| 摘要 |
PURPOSE:To improve the detection precision of a decreasing voltage by detecting a decreasing voltage detection signal from the secondary side of a constant voltage circuit. CONSTITUTION:A decreasing voltage detecting circuit 40 detects the continuity and cut-off state of a transistor TR3. Namely, when a source voltage B is high enough to obtain a constant voltage VCC, the TR3 is on and a TR4 is off. When the source voltage B drops until the constant voltage VCC is not obtained any more, the Zener voltage and Zener current of a Zener diode ZD2 drop and decrease respectively and the base-emitter voltage of the TR3 also drops. Then, the TR3 turns off and the TR4 turns on to apply an H-level signal to the inverted input terminal of the operational amplifier OP2 of a reset circuit 20 through a resistance R10. The output terminal of the amplifier OP2 is held at a level H and a reset signal appears at the reset terminal RES of a CPU. Thus, the cut-off state of the TR3 is detected from the decrease in the Zener voltage of the diode ZD2 of the constant voltage circuit 10 to detect the decreasing voltage.
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