As the zener diode detects a surpassed voltage beyond its limit, the circuit will get affected in a few microseconds while a fuse needs hundred milliseconds to react when it gets busted due to the exceeded current rating. The circuit must maintain a regulated power supply and lenient momentary voltages for both transistors consisting of low-power Schottky (LS) Transistor-Transistor Logic (TTL). The crowbar method used by the circuit showed the essence of a thyristor as it blows the fuse by short circuiting the supply because of the large current passing through the thyristor, thereby providing much protection.
The process happens in a few microseconds or less. The sensitive IC’s are saved when the zener diode performs under the excess voltage, toggling the thyristor and setting the output voltage to zero volts. Even if the fuse blows at this stage, it will not take an effect as the voltage has been lowered to zero volts.
In cases where high voltage regulators are used, a transformer with secondary voltage will be required. Overvoltage thresholds can be determined by deciding on a different regulator and zener diode.
Source:www.zen22142.zen.co.uk/Circuits/Power/power.html
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