)
2.
.....................................
On the diagram above, label the leads with their correct name.
Study the following two circuits carefully.
Circuit A
18
.....................................
Circuit B
[3]
[3]
Topic 1.7 – Switching Circuits.
Explain why the lamp in Circuit A does not light, but the one in Circuit B
does.
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3.
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[3]
Transistor circuits are sometimes referred to as switching circuits Why is this ?
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4.
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[2]
i)
Draw a circuit which will switch on a light when darkness falls.
[5]
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Unit E1 : Discovering Electronics
ii)
Describe how the circuit works.
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5.
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[3]
Study the following circuit carefully.
(Buzzer)
i)
What does the circuit do.
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[2]
ii)
Explain how the circuit works.
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[3]
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Topic 1.7 – Switching Circuits.
1.7.3
N-Channel MOSFET
In this section we are going to investigate the operation of a different type
of transistor which is called a MOSFET. This stands for Metal Oxide
Semiconductor Field Effect Transistor, which is a bit of a mouthful, so we
will simply refer to it as a MOSFET. There are many different types of
MOSFET, available but we will be concentrating only on one type in this
course, which is the n-channel enhancement MOSFET. You will not be asked
about any other version in the examination.
The symbol, and picture for an n-channel enhancement mode MOSFET is
shown below.
D, Drain
G, Gate
S, Source
The leads for this type of transistor are
labelled as Gate (G), Drain (D) and Source (S).
The Enhancement-mode MOSFET has the property of being normally "OFF"
when the gate bias voltage is equal to zero.
A drain current will only flow when a gate voltage (VGS) is applied to the gate
terminal. This voltage needs to be a positive voltage typically between 3V and
9V. The actual value of VGS depends on the type of MOSFET and the load
current flowing through the MOSFET. The exact calculation of this is beyond
the scope of this introductory course.
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GCSE Electronics.
Unit E1 : Discovering Electronics
Enhancement-mode MOSFET's make excellent electronic switches due to
their low "ON" resistance and extremely high "OFF" resistance and
extremely high gate resistance. Enhancement-mode MOSFET's are used
extensively to interface integrated circuit outputs to power high current
output devices because they can handle large currents and can be driven
directly by digital logic levels.
So let us look at how the MOSFET is used in a circuit.
Example : The following circuit shows a MOSFET being used to switch on a
high powered lamp from a light sensing circuit.
12V
12V
48W
Gate Voltage
0V
Notice that there is no need for a resistor, to connect the sensor to the gate
terminal, because virtually no current flows into the device due to its very
high resistance.
The gate voltage rises as it gets dark, and this is sensed by the MOSFET
which switches on when it reaches the appropriate level.
22
Topic 1.7 – Switching Circuits.
As far as the examination questions are concerned, these will be limited to:
i. identifying the symbol for a MOSFET,
ii. identifying the names of the three leads of the MOSFET,
iii. completing a diagram to show how a MOSFET can be connected to a logic system to
drive a high powered load,
iv. explaining that the MOSFET is able to drive high current loads directly from logic
gate because only a positive voltage is necessary at the gate terminal, no current is
necessary, making it an ideal interface for logic gates that have a very low output
current capability.
Summary of NPN Transistor and MOSFET characteristics.







Transistors are "Current Operated Devices" where a much smaller
Base current causes a larger Collector to Emitter current.
A transistor can also be used as an electronic switch to control
devices such as lamps, motors and solenoids etc.
The NPN transistor requires the Base to be more positive than the
Emitter.
MOSFET's are "Voltage Operated Devices"
MOSFET's have very high input resistances so very little or no
current (MOSFET types) flows into the input terminal making them
ideal for use as electronic switches.
The high input impedance makes the design of the sensing sub-system
easier, since we do not have to worry about current being drawn from
the sensing sub-system.
The high input impedance of the MOSFET means that static
electricity can easily damage MOSFET devices so care needs to be
taken when handling them.
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GCSE Electronics.
Unit E1 : Discovering Electronics
1.7.4
Thyristors
In the previous two sections we have discussed two types of output
switching device. One was a current controlled device, i.e. the NPN
transistor, and the other was a voltage controlled device, i.e. the MOSFET.
Both of these relied on the continuous provision of a base current or gate
voltage to keep the output on. The next device we are going to look at
which is also a voltage controlled device is the thyristor. This device has
the unique property that it is self latching, i.e. once triggered it remains on
until it is reset.
