The Logic overvoltage protection circuit is designed to protect the TTL logic ICs with supply voltages of 5 V from overvoltage conditions.
Zener diode ZD1 senses the power supply voltage, and if the supply voltage rise above 6 V, Q1 will-turn on. Therefore, Q2 conducts short the rail. Next events depend on the source supply. It will either shut down, go into current-limit or blow its supply fuse. None of these will damage the TTL chips. The rating of Q2 depends on the source supply, and whether it will be required to operate continuously in the event of failure. Its current rating has to be in excess of the source supply.
Logic overvoltage protection parts list: R1 : 330 ohm R2 : 47 ohm R3 : 1K R4 : 220 ohm Q1 : BC 107, 2N3704 Q2 : BD132 Q3 : Bc478, 2N3702 Led Zd1 : 6 V