The 5V regulated power supply is apply 74LS series integrated circuits which has to be really precise and tolerant of voltage transients. These IC’s are simply damaged by brief voltage spikes. A fuse will blow when its electric current rating is exceeded, but requires several hundred milliseconds to respond. This circuit will react in a few microseconds, triggered when the output voltage exceeds the limit of the zener diode.
This circuit uses the crowbar process, where a thyristor is employed and short circuits the supply, causing the fuse to blow. This can take spot in just a few microseconds or less, and so provides a lot greater protection than an ordinary fuse. If the output voltage exceed 5.6V, then the zener diode will conduct, switching on the thyristor (all in a few microseconds), the output voltage is therefore reduced to 0 Volts and sensitive logic IC’s will probably be saved. The fuse will still take a few hundred milliseconds to blow but this isn’t significant now mainly because the supply to the circuit is already at 0 Volts and no harm is usually completed. The DC input towards the regulator needs to be some volts greater than the regulator voltage. In the case of a 5V regulator power supply, it’s suggested to use a transformer with secondary voltage of 8-10Vs AC.
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