A Silicon-Controlled Rectifier (SCR) is a four-layer (p-n-p-n) semiconductor device that doesn't allow current to flow until it is triggered and, once triggered, will only allow the flow of current in one direction. It has three terminals: 1) an input control terminal referred to as a 'gate'; 2) an output terminal known as the 'anode'; and 3) a terminal known as a 'cathode', which is common to both the gate and the anode.

SCR's are generally used for switching and power control purposes in AC and high-power circuits.  The SCR is a device that falls under a group of devices known as 'thyristors', which refer to devices that have a 4-layer or p-n-p-n structure.  The term 'silicon-controlled rectifier' is a trade name used by General Electric in 1957 to refer to this type of thyristor.

Figure 1.  Photo of various SCR's (left) and the circuit symbol for an SCR (right)

An SCR may be thought of as a rectifier whose ability to conduct current can be controlled using a third terminal known as a 'gate'.  While untriggered, an SCR will prevent any current to flow through it, except for a very small leakage current caused by non-ideal conditions. The SCR is triggered to turn on if the voltage across its gate and its cathode exceeds a certain threshold level.

Once an SCR has been triggered, it will remain 'on' even if the triggering gate voltage is removed, until the current flowing through it falls below a level known as its 'holding current'. Thus, a conducting SCR will continue to conduct as long as the current flowing through it is greater than the holding current. In normal AC applications, an SCR is turned off automatically during the half-cycle wherein the voltage and current are below zero.

The p-n-p-n structure of an SCR may be modeled in terms of a PNP and an NPN transistor, as shown in Figure 2.  It can easily be seen from this diagram why an SCR remains 'on' once triggered, even if the triggering gate voltage is removed. Applying sufficient triggering voltage at the gate drives the NPN transistor to conduct. This, in turn, pulls down the PNP's base voltage, causing the PNP to conduct.  The conducting PNP then supplies the base current to the NPN transistor to keep it conducting.  Unless the supply of current to the base of the NPN is cut off, the circuit will continue conducting under this 'on' condition.

Figure 2.  The Equivalent Circuit (left) and Structure (right) of an SCR

SCR's, which can have voltage ratings of up to 2,500 volts and current ratings of up to 3,000 amperes, are encountered in many AC and high-power applications.  Examples of applications for SCR's include:  1) power switching; 2) phase control; 3) battery charging; 4) power inverters; 5) motor switching and control;  6) high-voltage DC conversion; etc.

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