Standby power system

Abstract

A standby AC electrical power system connected between a load device and the AC line voltage comprises a voltage responsive control circuit connected between the AC line voltage source and a DC to AC inverter circuit. The control circuit generates a continuously pulsating DC control voltage which has an envelope which responds to and follows but at a different rate the increase and decrease in the envelope of the output of a full wave rectifier fed by the AC line voltage. The control voltage remains above a given triggering level when the average value of the rectified output for the current half cycle involved stays above a given monitored level indicating that the average value of the current half cycle of the AC line voltage is above a given low level and drops to the triggering level when the average value of the current half cycle of the AC line voltage drops to the given low level. The circuit which generates this control voltage comprises a multi-branched capacitor charge and discharge circuit having a first relatively long time constant producing branch which provides a relatively slowly increasing envelope in response to the rise in the envelope of the rectified output of a relatively short time constant producing branch which provides a relatively rapidly decreasing envelope in response to the drop in the envelope of the rectified output. A trigger circuit responds to the dropping of the envelope of the control voltage to the triggering level by connecting a DC battery to the DC to AC inverter circuit which then supplies an AC voltage to the load device.