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.
Ordinary diodes can be split into two types: Signal diodes which pass
small currents of 100mA or less and Rectifier diodes which can pass
larger currents. In addition there are LEDs (which have their own page)
and Zener diodes (at the bottom of this page).
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 hand lens to read it.
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 (such as a crocodile clip) clipped to the lead between the joint and the diode body.
Rectifier diodes are quite robust and no special precautions are needed for soldering them.
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.
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 forward voltage drop
across a conducting diode. It is about 0.7V for most 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).
When a reverse voltage is applied a perfect diode does not conduct but real diodes
leak a very tiny current (typically a few µA). 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.
Signal diodes (small current)
Signal diodes are typically used to process information (electrical signals) in circuits, so they
are only required to pass small currents of up to 100mA.
General purpose signal diodes such as the 1N4148 are made from silicon and have a
forward voltage drop of 0.7V.
Germanium diodes such as the OA90 have a lower forward voltage drop of 0.2V and this makes
them suitable to use in radio circuits as detectors which extract the audio signal from the weak radio signal.
They are rarely used now and may be hard to find.
For general use, where the size of the forward voltage drop is less important, silicon diodes are better because
they are less easily damaged by heat when soldering, they have a lower resistance when conducting, and they have
very low leakage currents when a reverse voltage is applied.
Protection diodes for relays
Signal diodes are also used to protect transistors and ICs from the brief high voltage produced when a relay coil
is switched off. The diagram shows how a protection diode is connected 'backwards' across the relay coil.
Why is a protection diode needed?
Current flowing through a coil creates a magnetic field which collapses suddenly
when the current is switched off. The sudden collapse of the magnetic field induces a
brief high voltage across the coil which is very likely to damage transistors and ICs.
The protection diode allows the induced voltage to drive a brief current through the coil
(and diode) so the magnetic field dies away quickly rather than instantly. This prevents
the induced voltage becoming high enough to cause damage to transistors and ICs.
Rectifier diodes (large current)
Rectifier diodes are used in power supplies to convert alternating current (AC)
to direct current (DC), a process called rectification.
They are also used elsewhere in circuits where a large current must pass through the diode.
All rectifier diodes are made from silicon and therefore have a forward voltage drop of 0.7V.
The table shows maximum current and maximum reverse voltage for some popular rectifier diodes.
The 1N4001 is suitable for most low voltage circuits with a current of less than 1A.
There are several ways of connecting diodes to make a rectifier to convert AC to DC.
The bridge rectifier is one of them and it is available in special packages containing the four diodes required.
Bridge rectifiers are rated by their maximum current and maximum reverse voltage. They have four leads or terminals:
the two DC outputs are labelled + and -, the two AC inputs are labelled
The diagram shows the operation of a bridge rectifier as it converts AC to DC.
Notice how alternate pairs of diodes conduct.
Zener diodes are used to maintain a fixed voltage. They are designed to 'breakdown' in a reliable and
non-destructive way so that they can be used in reverse to maintain a fixed voltage across their terminals.
Zener diodes can be distinguished from ordinary diodes by their code and breakdown voltage
which are printed on them. Zener diode codes begin BZX... or BZY... Their breakdown voltage
is usually printed with V in place of a decimal point, so 4V7 means 4.7V for example.
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