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Thyristor

Thyristors are integrated-circuit components that are used to control the application of electrical power to a load. Typically, thyristors are comprised of multiple layers of P-type semiconductor material and N-type semiconductor material. Various types of thyristors include, among others, triacs, silicon controlled rectifiers (SCRs), sidacs, and diacs. Thyristors are often used as the device to carry the overcurrent, because the associated sensing circuit are of a very fundamental nature. Thyristors have been used as indispensable devices for large capacity power switching owing to the low ON-state voltage characteristic. Semiconductor switches are increasingly required to control large amounts of power while conforming to demanding power loss requirements. Such switches are typically used in motor control systems, uninterrupted power supplies, high-voltage DC transmission, induction heating, and many other high power applications. The thyristor is a well known power semiconductor switch that permits large currents to be switched at high voltages. The thyristor is turned on when carriers enter one of the inner layers. This is performed by injecting a small gate current pulse into one of the inner layers. The anode and cathode terminals can be connected between ground and the conductor carrying circuit current. The gate terminal can be connected to a resistor circuit which senses the circuit current. The value of the resistor generally sets the threshold of current carried by the circuit before the thyristor is gated into conduction to shunt the overcurrent from the conductor to ground. A silicon controlled rectifier (SCR), one type of thyristor, is a semiconductor device having a four-layered, P-N-P-N structure. If connected in series with a supply voltage and load, and the supply voltage is less than the rated voltage of the device and no trigger current is applied to the gate, the thyristor will remain off. Because of the configuration of the P-type and N-type semiconductor materials, thyristors are comprised of a plurality of PN junctions. Each PN junction has a small junction capacitance associated with it. A triac is a five layer semiconductor device that has three terminals. The triac functions as a bidirectional alternating-current switch, and the SCR functions as a uni-directional switch permitting rectified current flow in one direction only. Triacs and SCRs are found in many industrial and consumer products in such uses as light dimmers, motor speed controllers, and heater controllers.


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