Topic 1.8 – Applications of Diodes.
Learning Objectives:
At the end of this topic you will be able to;
understand that a diode will only conduct when forwardbiased;
understand that a diode can be used to allow a current inone direction and prevent current in the opposite directionin parts of a circuit;
appreciate that the forward volt drop across a forwardbiased silicon diode is about 0.7 V;
state that a diode is used to protect transistors andcomparators for circuits which drive motors andsolenoids;
incorporate diode protection for circuits which drivemotors and solenoids.
Applications of Diodes.
There are a number of different types of diodes in use in electronics and we have already used one of them earlier in this topic as an indicator. This was of course the light emitting diode or led.
In this section we are going to examine the silicon diode, which is probably the simplest of all the diode family. It is a two lead device, which has the following appearance and circuit symbol.
You should notice that the symbol looks a little bit like an arrow and this is helpful in understanding what role the diode has in an electrical circuit. A careful examination of the two circuits below should help you to understand the behaviour of the diode.
In the circuit on the left, the lamp lights, because current can flow in the direction of the arrow on the diode symbol. This is called forward biaswhen the anode is more positive than the cathode.
In the circuit on the right, the lamp does not light, because the current is blocked by the diode. This is called reverse biaswhen the cathode is more positive than the anode.
The diode therefore acts as a one-way door to electric current.
We can see this more clearly if we add some voltmeters to the previous circuit as shown below.
In the left hand circuit we can see that the voltage of the battery is split between the diode (0.7V) and the lamp (5.3V).
In the right hand circuit we can see that all of the voltage is across the diode, leaving zero volts across the lamp, so no current can be driven through the lamp.
The diode has a very unusual I-V characteristic curve, which can be investigated using the following circuit.
The following table shows a typical set of results from this arrangement.
1N4001 DiodeV (V) / I (mA)
0.7 / 16.4
0.67 / 7.9
0.64 / 3.7
0.62 / 2.5
0.61 / 1.7
0.59 / 1.1
0.57 / 0.6
0.55 / 0.48
0.54 / 0.3
0.53 / 0.2
0.51 / 0.1
0.49 / 0.08
When plotted as a graph this gives the following characteristic.
We can see from the characteristic that below 0.5V, no current flows through the diode. As the voltage increases from 0.5V the current flowing starts to increase, slowly at first and as the voltage reaches 0.7V the increase in current becomes much more significant. Indeed the current can increase much more, but the voltage across the diode does not increase much past 0.7V.
The diode is therefore a very non-linear component and as such does not obey ohms law, because its resistance changes as the voltage across it changes.
You will not be required in the examination to reproduce or apply this characteristic in any way, it has only been included to help explain how the diode behaves. What you can expect, is to be given some circuits containing diodes, lamps and/or some voltmeters and be asked to identify which lamps will light, what the voltage across diodes / lamps would be etc. We will now look at some examples of these, which will be followed by an exercise for you to complete.
Examples:
1.The circuit below contains a battery, three lamps and two diodes.
Complete the following table to indicate the state of L1, L2 and L3.
Lamp / State (On / Off)L1
L2
L3
2.The following circuit diagram shows a 9V battery connected to a diode, and a lamp.
Complete the following table to reading on voltmeters V1, V2 and V3.
Voltmeter / Voltage Reading (V)V1
V2
V3
3.The following circuit diagram shows a 6V battery connected to a diode, and a lamp.
Complete the following table to reading on voltmeters V1, V2 and V3.
Voltmeter / Voltage Reading (V)V1
V2
V3
Solutions to example questions:
1.
Lamp / State (On / Off) / ReasonL1 / On / Current can flow from battery, through L1 and L3, and D2 (which is forward biased) and back to battery.
L2 / Off / Diode D1 is reversed biased, therefore blocks any current flowing through this top part of the parallel circuit.
L3 / On / See L1 description above
2.
Voltmeter / Voltage Reading (V) / ReasonV1 / 9V / This voltmeter is connected to the battery.
V2 / 9V / The diode is reverse biased, so all battery voltage appears across the diode.
V3 / 0V / Since the diode is reverse biased there is no voltage left for the lamp because
V1 = V2 + V3, so V3 = 0V
3.
Voltmeter / Voltage Reading (V) / ReasonV1 / 6V / This voltmeter is connected to the battery.
V2 / 5.3V / The diode is forward biased, and will have a voltage of 0.7V across it, so the remaining battery voltage appears across the lamp. i.e. 6-0.7 = 5.3V
V3 / 0.7V / Since the diode is forward biased there will be a voltage of 0.7V across it.
