SCR or Silicon Controlled Rectifier is a 3 pin device, having three basic terminals- anode, cathode, and gate. The gate terminal is the control terminal for the application of anode-cathode voltage. Typically Silicon is used because of its low leakage current. The polarity of the voltages applied to the cathode and anode decides whether the device is in forwarding bias or reverse bias and the gate voltage decides the conduction of the SCR. In other words, when a forward bias is applied to the SCR after proper positive gate voltage is applied, the device starts conducting and is turned off only when the current through the device is made less than the holding current. Thus the SCR can be used as a switch. The application of GATE voltage is known as firing.
Types of SCR firing:
Generally, there are two types of firing:
Zero Voltage Cross Over firing: Zero-crossing control mode (also called fast cycling, integral cycle, or burst firing) operates by turning the SCR’s on only when the instantaneous value of the sinusoidal voltage is zero.
Phase angle control method: The phase angle is varied, i.e. the application of gate pulses is delayed by certain time and the conduction is controlled.
Types of firing circuit:
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