Crowbar Power Supply Overvoltage Fault Protector
This is a The Crowbar circuit. This circuit is used to prevent the load from damaging. This circuit utilize a zener diode. The regulator steps down the 12V input to 5V. The load will damage when the regulator is fail to step down the input voltage because the load receive 12 volts. Using the zener diode, the problem can be solved. Here is the schematic diagram of the circuit:
The zener diode will conduct when the regulator output goes above 5.1V. The voltage will trigger the gate of the thyristor, then the large current through the thyristor blows the fuse. It will make the load protected.