The circuit symbols with for and appearance of a thyristor is
or
The symbol on the left will be found in the ‘Livewire’ or ‘Circuit Wizard’ circuit
simulation packages which you may use to simulate the operation of these
devices. The symbol on the right is a more traditional symbol for a thyristor
which you might find in text books, and other printed diagrams.
For the purposes of the examination questions the symbol on the right will be
used.
24
Topic 1.7 – Switching Circuits.
The operation of the thyristor can be illustrated as follows.
The voltage at the Gate of the thyristor is dependent on the amount of light
falling on the LDR. The thyristor will switch on if the anode is more positive
than the cathode, and a positive voltage > 1V is applied to the gate. The resistor
R is to limit the current flowing in to the gate of the thyristor just like we used
a resistor to limit the current flowing into the base of the NPN transistor.
We will now consider a practical use for a Thyristor which is in a burglar alarm.
One of the key properties of a burglar alarm is once it has been triggered it
should remain on until it is reset. Many burglar alarms use a light sensor and a
an infra red light beam which is invisible to the human eye. This is usually
arranged around a doorway as shown below.
Door
Frame
Light
Source
Light
Beam
LDR
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GCSE Electronics.
Unit E1 : Discovering Electronics
When the light beam shines on the LDR, the
voltage across the LDR is very small and this
is not sufficient to switch on the thyristor,
and the lamp is off as shown opposite.
When a burglar walks through the doorway
the beam of light is broken, and the LDR
falls into darkness. In darkness the voltage
across the LDR increases and this is now
sufficient to switch the thyristor on, and so
the lamp lights, as shown opposite.
As the burglar moves through the doorway
the light beam is restored to the LDR, and
we can see that even though the light level
increases again, as shown opposite, we can
see the latching action of the thyristor
because the lamp remains on, even though
the voltage at the gate has fallen below the
level needed to switch the thyristor on.
The question now arises as to how to switch the thyristor off when it has
‘latched’ on.
26
Topic 1.7 – Switching Circuits.
The thyristor can be switched off by using the switch shown in the circuit
diagram. You should recognise this as a push to break switch from topic 1.4.
The action of the switch is to stop the current flowing through the lamp and
thyristor, which effectively resets the thyristor and switches the lamp off.
Closing the switch again after it has been opened does not cause the lamp to
come back on, the thyristor has been switched off. The lamp will only come on
again if the gate voltage rises above the required value. This method works by
stopping the current flowing through the thyristor.
This method of resetting the thyristor is acceptable when the current through
the load is less than about 5A. In industrial applications thyristors can handle
very high powered output devices that require very large currents up to about
100A, and breaking this sort of current with a simple push switch can be
dangerous because the current can jump between the contacts of the switch
and cause sparks to jump across the gap. This is called arcing, and if repeated
many times can destroy the contacts of the switch. So for industrial
applications alternative methods of resetting the thyristor are used. As these
are much more complex they will not be covered in this introductory syllabus.
As far as the examination questions are concerned, these will be limited to:
i. identifying the symbol for a thyristor,
ii. identifying the names of the three leads of the thyristor,
iii. completing a diagram to show how a thyristor can be used with sensing
sub-systems in simple alarm circuits for example.
iv. completing a diagram to show how a thyristor can be connected to a
logic system to drive a high powered load,
v. explaining that the thyristor is a latching device which will remain on
until reset once triggered, by a suitable gate voltage.
vi. explain that the thyristor can be reset by breaking the current flowing
through the load.
vii.the load current will be limited to applications requiring a maximum of
10A, which can be safely switched off using the method of breaking the
current described in part vi above.
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GCSE Electronics.
Unit E1 : Discovering Electronics
Student Homework 2:
Answer all questions in the spaces provided, continue on a separate piece of
paper if required.
1.
i)
ii)
What device is shown above ? .....................................
[1]
The three leads of this device have special names, what are they?
....................................
.....................................
.....................................
iii)
On the diagram above, label the leads with their correct name.
i)
What device is shown above? .....................................
2.
ii)
[3]
[1]
The three leads of this device have special names, what are they?
....................................
iii)
[3]
.....................................
.....................................
On the diagram above, label the leads with their correct name.
[3]
[3]
28
Topic 1.7 – Switching Circuits.
3.
Study the following two circuits carefully.
Circuit A
Circuit B
Circuit A and Circuit B perform similar functions.
(a)
What conditions are needed for the lamp to come on in Circuits A
and B.
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[1]
(b) How does circuit A differ from circuit B
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[3]
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GCSE Electronics.
Unit E1 : Discovering Electronics
4.
