This circuit blows a fuse (replaced by the 100milliohm resistor) when the input voltage is too high by triggering an SCR (the two transistors) to short-circuit the input. The zener diode controls the crowbar voltage. The crowbar stays on until the input voltage becomes zero due to the added 47nF capacitor. A load is simulated by the 100ohm resistor and capacitor. A diode with low forward voltage is recommended as to gracefully turn off the protected circuit and not discharge any capacitors.
