DIODES
A diode is a two terminal device which allows current to flow through it in
one direction only. The terminals are called the anode (A) and the cathode
(C). Fig 1 shows the symbols used to represent a diode.
A
C
A
C
A - Anode
C - Cathode
Fig. 1 Diode symbols
The triangle within the symbol indicates the direction in which conventional
current can easily flow through the diode i.e. from anode to cathode.
Electron flow is in the opposite direction.
FORWARD AND REVERSE BIAS
Fig 2a shows a forward biased silicon diode. When connected in this way,
the diode allows current to flow through it. Note that about 0.7V is dropped
across the diode. This is usually referred to as the ‘turn-on’ voltage. The
'turn-on' value varies between devices and manufacturers from 0.5V to
0.9V, but 0.7V is considered as a suitable approximation.
6V
0.7V
+
+
6V
5.3V
Fig. 2a Forward biased diode
6V
Fig. 2b Reverse biased diode
In Fig 2b the diode is reverse biased and has a very high resistance. In
this case, all of the supply voltage is dropped across the diode and the lamp
is off.
1
IDENTIFYING THE TERMINALS ON DIODES
The cathode lead (C) on plastic encapsulated diodes is indicated by a silver
band or a tapered end. (Fig 3a)
C
C
C
C
(a) Low power
(b) High power
Fig. 6.3
High power diodes (Fig 3b) are usually stud mounted on a heat sink. A diode
symbol is often shown on the case. They are available with either anode or
cathode stud mounting. One of the leads makes direct contact with the heat
sink, the other being available at the insulated terminal at the top of the
diode.
SELECTING A SILICON DIODE
A diode can be damaged by passing too much current through it under
forward bias or by applying a very high reverse voltage across it. Manufacturers
provide the following technical information about diodes:
•
the maximum forward current (I F) that the diode can carry.
•
the repetitive reverse maximum voltage (VRRM) that can be applied
to a diode. It must be remembered that reverse voltage is applied
on diodes in rectifier circuits.
Small plastic diodes are available with current carrying capacity of up to
about 3A. They are manufactured to withstand reverse voltages up to
1000V. Stud mounting diodes are designed to carry much larger currents
(up to 240A available).
2
We shall start this Activity by testing the Silicon Diodes, this should
be done before power is supplied to the device.
INFORMATION
A diode should provide a low resistance path when forward biased and a
high resistance path when reverse biased.
USING A DIGITAL MULTIMETER TO TEST A DIODE
The normal resistance range on a Digital Multimeter is not suitable for
checking a diode. You should find a special range for checking diodes
within the ‘ohm’ range. It is usually indicated by a diode symbol.
Procedure:
(a)
Set the range selector to the ‘diode check’ (
) position.
(b)
Connect the black lead to the cathode of the diode and the red lead
to the anode. Polarity is not reversed at the leads of Digital
Multimeters so the diode is forward biased. Fig 5 (a)
The meter displays the voltage dropped across the forward biased diode. It
should be between 0.4V and 0.7V for a silicon diode.
Fig 5 (a)
(c)
Fig 5 (b)
Connect the red lead to the cathode of the diode and the black lead
to the anode. The diode is now reverse biased and the display should
show a 1. Fig 5 (b)
3
ACTIVITY:
In this Activity we shall investigate forward and reverse biased diodes.
1a.
Set switch to the OFF position then adjust the DC supply so that
6V is available.
1b.
Connect up the circuit arrangement shown below then set the
switch to the ON position. The diode is forward biased.
+
6
Circuit Diagram
1c.
Short out the diode by connecting a lead across it.
Comment upon, and try to explain, any change in intensity.
_______________________________________________________________________
_______________________________________________________________________
1d.
Remove the short then measure the DC voltage drop across the
forward biased diode using your digital voltmeter.
Voltage drop across diode
(forward bias)
1e.
4
=
_______ V
Turn the diode around in the circuit. Explain any difference.