POWER SOURCE INTERRUPTION DETECTING SYSTEM

PURPOSE:To detect interruption of input easily based on detection of instantaneous interruption of AC commercial power source by identifying interruption of input if a large DC source voltage drop in a device is detected simultaneously with detection of instantaneous interruption of AC commercial power source. CONSTITUTION:An AC input is converted through a DC source circuit 1 in a device into DC power which is fed to various circuits and the voltage thereof is monitored through a voltage monitor circuit 4. AC input is monitored through an instantaneous interruption detecting circuit comprising a rectifier circuit 2 and a voltage monitor circuit 3 in parallel with DC voltage monitoring. DC source voltage in the device drops only slightly in case of instantaneous interruption but it drops considerably in case of interruption of input, and thereby interruption of input can be detected from a considerable droppage of DC source voltage when instantaneous interruption is detected. By such arrangement, interruption of AC commercial power source can be detected easily through existing instantaneous interruption detecting function and DC voltage drop detecting function in the device.