Diode

A diode can be thought of as the electronic version of a one-way valve. By restricting the direction of movement of charge carriers, it allows an electric current to flow in one direction, but essentially blocks it in the opposite direction.

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 rectifies the AM signal (i.e. it eliminates peaks of one polarity), 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 (originally a crystal earpiece, now more likely to be a loudspeaker), which generates sound.

Related Topics:
Amplitude modulated - Radio - Amplitude - Filter - Transducer - Crystal earpiece - Loudspeaker

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Power conversion

A half wave rectifier can be constructed from a single diode where it is used to convert alternating current electricity into direct current, by removing either the negative or positive portion of the AC input waveform.

Related Topics:
Rectifier - Alternating current - Direct current

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A special arrangement of four diodes that will transform an alternating current into a direct current, using both positive and negative excursions of a single phase alternating current, is known as a diode bridge, single-phase bridge rectifier, or simply a full wave rectifier.

Related Topics:
Alternating current - Direct current - Diode bridge - Bridge rectifier

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With a split (center-tapped) alternating current supply it is possible to obtain full wave rectification with only two diodes. Often diodes come in pairs, as double diodes in the same housing.

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When it is desired to rectify three phase power, one could rectify each of the three phases with the arrangement of four diodes used in single phase, which would require a total of 12 diodes. However, due to redundancy, only six diodes are needed to make a three phase full wave rectifier. Most devices that generate alternating current (such devices are called alternators) generate three phase alternating current.

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For example, an automobile alternator has six diodes inside it to function as a full wave rectifier for battery charge applications.

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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, and become forward-biased (conducting) when the voltage rises above its normal value. For example, diodes are used in stepper motor and relay circuits to de-energize coils rapidly without the damaging voltage spikes that would otherwise occur. 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 Diode types above).

Related Topics:
Stepper motor - Relay - Integrated circuits - Diode types

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Logic gates

Diodes can be combined with other components to construct AND and OR logic gates.

Related Topics:
AND - OR - Logic gate

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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.

Related Topics:
Semiconductor - Energetic - Electronics - Cosmic ray - Noise - Pulse

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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.

Related Topics:
Particle detector - Electron volt

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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.

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They have a finite life, especially when detecting heavy particle, because of radiation damage. Silicon and germanium are quite different in their ability to convert gamma rays to electron showers.

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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.

Related Topics:
Semiconductor detector - Energy loss fluctuation

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