Diodes and Maximum Reverse Voltage capacity

Diodes

Signal    diodes |Rectifier diodes | Bridge rectifiers |Zener diodes
Also see: LEDs |
AC and DC |
Power Supplies



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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Signal diodes (small current)

Signal diodes are 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.
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.
Current flowing through a relay 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 relay 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.
 

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Diode	Maximum Current	Maximum Reverse Voltage
1N4001	1A	50V
1N4002	1A	100V
1N4007	1A	1000V
1N5401	3A	100V
1N5408	3A	1000V

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.

Also see:
Power Supplies

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Bridge rectifiers

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.
Also see: Power Supplies





Various types of Bridge Rectifiers

Note that some have a hole through their centre for attaching to a heat
sink
Photographs ©
Rapid Electronics

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Zener diodes

Example:      
Circuit symbol:   


                  a = anode, k = cathode

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. The diagram
shows how they are connected, with a resistor in series to limit the current.

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 printed with V in place of a decimal point, so
4V7 means 4.7V for example.
Zener diodes are rated by their breakdown voltage and maximum power:

• The minimum voltage available is 2.4V.


• Power ratings of 400mW and 1.3W are common.