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A silicon controlled rectifier (SCR) is a semiconductor device designed to switch and control high current loads. The SCR is essentially a current controller which relies on an externally supplied trigger voltage to switch on. SCR devices also feature a preset holding current value below which they will not conduct even if switched on. This allows for controlling when the device switches on and the minimum current it will pass. Silicon controlled rectifiers are typically used in high current, high voltage application although they are often found in lower voltage systems such as lamp dimmers.
SCRs consist of four alternating layers of p and n semiconductor materials. An external lead, known as the gate, is connected to the second p layer. The gate is used to supply the trigger voltage which causes the SCR to turn on and start conducting. The semiconductor wafers are formed into a compact pellet contained in a molybdenum or tungsten casing. The casing is fitted with a threaded stud on one end to allow for attachment of a heat sink with the gate and main input leads located on the other end.
The silicon controlled rectifier will remain inactive or open until a predetermined gate or trigger pulse is received at which point the device will switch on and allow current to pass. Once switched on, the gate voltage may be removed and the SCR will continue to conduct as long as the current flow does not drop below a preset level. This preset minimum is known as the holding current of the device; even if the SCR receives a gate pulse, it will not turn on if the main supply current is below this level. These characteristics make the silicon controlled rectifier and ideal component for closely controlled current switching.
The rectifier's gate voltage is also a variable function in its operation. Each silicon controlled rectifier has a rated threshold gate voltage below which the device will not activate. This feature gives circuit designers additional control and flexibility over the rectifiers switching conditions. The gate control circuit is often fitted with a protection device known as a snubber. This prevents sudden increases in voltage applied to the rectifier from inducing capacitive coupling in the gate and inadvertently switching it on.
The SCR is generally used in circuits which feature very high current and voltage values. Modern SCRs can comfortably switch power ratings in excess of a million watts and have become an integral part of high voltage alternating current (AC) to direct current (DC) conversions. These heavy duty variants are often water cooled and suspended in huge stacks from the ceilings of power transfer facilities. An SCR may also be found in domestic applications utilizing far lower power levels such as motor controllers, heating controllers, and large light dimmers.
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