Silicon Controlled Rectifier (SCR) is a kind of semi conductor device used in switching applications. When a positive voltage is applied to its gate, SCR conducts and latches. It remains conductive even if its gate current is removed. The latched SCR can be switched off by removing its Anode current.
SCR is a solid state switch with two transistors and a resistor inside. Complementary transistors, NPN and PNP form the PNPN junction of SCR. Fig 1 shows the working of SCR. It is a complementary regenerative switch in which the base current of NPN transistor is derived from the PNP transistor. When a positive voltage is applied to the gate of SCR, the NPN transistor switches on and its collector pulls the base of PNP transistor and it conducts. Its collector current feeds the base of NPN transistor and the process repeats. This latches SCR even if the gate current is removed. Thus both the transistors feed each other and hence the process is called “Regenerative switching”.
Usually the SCR needs only a small high pulse in its gate to conduct. But sometimes line transients cause false triggering in SCR. This is called “ Rate Effect”. A finite amount of capacitance exists between the Anode and Gate of SCR. If a sharply rising voltage exists at the Anode, the internal capacitance can cause part of the rising voltage to break through the gate and thus triggers the SCR. This Rate effect can be eliminated using an “RC Snubber Network” between the Anode and Cathode of SCR.
Fig.2 shows a typical relay driver circuit using SCR. When push switch S1 is pressed, Relay turns on. When S2 is pressed relay turns off. Thus the SCR working like a “Soft Switch” for controlling the relay. When S1 is pressed, gate of SCR gets trigger pulse through R1 and it conducts and latches. Relay turns on. At the same time capacitor C1 charges via R3. When S2 is pressed, C1 dumps its voltage into the Anode of SCR which is oppositely polarized and it turns off. Relay thus turns off.
Pin assignment of some common SCRs. Keep the facing side of SCR on which the number printed and count 1,2 3 from left side. Then identify the Anode (A) ,Cathode (K) and Gate (G) as per the chart.