Resettable Shutdown circuits

If your circuits experience frequency overvoltage conditions, continually replacing blown fuses can get pretty expensive. However, this shutdown circuit overcomes that deficiency by replacing the fuse with a relay and a low-current SCR. When the input voltage rises above the threshold set by the Zener diode (Dl), a current of sufficient magnitude is applied to the gate of SCR1, which turns it on. That draws current through the relay coil and energizes it, which swings its commutator to its normally open contact, and disrupts power to the circuit under power. Switch SI, a normally closed pushbutton switch, is used to reset the circuit;
Resettable Shutdown circuits - schematic

it does so by interrupting power to the relay. When SI is pressed, the relay`s wiper arm returns to the normally closed position, restoring power to the connected circuit. If you deal with a number of circuits that have different burn-out levels, try the circuit in B. That circuit variation, a variable trip-point shutdown circuit, allows you to adjust the shutdown threshold to whatever level you desire. The circuit adjustment allows for the 30% variance in the trip point. The zener diode should be selected to have a voltage rating that is slightly lower than the minimum desired threshold voltage.

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