A silicon controlled rectifier (SCR) is a semiconductor device used to control high current loads. It has a preset holding current value and requires an externally supplied trigger voltage to turn on. SCRs consist of four alternating layers of p and n semiconductor materials and have a gate to provide the trigger voltage. They are used in high current, high voltage applications and low voltage systems such as lamp dimmers. SCRs can switch power ratings in excess of a million watts and are used in motor controllers, heater controllers, and large light dimmers.
A silicon controlled rectifier (SCR) is a semiconductor device designed to switch and control high current loads. The SCR is essentially a current controller that relies on an externally supplied trigger voltage to turn on. SCR devices also have a preset holding current value below which they will not conduct even when turned on. This allows you to control when the device turns on and the minimum current it will pass. Silicon controlled rectifiers are typically used in high current, high voltage applications, although they are often found in low voltage systems such as lamp dimmers.
SCRs consist of four alternating layers of p and n semiconductor materials. An external lead, known as a gate, is connected to the second layer p. The gate is used to provide the trigger voltage which causes the SCR to turn on and start conducting. Semiconductor wafers are formed into a compact pellet contained in a molybdenum or tungsten shell. The enclosure features a threaded stud on one end to allow a heat sink to be connected with the gate and main input cables located on the other end.
The silicon controlled rectifier will remain idle or open until a predetermined gate or trigger pulse is received, at which point the device will turn on and allow current to flow. Once turned on, the gate voltage can be removed and the SCR will continue to conduct until the current flow drops below a preset level. This preset minimum is known as the device holding current; even if the SCR receives a gate pulse, it will not fire if the main supply current is below this level. These characteristics make the silicon controlled rectifier and the ideal component for tightly controlled current switching.
The rectifier gate voltage is also a variable function in its operation. Each silicon controlled rectifier has a nominal threshold gate voltage below which the device will not fire. This feature gives circuit designers additional control and flexibility over the switching conditions of the rectifiers. The gate control circuit is often equipped with a protective device known as a snubber. This prevents sudden increases in voltage applied to the rectifier from inducing capacitive coupling in the gate and inadvertently turning it on.
SCR is generally used in circuits that have 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 can also be found in home applications using much lower power levels such as motor controllers, heater controllers, and large light dimmers.
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