| 摘要 |
PURPOSE:To detect the power failure of an AC power supply properly to prevent a computer or the like from the generating malfunction due to the power failure by comparing the voltage of the power supply with an AC reference voltage having specific effective value and waveform and deciding the power failure on the basis of the continuation of the compared result. CONSTITUTION:When a mode changing switch 15 is turned to an initializing mode, a computer 31 counts up AC input voltages successively outputted from a timer 34 through a power supply transformer 11, a full wave rectifying circuit 12 and an A/D converter 14 until a zero crossing point and the succeeding zero crossing point are detected by a zero crossing detecting circuit 13. Then, the computer 31 calculates an effective value, determines the ratio of the effective value to a set effective value corresponding to the minimum voltage driving the computer or the like and stores a reference voltage having waveform similar to that of the AC input voltage in a non-volatile memory 35 on the basis of the set effective value. In a power failure detecting mode, the AC input voltage is compared with the corresponding reference voltage of the memory 36, and when the continuous time for setting the input voltage less than the reference voltage exceeds a prescribed value, a power failure detecting signal is outputted and the computer or the like can be prevented from the generation of malfunction due to the power failure.
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