The output of a logic system is required to switch on a high power motor
which requires a current of 10A. Complete the diagram below to show a
suitable switching device and any necessary connections.
12V
Motor
12V, 10A
Logic
System
0V
[2]
30
Topic 1.7 – Switching Circuits.
5.
A student made the following circuit to switch on a fan (driven by the
motor) in an air conditioning system for a factory when it gets light, so
that is not too hot for the workers when they arrive.
a)
Identify three faults with the students design.
i)
...................................................................................................................
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ii)
...................................................................................................................
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iii)
b)
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[3]
Draw the circuit diagram for a solution to this problem that does
not suffer from the faults identified.
[3]
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GCSE Electronics.
Unit E1 : Discovering Electronics
6.
The following circuit is used as a burglar alarm, with a pressure pad
located under a doormat.
The circuit suffers from the disadvantage that once switched on the
buzzer will not go off. In the space below draw a circuit diagram that
includes the modification(s) to the circuit so that the alarm can be turned
off once it has been switched on.
[3]
32
Topic 1.7 – Switching Circuits.
1.7.5 Voltage Comparators
In the previous three sections when we have discussed the operation of the
NPN transistor, N-channel MOSFET, and Thyristor. We have concentrated on
their ability to enable large output currents to flow, from relatively small input
currents or voltages. All three devices operate best when their input current
or voltage change rapidly, i.e. like the output from a logic gate switching on
from its off state. This rapid transition drives the output switch hard from its
off state to the on state very quickly.
Sometimes the output signal connected to these electronic switches takes
much longer to increase and this can lead to problems with the transistor,
MOSFET or Thyristor not fully switching on and then they become very hot.
The reasons for this are beyond the scope of this introductory unit, but it is a
situation that we need to avoid.
The sort of signals that are slow changing like this are the ones that come from
sensing circuits involving LDRs and thermistors primarily. These tend to have
slowly changing outputs because light level and temperature don’t usually
change very quickly. We need a device that can change these slow changing
signals into fast changing signals in order to drive the output switching devices
hard into their on or off states. The device that allows us to do this is the
voltage comparator.
The voltage comparator is an integrated circuit that has five connections. At
first it may appear to be quite complex but it has very simple rules about how
the output is determined. The circuit configuration (left) and pin-out (right)
is shown below. Note: some comparators have a different pin-outs. For the
purposes of this examination the pinout diagram on the right will be used.
+ve Supply Voltage
+
_
Non-Inverting
Input Voltage,
V+
0V
0V
VOUT
Inverting
Input Voltage,
V33
GCSE Electronics.
Unit E1 : Discovering Electronics
In most practical circuits the power supply connections to the comparator are
not shown, which simplifies the diagram to the one below:
+
_
Non-Inverting
Input Voltage,
V+
Inverting
Input Voltage,
V-
VOUT
0V
Remember that if you are setting this circuit up in practice you must
connect the power supply to the comparator otherwise it will not work.
The operation of the comparator is such that it amplifies any difference
between the two input voltages by an enormous amount, causing the output to
be at one of the extremes of the power supply connected to it. This means
that the output voltage will fall into one of the following categories.
Case 1 : If V+ > V- then VOUT will be at the positive saturation voltage.
Case 2 : If V+ < V- then VOUT will be 0V.
A difference of just a few microvolts between the two inputs is enough to
cause the output to swing rapidly from one state to another. The rapid
transition makes the voltage comparator an ideal device to use with circuits
employing slow response sensors like LDRs and thermistors.
If a slow response sensor is to be used with digital circuits then the voltage
comparator will be essential if these are to be connected to digital logic
circuits, which do not react very well to input voltages which are not very
close to either Logic 0 or Logic 1, or to switching circuits that require a rapid
transition between voltage levels to enable clean switching of an output.
34
Topic 1.7 – Switching Circuits.
As we have discovered in topic 1.5, LDRs and thermistors are used as part of
a voltage divider, and depending on the orientation of the sensing component
will provide either a rising or falling voltage at the output.
To incorporate this type of sensing circuit with a comparator we actually use
a second voltage divider to provide a reference voltage which controls the
voltage at which the output of the comparator circuit changes. In GCSE
examinations this reference voltage will be connected to the inverting
input.
The circuit diagram, which may appear complicated at first glance is shown
below.
+9V
R1 = 5k
+
_
R3 = 10k
R2 = 5k
VOUT
0V
Temperature Sensing
Sub-system
Reference Voltage
Sub-system
The first voltage divider shown in the ‘blue’ box you should recognise as the
temperature sensing circuit, discussed at length in topic 1.5. You should be
able to work out that the voltage at the non-inverting ‘+‘ input of the
comparator will increase as the temperature rises.
