| 摘要 |
PROBLEM TO BE SOLVED: To surely detect a momentary power failure of a power source without being influenced by disturbances, by varying a detection time for detecting the presence/absence of a zero cross point of the power source in accordance with a source frequency. SOLUTION: A microcomputer is provided with a power failure detection means 43 and a detection time-varying means 44. The power failure detection means 43 judges as a momentary stoppage of power supply from an input power source(PS) when a zero cross point(ZC) of a source voltage is not detected by a zero cross detection circuit 51 for a predetermined detection time T1. The varying means 44 varies the detection time T1 to be shorter as a source frequency is higher. The circuit 51 detects the presence/absence of the ZC of the source voltage. A zero cross detection signal is output to the microcomputer every time the ZC is detected, whereby an interruption process is carried out. On the other hand, when the power source PS momentarily fails, the ZC of the voltage is not detected by the circuit 51 even before a momentary power failure detection counter (t) becomes 0. Since the interruption process is not carried out in this case, this is detected as the momentary power failure. |
