| 摘要 |
PROBLEM TO BE SOLVED: To obtain a power failure detection circuit for accurately detecting the power failure of an AC power supply without outputting an erroneous power failure detection signal in normal operation even if the current transmission rate of a photocoupler is low. SOLUTION: An AC power supply system circuit that is serially connected from one terminal of a diode bridge 2 to the other terminal via a first light- emitting diode 4a, a second light-emitting diode 50a, and a zener diode 5 and allows a second photo transistor 50b to short-circuit the zener diode 5 while current flows to the second light-emitting diode 50a and a DC power supply 6 that is connected to the input terminal of the first photo transistor 4b while current flows to the first light-emitting diode 4a charge a capacitor 9 that is connected to an output terminal, and a voltage comparator 12 compares the charge voltage of the capacitor 9 with a specific preset voltage, thus detecting the power failure of an AC power supply 1.
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