Download Applications of Diodes Word Document

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 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.
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GCSE Electronics.
Unit E1 : Discovering Electronics
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.
Anode
Cathode
Conventional current flows in this direction.
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 bias when
the anode is more positive than the cathode.
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Topic 1.8 – Applications of Diodes.
In the circuit on the right, the lamp does not light, because the current is
blocked by the diode. This is called reverse bias when 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.
6.0V
0.7V
5.3V
0.0V
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.
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GCSE Electronics.
Unit E1 : Discovering Electronics
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 Diode
V (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.
1N4001 Diode
Current (mA)
20
15
10
5
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Voltage (V)
4
0.7
0.8
0.9
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Topic 1.8 – Applications of Diodes.
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
L1
L2
L3
State (On / Off)
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GCSE Electronics.
Unit E1 : Discovering Electronics
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
V1
V2
V3
3.
Voltage Reading (V)
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
V1
V2
V3
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Voltage Reading (V)
Topic 1.8 – Applications of Diodes.
Solutions to example questions:
1.
Lamp
State (On / Off)
L1
On
L2
Off
L3
On
Reason
Current can flow from battery,
through L1 and L3, and D2 (which is
forward biased) and back to
battery.
Diode D1 is reversed biased,
therefore blocks any current
flowing through this top part of the
parallel circuit.
See L1 description above
2.
Voltmeter
Voltage Reading (V)
V1
9V
V2
9V
V3
0V
Voltmeter
Voltage Reading (V)
V1
6V
V2
5.3V
V3
0.7V
Reason
This voltmeter is connected to
the battery.
The diode is reverse biased, so all
battery voltage appears across
the diode.
Since the diode is reverse biased
there is no voltage left for the
lamp because
V1 = V2 + V3, so V3 = 0V
3.
Reason
This voltmeter is connected to
the battery.
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
Since the diode is forward biased
there will be a voltage of 0.7V
across it.
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GCSE Electronics.
Unit E1 : Discovering Electronics
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, MOSFET and 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:
9V
Protection
diode
Load
0V
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 or npn transistor, there would be no change to position or
orientation of the protection diode. The load represents either a motor or
solenoid.
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Topic 1.8 – Applications of Diodes.
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
L1
L2
L3
2.
State (On / Off)
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
V1
V2
V3
Voltage Reading (V)
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GCSE Electronics.
Unit E1 : Discovering Electronics
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
L1
L2
L3
4.
State (On / Off)
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
V1
V2
V3
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Voltage Reading (V)
Topic 1.8 – Applications of Diodes.
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
L1
L2
L3
State (On / Off)
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GCSE Electronics.
Unit E1 : Discovering Electronics
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
V1
V2
V3
8.
Voltage Reading (V)
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
L1
L2
L3
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State (On / Off)
Topic 1.8 – Applications of Diodes.
9.
The following diagram shows a circuit to operate a solenoid.
9V
W
Y
X
Solenoid
Z
0V
(a)
(b)
What is the name of :
(i)
component W
........................................................
(ii)
component X
........................................................
(iii) component Y
........................................................
(iv)
........................................................
component Z
What is the purpose of component ‘W’?
.............................................................................................................................
.............................................................................................................................
(c)
What is the purpose of component ‘Y’?
.............................................................................................................................
.............................................................................................................................
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GCSE Electronics.
Unit E1 : Discovering Electronics
Solutions to Student Exercise 1.
1.
Lamp
L1
L2
L3
State (On / Off)
On
On
Off
2.
Voltmeter
V1
V2
V3
Voltage Reading (V)
5
5
0
3.
Lamp
L1
L2
L3
State (On / Off)
On
On
On
4.
Voltmeter
V1
V2
V3
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Voltage Reading (V)
0.7
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11.3
Topic 1.8 – Applications of Diodes.
5.
6.
Lamp
L1
L2
L3
State (On / Off)
Off
Off
Off
7.
Voltmeter
V1
V2
V3
Voltage Reading (V)
9
9
0
8.
Lamp
L1
L2
L3
State (On / Off)
Off
On
On
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GCSE Electronics.
Unit E1 : Discovering Electronics
9.
(a)
(i)
component W
Push to Break Switch.
(ii)
component X
Light Dependent Resistor
(ii)
component Y
Silicon Diode
(iii) component Z
Thyristor
(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.
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Topic 1.8 – Applications of Diodes.
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.
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
2.
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
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