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SCR's are used for overvoltage protection for digital circuits.
The SCR is normally nonconducting as long as the gate is at a lower voltage than the cathode. If the gate voltage becomes larger than the cathode voltage, then the SCR becomes a short and lets current pass from the anode to the cathode.
Circuit for overvoltage protection
If the input voltage exceeds the zener breakdown voltage of approximately 6 V, current passes through the zener diode and produces a voltage on the SCR gate. The SCR then shorts the input power supply to ground protecting the rest of the circuit.
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