Safety devices like fuses provide protection against excess current, but do nothing for transients and short duration spikes of high voltahe on the power supply lines.This circuit uses the “crowbar” method and provides fast protection against transient voltage spikes, transients that could cause damage to sensitive components. The thyristor will trigger in a few microseconds. This is over 1000 times faster than an ordinary quick blow fuse. If the output voltage exceeds the limit set by the zener, then it will conduct. The voltage across the 4.7k resistor will rise, the thyristor switches on and the power rails are short circuited.
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