Load protection circuit

Abstract

A battery operated standby inverter power supply uses an electromechanical relay switching network to connect and disconnect line power between input and output terminals. An overcurrent relay pulser rapidly opens the switching system relay contacts upon transition to battery mode, and a breakover diode network accelerates field collapse in the relay coil attendant to reversion of the relay to its normally closed condition attendant to each battery to line transition. A fault detector establishes acceptable line voltage thresholds, the exceeding of either of which triggers operation from line to battery mode. These triggering thresholds are automatically pulled closer together during initial system startup. During system startup and attendant to each battery-to-line transition, the fault detector is momentarily latched to indicate acceptable line voltage conditions, thereby removing its susceptibility to transient retriggering. During start-up mode a low-current transformerless power supply connected to the power line input terminals charges a storage capacitor sufficiently to power the fault detector through a single interrogation cycle, to actuate the switching circuitry to line mode operation if proper line voltage conditions are sensed. Collapse of this capacitor voltage during this operation terminates the interrogation cycle, a new cycle begins when the capacitor has been suitably charged. An overload detector senses the load current in line mode and prevents actuation to battery mode if the load connected to the power supply output exceeds the rated current value of the inverter.