Introduction

Within the family of PNPN devices, the silicon-controlled rectifier (SCR) is definitely of the best interest nowadays. it had been initially introduced in 1956 by Bell phone Laboratories. many of the lots of 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, protecting circuits, heater controls, and section controls. In recent years, SCRs are designed to regulate powers as high as ten MW with individual ratings as high as 2000 A at 1800 V. Its frequency vary of application has additionally been extended to concerning fifty kc, allowing some high-frequency applications like heating and inaudible cleanup.

BASIC SILICON-CONTROLLED RECTIFIER OPERATION

As the nomenclature indicates, the SCR may be a rectifier created of chemical element material with the 3rd terminal for management functions. the chemical element was chosen attributable to its heat and power capabilities. the essential operation of the SCR is completely different from the basic two-layer semiconductor therein a 3rd terminal, known as a gate, determines once the rectifier switches from the open-circuit to short-circuit state. it’s not enough to easily forward-bias the anode-to-cathode region of the device. within the physical phenomenon region, the dynamic resistance of the SCR is usually zero.01 to 0.1 ohm. The reverse resistance is usually a hundred k ohm or a lot of. The written 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 physical phenomenon is to be established, the anode should be positive with reference to the cathode. this can be not, however, an ample criterion for turning the device on. A pulse of ample magnitude should even be applied to the gate to ascertain a stimulation gate current, diagrammatical symbolically by IGT.

Circuit Symbol and Terminal Identification

SCR / Thyristor

An SCR (Thyristor) may be a “controlled” rectifier (diode). management the physical phenomenon underneath forwarding bias by applying a current into the Gate terminal. underneath reverse bias, sounds like typical PN junction diode.4-layer (PNPN) device.Anode, Cathode as for a traditional PN junction diode. Cathode Gate brought out for dominant input.

SCR Apply Biasing Transistor

With the Gate terminal OPEN, each transistors area unit OFF. because the applied voltage will increase, there’ll be a “breakdown” that causes each transistor to conduct (saturate) creating IF > zero and VAK = zero.
VBreakdown = VBR(F).