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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).
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.
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!
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 with relays to protect transistors and integrated circuits from
the brief high voltage produced when the relay coil is switched off. The diagram shows how a
protection diode is connected across the relay coil, note that the diode is connected
'backwards' so that it will normally NOT conduct. Conduction only occurs when the relay coil
is switched off, at this moment current tries to continue flowing through the coil and it is
harmlessly diverted through the diode. Without the diode no current could flow and the
coil would produce a damaging high voltage 'spike' in its attempt to keep the current flowing.
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
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
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 2.7V.
- Power ratings of 400mW and 1.3W are common.
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