High Voltage Emergency Mains Cut-OFF Circuit

In this post we learn how to make a simple high voltage emergency mains cut-off circuit for intentionally blowing a fuse during a catastrophic high voltage tendency, making sure that the attached fuse blows first rather than damaging the valuable appliances

If the voltage of the mains supply of a computer goes up too much, components on the printed circuit boards can potentially be destroyed or even cracked.

This emergency cut-out placed between the mains supply and the load interrupts the supply in case the voltage levels crosses a specific value.

For many reasons it can be possible for. the output voltage of a power supply to rise to a hazardous level. The emergency cut-out described here has been set to the maximum supply voltage of 5.25 V that is mentioned by the manufacturers of TTL lCs. Zener diode D1 starts operating just before the stated zener voltage is hit.

A tiny current flows in the anode-gate circuit of thryristor Th1; the measure of this current can be fixed with preset potentiometer Pl hooked up in parallel with the gate- cathode circuit of Thl. Whenever the mains supply rises, the current through the zener diode becomes large enough to cause the thyristor to fire. The firing degree can be found between 5.2... 6 V.

The moment the thyristor fires, the mains supply voltage lowers significantly due to the fact the thyristor practically short-circuits the mains supply.

ln the case of a supply without current restricting, fuse F1 inhibits the current getting too high a value. The rating of the fuse depends, obviously, on the load necessity. Throughout testing and adjustment of the circuit, it is critical that the thyristor continues to conduct after it is fl red until its current has decreased to zero.

The firing voltage level may be fixed through a mains supply with a current limiter before it is put into use, in case it proves impossible, perhaps because of the tolerances of the zener diode, to fix the firing voltage to the required value, you may try a 5.1 V zener diode.

The complete circuit diagram for the high voltage emergency mains cut-off circuit is shown below.