Protecting Devices from switch off voltages
In topic 1.7 we introduced the idea that npn transistors, MOSFET’s and thyristors can be used to switch on high powered output devices namely motors and solenoids.
There is one issue that we did not mention in the last topic which is that these high powered output devices can damage npn transistors, MOSFET’s and thyristors when they switch off because they generate a very high reverse voltage.
You may think that protecting the npn transistor, MOSFETand thyristor would be very complicated but in reality it is very easy, and requires simply the insertion of a silicon diode to the circuit as shown below:
When the motor or solenoid switches off, it generates a very large reverse voltage which left alone would damage the MOSFET, transistor etc permanently. The diode ensures that this voltage does not rise above 0.7V which protects the MOSFET transistor etc from any damage.
The circuit has been shown here with a MOSFET, but it could just as easily be a thyristor ornpntransistor, there would be no change to position or orientation of the protection diode. The load represents either a motor or solenoid.
Student Exercise 1.
1.The circuit below contains a battery, three lamps and two diodes.
Complete the following table to indicate the state of L1, L2 and L3.
Lamp / State (On / Off)L1
L2
L3
2.The following circuit diagram shows a 5V battery connected to a diode, and a lamp.
Complete the following table to reading on voltmeters V1, V2 and V3.
Voltmeter / Voltage Reading (V)V1
V2
V3
3.The circuit below contains a battery, three lamps and two diodes.
Complete the following table to indicate the state of L1, L2 and L3.
Lamp / State (On / Off)L1
L2
L3
4.The following circuit diagram shows a 12V battery connected to a diode, and a lamp.
Complete the following table to reading on voltmeters V1, V2 and V3.
Voltmeter / Voltage Reading (V)V1
V2
V3
5.The following circuit diagram shows part of a transistor circuit.
Add a component to the circuit diagram to protect the transistor when the motor comes on.
6.The circuit below contains a battery, three lamps and two diodes.
Complete the following table to indicate the state of L1, L2 and L3.
Lamp / State (On / Off)L1
L2
L3
7.The following circuit diagram shows a 9V battery connected to a diode, and a lamp.
Complete the following table to reading on voltmeters V1, V2 and V3.
Voltmeter / Voltage Reading (V)V1
V2
V3
8.The circuit below contains a battery, three lamps and two diodes.
Complete the following table to indicate the state of L1, L2 and L3.
Lamp / State (On / Off)L1
L2
L3
9.The following diagram shows a circuit to operate a solenoid.
(a)What is the name of :
(i)component W......
(ii)component X......
(iii)component Y......
(iv)component Z......
(b)What is the purpose of component ‘W’?
......
......
(c)What is the purpose of component ‘Y’?
......
......
Solutions toStudent Exercise 1.
1.
Lamp / State (On / Off)L1 / On
L2 / On
L3 / Off
2.
Voltmeter / Voltage Reading (V)V1 / 5
V2 / 5
V3 / 0
3.
Lamp / State (On / Off)L1 / On
L2 / On
L3 / On
4.
Voltmeter / Voltage Reading (V)V1 / 0.7
V2 / 12
V3 / 11.3
5.
6.
Lamp / State (On / Off)L1 / Off
L2 / Off
L3 / Off
7.
Voltmeter / Voltage Reading (V)V1 / 9
V2 / 9
V3 / 0
8.
Lamp / State (On / Off)L1 / Off
L2 / On
L3 / On
9.(a)(i)component WPush to Break Switch.
(ii)component XLight Dependent Resistor
(ii)component YSilicon Diode
(iii)component ZThyristor
(b)The push to break switch allows the thyristor to be switched off.
(c)The diode protects the thyristor from the very high voltage produced by the solenoid when it switches off.
No examination questions have been set on this topic as the student exercise effectively covers all of the options available for examination questions. These would usually form part of a larger question on npn transistors, MOSFET’s or thyristors.
Self Evaluation Review
Learning Objectives / My personal review of these objectives: / /
understand that a diode will only conduct when forward biased;
understand that a diode can be used to allow a current in one direction and prevent current in the opposite direction in parts of a circuit;
appreciate that the forward volt drop across a forward biased silicon diode is about 0·7 V;
state that a diode is used to protect transistors and comparators for circuits which drive motors and solenoids;
incorporate diode protection for circuits which drive motors and solenoids.
incorporate diode protection for circuits which drive motors and solenoids.
Targets:1.………………………………………………………………………………………………………………
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- ………………………………………………………………………………………………………………
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