The second voltage divider shown in the ‘red’ box is a simple voltage divider
containing two equal resistors. Again by this stage you should be able to work
out that the voltage at the inverting ‘-’ input of the comparator will be 4.5V.
35
GCSE Electronics.
Unit E1 : Discovering Electronics
When the temperature is low, the resistance of the thermistor will be very
high, the voltage at the output of the temperature sensing circuit will be low,
and the output of the comparator will be at the minimum voltage of the power
supply, because the voltage at the inverting ‘-’ input will be higher than the
voltage at the non-inverting ‘+‘ input.
As the temperature rises, the resistance of the thermistor begins to fall,
this causes the voltage at the non-inverting ‘+‘ input to start to rise. When
this voltage reaches just over 4.5V, the voltage at the non-inverting ‘+‘ input
will be bigger than the voltage at the inverting input and the output will
increase to 9V, since V+>V-.
This circuit therefore performs the simple operation of providing a high
output signal when the temperature is high, and could possibly be used as a
simple fire alarm.
As it stands the circuit has no adjustment of the temperature at which the
output of the comparator switches from high to low. It turns out that it is
quite easy to make this circuit adjustable, simply by making any one of the
three resistors R1, R2 or R3 variable. Whichever one is chosen to be variable
it will have the desired effect of either adjusting the voltage range of the
temperature sensing circuit or changing the reference voltage at which the
comparator switches.
36
Topic 1.7 – Switching Circuits.
An alternative way of producing the reference voltage is to use a
potentiometer as shown in the following diagram. This has the advantage that
the reference voltage can be varied over the full voltage supply range, making
the circuit extremely flexible, and most importantly very sensitive.
+9V
+
_
VR1 =
20k
VOUT
R2 = 10k
0V
Temperature
Sub-system
Reference Voltage
Sub-system
Just as it is quite straight forward to make this circuit adjustable it is also
just as straight forward to switch the function of the circuit to provide the
opposite behaviour, i.e. switch the output of the comparator on when the
temperature decreases, as would be the case for an ice alarm.
There are two ways in which this can be achieved, firstly by reversing the
position of the thermistor in the sensing circuit as shown below.
+9V
R3 = 10k
R1 = 5k
+
_
R2 = 5k
VOUT
0V
Temperature
Sub-system
Reference Voltage
Sub-system
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GCSE Electronics.
Unit E1 : Discovering Electronics
Secondly we can reverse the inputs to the comparator, which will also have
the same effect, as shown below:
+9V
R1 = 5k
+
_
R3 = 10k
R2 = 5k
VOUT
0V
Temperature sensing
Sub-system
Reference Voltage
Sub-system
The remarkable flexibility of this circuit means that in the design of any
circuit there is usually more than one correct solution, sometimes in an
examination, part of the circuit is drawn for you, so that the number of
correct solutions are limited, but if you are designing your own circuit, then
you are free to choose whatever solution you want provided that it satisfies
your own design criteria.
In all of the above circuits we have not considered possible output devices
that could be connected to the comparator. The output current of many
comparators is limited to a range of between 30mA and 300mA. This is of no
use for driving high powered output devices like motors, or solenoids.
However it is capable of driving LEDs with an appropriate series resistance,
to limit the voltage across the LED to approximately 2V, buzzers, and some
low power lamps.
38
Topic 1.7 – Switching Circuits.
Consider the following circuit built from a comparator i.c. which saturates at
the supply voltage, i.e. 9V:
+9V
R1 = 3k
+
_
RX
R3 = 10k
R2 = 6k
0V
Light Sensing
Sub-system
Reference Voltage
Sub-system
This circuit uses an LDR as part of the light sensing circuit. From our work in
previous topics you should be able to determine that the voltage at the noninverting ‘+’ input of the comparator will rise as the light intensity falling on
the LDR increases.
When this voltage reaches just over 6V, the output of the comparator will go
high (9V) since the voltage from the sensor will be higher than that of the
reference circuit, and the LED will light.
The system therefore switches on the LED when it gets light.
If we need to switch on higher power devices like motors and solenoids, then
we can simply connect the transistor, MOSFET, or thyristor circuit from
earlier in this topic to the output of the comparator.
Whilst the comparator can be considered a switching device because it can
switch on and off low power outputs, it’s main function is to act as an analogue
to digital converter to provide fast changing signals to drive more traditional
output switching devices like the transistor, MOSFET or thyristor into one
state or another, as on the following page.
