Patent ID: 7684164
Filing Date: 2010-03-23
Classification: H02H

Abstract:
1. An automatic power-off protection apparatus comprising: an alternating current (AC) input; an AC output connected to the AC input via power wires that comprises a naught wire and a live wire; a switching unit for enabling and disabling a connection between the AC input and the AC output; a first pair of coupling coils comprising a first primary coil connected on the live wire and coupled to a first secondary coil for producing a first induced AC voltage thereacross when AC current flows through the first primary coil; a second pair of coupling coil comprising a second primary coil connected on the naught wire and coupled to a second secondary coil for producing a second induced AC voltage thereacross when AC current flows through the second primary coil; an operational amplifier for amplifying a voltage difference between induced voltages across the first secondary coil and the second secondary coil; a first comparator for comparing the amplified voltage difference with a first reference voltage and controlling the switching unit to switch off if the amplified voltage difference is higher than the first reference voltage; a second comparator for comparing the amplified voltage difference with a second reference voltage lower than the first reference voltage and controlling the switching unit to switch off if the amplified voltage difference is lower than the second reference voltage.