VOLTAGE DROP DETECTION CIRCUIT

PURPOSE:To bring the delay of a response time based on the time constant of a smoothing circuit to zero and to increase the speed of a detection time, by separating the phase of the AC signal of a commercial power supply into many phases and superposing each of phases to compare the same with a reference value. CONSTITUTION:In the input of AC to a DC forming circuit 5, predetermined phase shifting quantity is set by phase shifting circuits 2a, 2b and the AC is rectified by rectifying circuits 3a, 3b. As a result, the output of the DC forming circuit 5 becomes signals on which the output signals Sa, Sb of polyphase setting circuits 4a, 4b are superposed to be compared with a preset reference signal Ss by a comparing circuit 6. By this method, a smoothing circuit becomes unnecessary and the output of the DC forming circuit 5 is immediately lowered by the generation of voltage drop and the voltage drop is instantaneously detected by the comparing circuit 6.