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GCSE Electronics.
Unit E1 : Discovering Electronics
+9V
M
R1 = 3k
+
_
R3 = 10k
R2 = 6k
0V
Light Sensing
Sub-system
Reference Voltage
Sub-system
Now it’s time for you to have a go at a few circuits.
40
Topic 1.7 – Switching Circuits.
Student Homework 3.
1.
The following circuit shows a comparator. The resistance of the
thermistor at 25°C is 50kΩ, and at 100°C is 5kΩ. The output voltage of
the comparator will be either +6V or 0V.
+6V
R1 = 3k
R3 = 10k
P
+
_
Q
RX
R2 = 6k
0V
a)
Calculate the voltage at point P when the temperature is
i)
25°C.
.............................................................................................................................
.............................................................................................................................
.............................................................................................................................
ii)
100°C.
.............................................................................................................................
.............................................................................................................................
.............................................................................................................................
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GCSE Electronics.
Unit E1 : Discovering Electronics
b)
Calculate the voltage at point Q.
.............................................................................................................................
.............................................................................................................................
.............................................................................................................................
c)
The LED has a maximum forward voltage of 2V. Calculate the
series resistance Rx required to limit the current through the LED
to 20mA.
.............................................................................................................................
.............................................................................................................................
.............................................................................................................................
d)
Hence describe the function of the circuit given.
.............................................................................................................................
.............................................................................................................................
.............................................................................................................................
e)
42
Show on the diagram how the reference voltage, can be made
adjustable.
Topic 1.7 – Switching Circuits.
f)
Draw two new arrangements for this circuit which provide the
opposite function to the one given.
Solution 1:
Solution 2:
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GCSE Electronics.
Unit E1 : Discovering Electronics
Solutions to Student Exercises
Homework Questions 1.
1.
i)
NPN.
ii)
Collector, Base, & Emitter.
iii)
2.
In circuit A the base lead is connected to the 0V line. When this occurs
there is no voltage difference between the base and emitter terminals
of the transistor, and the transistor is off. No current can flow through
the collector/emitter junction and so the lamp remains off. In circuit B
the base is connected to the +6V line and a base-emitter voltage of 0.7V
is achieved. The transistor is on and a collector-emitter current flows
and hence the lamp lights.
3.
Transistor circuits are sometimes referred to as switching circuits
because a small current entering the base of the transistor produces a
much larger current in the collector. If no base current flows then
there is no collector current, so the transistor can be considered as a
switch since current in the collector circuit can be switched on by a
small base current.
44
Topic 1.7 – Switching Circuits.
4.
5.
i)
ii)
In daylight the resistance of the LDR is low, compared to the
10kΩ resistor, therefore there will be a low voltage across the
LDR. This ensures that the base-emitter voltage does not rise
above 0.7V and so the transistor remains off, so the lamp is off.
When darkness begins to fall the resistance of the LDR begins to
increase and so therefore does the voltage across it. When this
voltage increases above 0.7V the transistor starts to switch on, a
collector current flows causing the lamp to come on.
i)
The circuit switches on a buzzer when the temperature being
monitored by the thermistor goes above a certain value.
ii)
When it is cold the resistance of the thermistor is high, compared
to the 10kΩ resistor, therefore there will be a low voltage across
the 10kΩ. This ensures that the base-emitter voltage does not
rise above 0.7V and so the transistor remains off, so the buzzer
does not sound. When the temperature increases the resistance of
the thermistor begins to fall and so therefore does the voltage
across it. The voltage across the 10kΩ resistor rises and when
this voltage increases above 0.7V the transistor starts to switch
on, a collector current flows causing the buzzer to come on.
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GCSE Electronics.
Unit E1 : Discovering Electronics
Homework Questions 2:
1.
i)
Thyristor
ii)
Gate, Anode and Cathode (any order)
iii)
Anode
[1]
[3]
Gate
Cathode
[3]
2.
i)
MOSFET
ii)
Source, Gate and Drain (any order)
[1]
[3]
iii)
Source
Gate
Drain
[3]
3.
(a)
(b)
46
The lamp will come on in Circuits A and B when the temperature
rises above a particular value.
[1]
Circuit A differs from Circuit B because it contains a thyristor.
Once the temperature goes over its set limit and the lamp
switches on, it will remain on, even if the temperature falls again.
The lamp has been latched on. In circuit B containing a MOSFET,
if the temperature decreases again, then the lamp will go off.
