A crowbar circuit is a power supply protection circuit that rapidly short-circuits ("crowbars") the supply line if the voltage and/or current exceeds defined limits. It is used to prevent an overvoltage condition of a power supply unit from damaging the circuits attached to the power supply. In practice, the resulting short blows a fuse or triggers other protection, effectively shutting down the supply.
Crowbar circuits are frequently implemented using a thyristor (also called an SCR) or a trisil or thyratron as the shorting device. The following schematic shows a A Typical Crowbar Circuit Diagram. This crowbar circuit has an 8V power supply, and triggers at 9.1V. To change the power supply rating, the zener diode, ZD1, needs to be changed to reflect the new trigger voltage. It should be about 1V higher than the nominal supply voltage.