Method and apparatus for detection of ac power failure conditions

Abstract

An apparatus and method for detecting an AC power failure condition employs a fast attack, slow decay, energy storage circuit for tracking an AC input signal and for providing a slowly decaying output based upon a last received peak voltage input value. A current detection circuit monitors the current flow to the storage circuit from the AC mains and generates a current detection signal in response thereto. A power failure condition is declared when either the voltage across the energy storage circuit decays below a selected threshold value or current is not detected flowing to the energy storage circuit at a selected time. A particular current detection circuit employs a high permeability core which is saturated by a current in the sensed line having a current value below an expected peak current flowing to the energy storage circuit. An interrogation pulse periodically energizes an interrogation winding which, in the absence of current in the sensed line, induces a signal across an output winding. When the current being sensed is sufficient to saturate the core, no output signal is generated across the output winding. Circuitry monitors the voltage across the output winding and generates a conditioning signal when even a selected minimum current is not detected in the line at the time of the interrogation pulse.