This is a Overvoltage Protected 5V Power Supply circuit. This circuit can be used to supply 74LS series integrated circuit and TTL. This circuit can protect the TTL or 74Ls series integrated circuit from damage because these ICs are easily damaged by short voltage spikes. This circuit uses a fuse which if the current rating is exceeded, the fuse will blow. However, the fuse need several milliseconds to respond, so the reaction of this circuit is rather slow. This circuit uses the zener diode as voltage exceed indicator which will trigger the circuit when the limit of the zener diode is exceeded. Here is the circuit:
To solve the problem, a thyristor is used and short the circuits the supply, it will make the fuse to blow. It’s can reduce the time reaction of the circuit. It can provide better protection than an ordinary fuse. This method is called crowbar method. When the output voltage is greater than the limit of zener diode, it will switch on the thyristor which will reduce the input voltage of 7805 IC) to 0V while blowing the fuse. The regulator’s DC input should be higher than the regulator voltage. For 5V regulator, the good transformer is the transistor with secondary voltage of 8-10V AC.