[3]
Topic 1.7 – Switching Circuits.
4.
The output of a logic system is required to switch on a high power motor
which requires a current of 10A. Complete the diagram below to show a
suitable switching device and any necessary connections.
12V
Motor
12V, 10A
Logic
System
0V
5.
a)
b)
i)
The fan would switch on in the dark, not in the light.
ii)
Once switched on the fan could not be switched off.
iii)
The fan would switch on even if it was cold, as it is not
dependent on temperature only light.
[2]
[3]
[3]
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GCSE Electronics.
Unit E1 : Discovering Electronics
6.
[3]
48
Topic 1.7 – Switching Circuits.
Homework Questions 3.
1.
a)
i)
25°C.
50k
 6V
50k  10k
50

 6  5V
60
VP 
ii)
100°C.
5k
 6V
5 k  10k
5

 6  2V
15
VP 
b)
6k
 6V
3k  6k
6
  6  4V
9
VQ 
c)
62
20  10 3
4

 200
20  10 3
RX 
d)
At 25°C the output of the comparator is on, because the voltage
from the temperature sensing circuit ‘P’ is higher than the
reference voltage ‘Q’, and so the LED will be on.
As the temperature rises the voltage at ‘P’ will be falling until it
reaches 2V at 100°C. When the voltage falls below 4V, the voltage
at ‘Q’ the comparator output will switch to 0V and the LED will
switch off.
The circuit is therefore behaving as a low temperature warning
system.
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GCSE Electronics.
Unit E1 : Discovering Electronics
e)
Either
+6V
R1 = 3k
R3 = 10k
P
+
_
Q
200Ω
R2 = 6k
0V
or
Or
+6V
R1 = 3k
R3 = 10k
P
+
_
Q
200Ω
R2 = 6k
0V
f)
Option 1:
+6V
R1 = 3k
+
_
200Ω
R3 = 10k
0V
50
R2 = 6k
Topic 1.7 – Switching Circuits.
Option 2:
+6V
R3 = 10k
R1 = 3k
+
_
200Ω
R2 = 6k
0V
Now for some examination style questions.
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GCSE Electronics.
Unit E1 : Discovering Electronics
Examination Style Questions
1.
(a)
The diagram shows a comparator IC shown from above.
(i)
Label pin 1 with the number 1.
(ii)
Label pin 7 with the number 7.
[2]
(b)
A warning system is used in a shop to warn the assistant if the temperature in the freezer
becomes too warm. The circuit diagram for this is shown below.
(i)
(ii)
The circuit makes use of a thermistor. Draw the circuit symbol for a thermistor in the
space below.
[1]
The output VOUT of the comparator saturates at +6V and 0V. Complete the table for
the given values of the input voltages.
Input 1 (V)
Input 2 (V)
3.2
4.0
4.5
2.1
Output VOUT (V)
[2]
52
Topic 1.7 – Switching Circuits.
2.
Part of a transistor switching circuit is shown below.
(a)
On the diagram, label the base of the transistor with the letter ‘b’
[1]
(b)
At a certain time VIN is measured to be 0.4V.
(i)
Is the transistor on or off ?
..................................
(ii)
Is the buzzer on or off ?
..................................
[2]
(b)
Some time later VIN is measured to be 2.5V.
(i)
Is the transistor on or off ?
..................................
(ii)
Is the buzzer on or off ?
..................................
[2]
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GCSE Electronics.
Unit E1 : Discovering Electronics
3.
Here is part of a comparator circuit.
(a)
What is the combined resistance of R1 and R2?
......................................................................................................................................
[1]
(b)
Use the equation given in the information sheet to calculate the value of voltage V1.
......................................................................................................................................
......................................................................................................................................
......................................................................................................................................
[2]
(c)
The output Vo of the comparator saturates at 9V and 0V.
Complete the table below to show what the output voltage will be for the given conditions:
V1 (V)
V2 (V)
5.0
7.3
5.0
2.1
Output VOUT (V)
Is LED on or off?
[3]
4.
The following symbol is for a switching device.
(a)
What is the name of this device?
............................................
(b)
Label the three connecting leads by choosing from the following options.
[1]
base, drain, collector, anode, source, emitter, gate, cathode
[3]
54
Topic 1.7 – Switching Circuits.
5.
(a)
The diagram shows a comparator IC shown from above.
(i)
Label pin 1 with the number 1.
(ii)
Label pin 5 with the number 5.
[2]
(b)
A following diagram shows a circuit which indicates when the temperature in a greenhouse is
too high.
