A silicon-controlled rectifier (or semiconductor-controlled rectifier) is a four-layer solid state current controlling device. The SCR is a power-switching device commonly used for lighting control, motor speed control and other variable power applications. SCRs are unidirectional devices (i.e. can conduct current only in one direction) as opposed to TRIACs which are bidirectional (i.e. current can flow through them in either direction). SCRs can be triggered normally only by currents going into the gate as opposed to TRIACs which can be triggered normally by either a positive or a negative current applied to its gate electrode.
Fig 1. SCR Schematic Symbol
Construction
The Silicon Control Rectifier (SCR) consists of four layers of semiconductors, which form NPNP or PNPN structures. It has three junctions, labeled J1, J2, and J3 and three terminals. The anode terminal of an SCR is connected to the P-Type material of a PNPN structure, and the cathode terminal is connected to the N-Type layer, while the gate of the Silicon Control Rectifier SCR is connected to the P-Type material nearest to the cathode.
An SCR consists of four layers of alternating P and N type semiconductor materials. Silicon is used as the intrinsic semiconductor, to which the proper dopants are added. The junctions are either diffused or alloyed. The planar construction is used for low power SCRs (and all the junctions are diffused). The mesa type construction is used for high power SCRs. In this case, junction J2 is obtained by the diffusion method and then the outer two layers are alloyed to it, since the PNPN pellet is required to handle large currents. It is properly braced with tungsten or molybdenum plates to provide greater mechanical strength. One of these plates is hard soldered to a copper stud, which is threaded for attachment of heat sink. The doping of PNPN will depend on the application of SCR, since its characteristics are similar to those of the thyratron. Today, the term thyristor applies to the larger family of multilayer devices that exhibit bistable state-change behaviour, that is, switching either ON or OFF.
The operation of a SCR and other thyristors can be understood in terms of a pair of tightly coupled bipolar junction transistors, arranged to cause the self-latching action:
Working
The silicon-controlled rectifier is like a junction transistor with a fourth layer and therefore three p-n junctions. The two outer junctions are forward biased by the voltage as shown, but the inner junction is reverse biased. A small current in the gate electrode can turn on the current, and it will stay on until the driving voltage is removed. It is called a rectifier because it conducts current in only direction. If AC voltage is applied, then it can be turned on by a pulse and remain on until the end of that half cycle. Timed 60 Hz triggers can by used to control power by changing the trigger point within the half cycle.
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