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.
Copyright © 1996 by Brian M. Tissue