(i)
The output VOUT of the comparator saturates at +9V and 0V. Complete the table for
the given values of the input voltages.
Input A (V)
Input B (V)
4.5
2.3
4.5
4.8
Output VOUT (V)
[2]
(ii)
The temperature at which the bulb comes on needs to be adjustable. Modify the circuit
diagram above to allow this.
[1]
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GCSE Electronics.
Unit E1 : Discovering Electronics
6.
This is the symbol for a comparator.
Choose the correct name for connections A and B from the following list.
Ground
Inverting Input
Non-inverting input
Output
Positive Supply
Write your answers in the spaces provided on the diagram.
[2]
7.
(a)
Here is a voltage divider.
(i)
What is the combined resistance of R1 and R2?
(ii)
......................................................................................................................................
[1]
Use the equation given in the information sheet to calculate the value of voltage V1.
......................................................................................................................................
......................................................................................................................................
......................................................................................................................................
[2]
56
Topic 1.7 – Switching Circuits.
(b)
The voltage divider is now connected to a comparator circuit as follows:
The output Vo of the comparator saturates at 10V and 0V.
Complete the table below to show what the output voltage will be for the given conditions:
Output Vo (V)
V2 bigger than V1
V2 smaller than V1
[2]
(c)
A sensing subsystem is now connected to the comparator so that the voltage Vo is high
when the temperature is high.
Complete the diagram to show this circuit.
Remember to use the correct symbols for the components.
[3]
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GCSE Electronics.
Unit E1 : Discovering Electronics
8.
Here is part of a comparator circuit.
(a)
What is the combined resistance of R1 and R2?
......................................................................................................................................
[1]
(b)
Use the equation given in the information sheet to calculate the value of voltage V1.
......................................................................................................................................
......................................................................................................................................
......................................................................................................................................
(c)
[2]
The output Vo of the comparator saturates at 9V and 0V.
Complete the table below to show what the output voltage will be for the given conditions:
V1 (V)
V2 (V)
5.0
2.3
5.0
7.1
Output Vo (V)
Is LED on or off?
[3]
58
Topic 1.7 – Switching Circuits.
(d)
A sensing sub system is now connected to the comparator so that the LED is ON when it is
light.
Complete the diagram to show the circuit.
[3]
9.
The following symbol is for a switching device.
(a)
What is the name of this device?
............................................
[1]
(b)
Label the three connecting leads by choosing from the following options.
base, drain, collector, anode, source, emitter, gate, cathode
[3]
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GCSE Electronics.
Unit E1 : Discovering Electronics
10.
Part of a transistor switching circuit is shown below.
(a)
On the diagram, label the collector of the transistor with the letter ‘c’
[1]
(b)
At a certain time VIN is measured to be 2.8V.
(i)
Is the transistor on or off ?
(ii)
Is the lamp on or off ?
..................................
..................................
[2]
(b)
Some time later VIN is measured to be 0.5V.
(i)
Is the transistor on or off ?
..................................
(ii)
Is the buzzer on or off ?
..................................
[2]
60
Topic 1.7 – Switching Circuits.
11.
(a)
The diagram shows a comparator IC shown from above.
(i)
Label pin 3 with the number 3.
(ii)
Label pin 6 with the number 6.
[2]
(b)
The following incomplete circuit diagram shows a circuit which should indicate when the
light level is too low.
(i)
The output VOUT of the comparator is either 12V or 0V.
Complete the table for the given values of the input voltages.
Input X (V)
Input Y (V)
6.1
7.3
9.5
7.3
Output VOUT (V)
Bulb On/Off
[3]
(ii)
Complete the circuit diagram above to make:


The voltage at X high when the light level is too low.
The voltage at the input Y adjustable.
[2]
(iii)
Name one use for this circuit. ..............................................................................
[1]
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GCSE Electronics.
Unit E1 : Discovering Electronics
12.
(a)
The diagram shows a comparator IC seen from above.
(i)
What is the pin number of the non-inverting input of the comparator?
..............................................................
(ii)
What is the pin number of the output of the comparator?
..............................................................
[2]
(b)
The following diagram shows part of a circuit which indicates when the temperature in a
green house is too high.
The output VOUT of the comparator saturates at 9V and 0V
(i)
What does the comparator do in this circuit ?
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
.............................................................................................................................................
[1]
62
Topic 1.7 – Switching Circuits.
