emergency mains cut-out

19 emergency mains cut-out If the voltage of the mains supply of a computer rises too high, compo- nents on the printed circuit boards can easily be damaged or even de- stroyed. This emergency cut-out placed between the mains supply and the load interrupts the supply when the voltage level exceeds a predeter- mined value. For mant" reasons it is possible for the output voltage of a power supply to rise to a dangerous level. The emergency cut-out described here has been set to the maximum supply TIC 106D F, 7400 P C: voltage of 5.25 V that is stated by the manufacturers of TTL ICs. Zener diode Dl starts conducting just before the stated zener voltage is reached. A small current flows in the anode-gate circuit of thryristor Th1; the level of this current can be set with preset potentiometer Pl con- nected in parallel with the gate- cathode circuit of Th1. When the mains supply rises, the current through the zener diode becomes large enough to cause the thyristor to fire. The firing ...