Low voltage protection circuit

Claim 1

In a load-protection circuit operative with an alternating-current voltage-source including circuit means for sensing an undervoltage condition to control the voltage supplied to the load upon occurrence of said undervoltage condition, the improvement comprising: a transformer having input and output windings, said output winding having two output taps and at least one additional tap, said input winding being connected to said voltage source, the load being connected across one of said output taps and said additional tap, the other of said taps and said additional tap being connected to said circuit means for causing the latter to switch the load from said additional tap to said other of said taps upon occurrence of said undervoltage condition, wherein the load has two terminals and said circuit means further includes: a rectifier connected across said alternating current voltage source for providing a direct-current potential; a resistor in series with a zener diode connected across said direct-current potential; a potentiometer shunted across said direct current potential for providing a variable direct-current voltage output; a difference amplifier having two inputs and output terminals, respectively, said input terminals being connected across the junction of said resistor and said zener diode, and said variable direct-current voltage output, respectively; and a relay having a winding and two single pole, double throw contacts, said winding being connected across said output terminals, said single pole contact being connected to one of said terminals of said load, one of said double throw contacts being connected to said additional tap, the other of said double throw contacts being connected to said other of said taps, whereby the voltage across the load is substantially restored to a normal condition.