APPLICATIONS OF DIODE
Applications
Radio demodulation
The first use for the diode was the demodulation of amplitude modulated (AM)
radio broadcasts. The history of this discovery is treated in depth in
the radio article. In summary, an AM signal consists of alternating positive and
negative peaks of voltage, whose amplitude or “envelope” is proportional to the
original audio signal, but whose average value is zero. The diode (originally a
crystal diode) rectifies the AM signal, leaving a signal whose average amplitude is
the desired audio signal. The average value is extracted using a simple filter and
fed into an audio transducer, which generates sound.
Power conversion
Rectifiers are constructed from diodes, where they are used to convert alternating
current (AC) electricity into direct current (DC). Automotive alternators are a
common example, where the diode provides better performance than the
commutator of earlier dynamo.
Similarly, diodes are also used in Cockcroft–Walton voltage multipliers to
convert AC into higher DC voltages.
Over-voltage protection
Diodes are frequently used to conduct damaging high voltages away from sensitive
electronic devices. They are usually reverse-biased (non-conducting) under normal
circumstances. When the voltage rises above the normal range, the diodes become
forward-biased (conducting). For example, diodes are used in ( stepper motor and
H-bridge ) motor controller and relay circuits to de-energize coils rapidly without
the damaging voltage spikes that would otherwise occur. (Any diode used in such
an application is called a flyback diode). Many integrated circuits also incorporate
diodes on the connection pins to prevent external voltages from damaging their
sensitive transistors. Specialized diodes are used to protect from over-voltages at
higher power (see above).
Logic gates
Diodes can be combined with other components to construct AND and OR logic
gates. This is referred to as diode logic.
Ionising radiation detectors
In addition to light, mentioned above, semiconductor diodes are sensitive to more
energetic radiation. In electronics, cosmic rays and other sources of ionising
radiation cause noise pulses and single and multiple bit errors. This effect is
sometimes exploited by particle detectors to detect radiation. A single particle of
radiation, with thousands or millions of electron volts of energy, generates many
charge carrier pairs, as its energy is deposited in the semiconductor material. If the
depletion layer is large enough to catch the whole shower or to stop a heavy
particle, a fairly accurate measurement of the particle’s energy can be made,
simply by measuring the charge conducted and without the complexity of a
magnetic spectrometer or etc. These semiconductor radiation detectors need
efficient and uniform charge collection and low leakage current. They are often
cooled by liquid nitrogen. For longer range (about a centimetre) particles they need
a very large depletion depth and large area. For short range particles, they need any
contact or un-depleted semiconductor on at least one surface to be very thin. The
back-bias voltages are near breakdown (around a thousand volts per centimetre).
Germanium and silicon are common materials. Some of these detectors sense
position as well as energy. They have a finite life, especially when detecting heavy
particles, because of radiation damage.
Silicon and germanium are quite different in their ability to convert gamma rays to
electron showers.
Semiconductor detectors for high energy particles are used in large numbers.
Because of energy loss fluctuations, accurate measurement of the energy deposited
is of less use.
Temperature measuring
A diode can be used as a temperature measuring device, since the forward voltage
drop across the diode depends on temperature. From the Shockley ideal diode
equation given above, it appears the voltage has a positive temperature coefficient
(at a constant current)but depends on doping concentration and operating
temperature. The temperature coefficient can be negative as in typical thermistors
or positive for temperature sense diodes down to about 20 degrees Kelvin.
Current steering
Diodes will prevent currents from flowing in unintended directions. To supply
power to an electrical circuit during a power failure, the circuit can draw current
from a battery. An Uninterruptible power supply built in this may use diodes to
ensure that current is only drawn from the battery when necessary.
Similarly, small boats typically have two circuits each with their own
battery/batteries: one used for engine starting; one used for domestics. Normally
both are charged from a single alternator, and a heavy duty split charge diode is
used to prevent the higher charge battery (typically the engine battery) from
discharging through the lower charged battery when the alternator is not running.
Source: http://www.juliantrubin.com/encyclopedia/electronics/diode.html
