JPS6459513A - Power failure detecting circuit - Google Patents

Abstract

PURPOSE:To guarantee a stable output operation, by outputting a power failure detecting signal by the time up of a timer by starting up the timer when a voltage being lowered at the time of power failure passes a high threshold value, but resetting the timer when the voltage arrives at a low threshold value by dropping steeply before that. CONSTITUTION:To monitor the power failure, a monitoring voltage detection circuit 21 which detects the DC voltage of an input smoothing capacitor 20, a first comparator 23 provided with a first threshold value and outputs a set signal when a monitoring voltage decreases less than the first threshold value, a second comparator 24 provided with a second threshold value with a level less than that of the first threshold value and outputs a reset signal by deciding the dropping of the monitoring voltage less than the second threshold value as the abnormality of a power source, and the timer 26 set at an operating state by the set signal and reset by the reset signal and outputs the power failure detecting signal at the time of the arrival of a timer setting time are provided. In such a way, it is possible to change the state of a load or the output timing of the power failure detecting signal appropriately corresponding to the level of an input AC voltage before the generation of the power failure, and to bracket the abnormality of the power source from the power failure easily.