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Within the family of PNPN devices, the silicon-controlled rectifier (SCR) is unquestionable of the most effective interest today. it had been ab initio introduced in 1956 by Bell phone Laboratories. several of the ample common areas of application for SCRs embrace relay controls, time-delay circuits, regulated power suppliers, static switches, motor controls, choppers, inverters, cyclo converters, battery chargers, protective circuits, heater controls, and section controls. In recent years, SCRs area unit designed to manage powers as high as 10 MW with individual ratings as high as 2000 A at 1800 V. Its frequency vary of application has in addition been extended to regarding fifty rate, permitting some high-frequency applications like heating and unhearable cleanup.


As the word indicates, the SCR could also be a rectifier created of substance material with the third terminal for management functions. the substance was chosen due to its heat and power capabilities. the essential operation of the SCR is totally totally different from the fundamental two-layer semiconductor in this a third terminal, referred to as a gate, determines once the rectifier switches from the open-circuit to short-circuit state. it’s not enough to simply forward-bias the anode-to-cathode region of the device. at intervals the natural phenomenon region, the dynamic resistance of the SCR is sometimes zero.01 to 0.1 ohm. The reverse resistance is sometimes 100 k ohm or loads of. The printed symbol for the SCR is shown in Fig. 21.1 with the corresponding connections to the four-layer semiconductor structure. As indicated in Fig. 21.1a, if the forward natural phenomenon is to be established, the anode ought to be positive with respect to the cathode. this may be not, however, associate ample criterion for turning the device on. A pulse of ample magnitude ought to even be applied to the gate to establish a stimulation gate current, delineated  symbolically by IGT.

Circuit image and Terminal Identification

SCR / Thyristor

An SCR (Thyristor) could also be a “controlled” rectifier (diode). management the natural phenomenon beneath forwarding bias by applying a current into the Gate terminal. beneath reverse bias, feels like typical PN junction diode.4-layer (PNPN) device.Anode, Cathode as for a conventional PN junction diode. Cathode Gate brought out for dominant input.

SCR Apply Biasing junction transistor

With the Gate terminal OPEN, every transistors unit OFF. as a result of the applied voltage can increase, there’ll be a “breakdown” that causes every junction transistor to conduct (saturate) making IF > zero and VAK = zero.

VBreakdown = VBR(F).