Example:
Circuit symbol:
Function
Diodes allow electricity to flow in only one
direction. The arrow of the circuit symbol shows
the direction in which the current can flow.
Diodes are the electrical version of a valve
and early diodes were actually called valves.
Forward Voltage Drop
Electricity uses up a little energy pushing
its way through the diode, rather like a person
pushing through a door with a spring. This means
that there is a small voltage across a conducting
diode, it is called the forward voltage drop
and is about 0.7V for all normal diodes which
are made from silicon. The forward voltage drop
of a diode is almost constant whatever the current
passing through the diode so they have a very
steep characteristic (current-voltage graph).
Reverse Voltage
When a reverse voltage is applied a perfect
diode does not conduct, but all real diodes
leak a very tiny current of a few µA or
less. This can be ignored in most circuits because
it will be very much smaller than the current
flowing in the forward direction. However, all
diodes have a maximum reverse voltage (usually
50V or more) and if this is exceeded the diode
will fail and pass a large current in the reverse
direction, this is called breakdown.
Ordinary diodes can be split into two types:
Signal diodes which pass small currents of 100mA
or less and Rectifier diodes which can pass
large currents. In addition there are LEDs (which
have their own page) and Zener diodes (at the
bottom of this page).
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Connecting and soldering
Diodes must be connected the correct way round,
the diagram may be labelled a or + for anode
and k or - for cathode (yes, it really is k,
not c, for cathode!). The cathode is marked
by a line painted on the body. Diodes are labelled
with their code in small print, you may need
a magnifying glass to read this on small signal
diodes! 
Small signal diodes can be damaged by heat when
soldering, but the risk is small unless you
are using a germanium diode (codes beginning
OA...) in which case you should use a heat sink
clipped to the lead between the joint and the
diode body. A standard crocodile clip can be
used as a heat sink.
Rectifier diodes are quite robust and no special
precautions are needed for soldering them.
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Testing diodes
You can use a multimeter or a simple tester
(battery, resistor and LED) to check that a
diode conducts in one direction but not the
other. A lamp may be used to test a rectifier
diode, but do NOT use a lamp to test a signal
diode because the large current passed by the
lamp will destroy the diode!
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