HYSTERESIS CIRCUIT AND POWER SUPPLY CIRCUIT

PROBLEM TO BE SOLVED: To provide a hysteresis circuit that requires no power supply, and a power supply circuit that uses the hysteresis circuit. SOLUTION: The hysteresis circuit includes zener diodes 2, 3 connected in series between an input terminal 10 and an input terminal 11; an FET 4 provided between the input terminal 11 and an output terminal 12 and which is turned on when the total voltage of voltages applied to the zener diodes 2, 3 exceeds a first predetermined voltage, outputs the voltage of the negative input terminal 11 from the output terminal 12; and a photocoupler 7 connected in parallel with the zener diode 2 and which applies the total voltage to the zener diode 3 when the FET 4 is turned on. After the FET 4 is turned on, when the voltage applied to the zener diode 3 becomes lower than a second predetermined voltage smaller than the first predetermined voltage, the FET 4 is turned off and the voltage outputted from the output terminal 12 is switched to the voltage of the positive input terminal 10. COPYRIGHT: (C)2007,JPO&INPIT