(i)
Complete the table for the given values of the input voltages
Input A (V)
Input B (V)
4.2
1.7
4.2
4.8
Output VOUT (V)
[2]
(ii)
Complete the circuit diagram opposite to make:


The voltage at input B high when it is warm,
The voltage at input A adjustable.
[3]
(c)
The gardener decides to improve his system by connecting the output to a heavy duty motor
to open the windows of the greenhouse when the temperature gets too high.
Complete the circuit diagram for the output of the new system, capable of switching on a
motor requiring 8A to operate properly.
[3]
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GCSE Electronics.
Unit E1 : Discovering Electronics
13.
A tennis umpire uses a system to show when the light level is too low to play. An incomplete
circuit diagram for this is shown below.
(a)
Which of the following graphs A, B C or D best shows the way in which the resistance of
the LDR depends on the light level?
..................................................................................................................................................
(b)
What happens to the voltage V1 as the resistance of the LDR increases?
..................................................................................................................................................
[1]
64
Topic 1.7 – Switching Circuits.
(c)
The output V2 of the comparator saturates at 9V and 0V.
Complete the table to show what the output voltage will be for the given values of the
input voltages.
Input 1 (V)
Input 2 (V)
6.1
7.3
9.5
7.3
Output V2 (V)
Is LED On or Off?
[2]
(d)
Complete the circuit diagram opposite by:
(i)
adding a voltage divider to provide a variable reference voltage for the comparator.
(ii)
(e)
adding any other connections so that the LED lights when the light level is too low
to play.
[4]
The current through the LED is 10mA and the forward voltage drop across it is 2V.
(i)
What is the voltage drop across resistor R when V2 = 9V?
(ii)
......................................................................................................................................
[1]
What is the current flowing through resistor R when R when V2 = 9V?
(iii)
......................................................................................................................................
[1]
Calculate a suitable resistance for resistor R.
......................................................................................................................................
(iv)
......................................................................................................................................
[2]
Choose a suitable preferred value for resistor R from the E24 series in the
information sheet.
......................................................................................................................................
[1]
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GCSE Electronics.
Unit E1 : Discovering Electronics
14.
A warning system is used on a car to warn the driver if the temperature outside falls below freezing,
and therefore there is a risk of finding ice on the roads. An incomplete circuit diagram for this is
shown below.
(a)
The output VOUT of the comparator saturates at +6V and 0V
Complete the table for the given values of input voltages.
Input 1 (V)
Input 2 (V)
3.2
4.0
4.5
2.1
Output VOUT (V)
[2]
(b)
Complete the diagram above to show the sensing circuit used to ensure the voltage into
input 1 of the comparator is high when it is cold.
Remember to use the correct circuit symbols for the components.
[3]
15.
The following symbol is for a switching device.
(a)
What is the name of this device?
............................................
(b)
Label the three connecting leads by choosing from the following options.
[1]
base, drain, collector, anode, source, emitter, gate, cathode
[3]
66
Topic 1.7 – Switching Circuits.
Self Evaluation Review
My personal review of these objectives:
Learning Objectives



1.7.1 – Introduction.
Understand that switching circuits are often used:
as an interface between analogue and digital sub-systems;
as a transducer driver to drive output devices;
1.7.2 – NPN Transistor.
Draw a circuit diagram to show how npn transistors can be used
as part of a switching circuit;
Identify the base, collector and emitter leads on a transistor;
State that a small base current can be used to control a much
larger load current;
Apply the following rules to a given transistor switching circuit:
For VIN < 0.7V, the transistor is off
For VIN > 0.7V, the transistor is on;
1.7.3 – N Channel MOSFET.
Draw a circuit diagram to show how a MOSFET can be used as
part of a switching circuit.
Identify the gate, source and drain on a MOSFET;
State that a gate voltage can be used to control a large load
current;
Explain why a MOSFET should be used to drive heavy loads
due to the limited output capability of logic systems;
1.7.4 - Thyristor
Identify the gate, anode and cathode on a thyristor;
State that a small gate voltage can be used to latch a large load
current;
Draw a circuit diagram to show how a thyristor can be used as a
latching transducer driver incorporating a reset switch.
1.7.5 – Voltage Comparators.
Know that comparators have a greater sensitivity than
transistor switches;
Use data sheets to identify pin connections on a dedicated
comparator IC;
Predict the output voltage given the input voltages in a
comparator circuit;
Appreciate the current driving limitations of comparators;
Design comparator circuits which cause output devices to
respond to information from sensors;
Design circuits to increase the output capabilities of a
comparator circuit by the addition of a transistor switch.
Targets:
1.
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
2.
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
67