Loss-Free Supply Protector

EE JULY/AUGUST 1987 by R Kambach Any diode-based circuit that protects against reversal of the supply polarity introduces a certain voltage drop. Also, when relatively high currents are involved, the choice of a suitable diode, and its dissi- pation, may become problem- atic. This circuit utilizes a relay con- tact to break the positive supply line when the input voltage has the wrong polarity. The coil voltage of the relay may be lower than the input voltage, because Re is activated within a few milliseconds, and then receives the correct coil volt- age via Ti -Di. Since the hold voltage of a relay is generally lower than the actuation volt- age, D2 can be dimensioned such that the relay operates reliably with a minimum of zener current taken from the supply. 0 LOSS-FREE SUPPLY PROTECTOR 87102