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DIODES
Bill Naylor, Electronworks Ltd
This article explains how diodes work and some of
their applications. You can use one of our
electronic kits to evaluate the theory.
If you have any suggestions for improving this
application note, please drop us a line at:
enquiries@electronworks.co.uk
A diode is the simplest of all semiconductor
devices. Its function in life is to conduct
electricity one way and not the other. So how is
this useful?
Consider the design of a power supply. AC mains is
applied at 240V. This is reduced down in size to
something a bit more sensible (eg 12V) using a
transformer, but this waveform is still ac. Placing
a diode in the path of the ac waveform causes only
the positive half cycles to conduct, thus creating a
dc voltage. See our article on Designing a Linear
Power Supply to see diodes in action inside a power
supply.
How do diodes work?
The symbol for a diode is shown in FIG 1. The anode
and cathode are labelled. If the voltage on the
anode is greater than the cathode by more than 0.6V
then the diode switches on and current flows from
the anode to the cathode. If the voltage is less
than 0.6V or indeed if the anode is negative with
respect to the cathode then the diode does not
conduct.
FIG 1
So where can we use a diode? Imagine a system that
has a main 5V battery and a 3V backup battery. The
main battery supplies power to the circuit
(connected to ‘Output’), but when the main battery
is being changed you still need some power to keep
the circuit alive. The circuit in FIG 2 will do
this.
FIG 2
If there is 0.6V across diode D1 its cathode will be
at 4.4V. Since the cathode of D1 is at the same
voltage as the cathode of D2, diode D2 will be
switched off since its anode is at 3V (lower than
its cathode). This means that all of the current
required by the circuit will be provide by the 5V
supply
When the main supply is removed, the reverse bias
across D2 is also removed. The cathodes of both
diodes now drop to 2.4V which is 0.6V below the
voltage on the anode of D2 (3V) and the load
connected to the output is now powered from the 3V
supply. Once the main supply is reconnected, the
voltage at ‘output’ raises to 4.4V, D2 is reverse
biased and the current is supplied via D1 again.
In many electronic circuits we need to keep a record
to time - a PC is such an application. There is
normally a silicon chip in the design that keeps a
record of the time. This chip must always have a
power supply connected to it (or the clock gets
reset). The circuit above is used in many such
applications.
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