PURPOSE: To realize operation of polarized magnet with an AC by controlling a control electrode of a semiconductor switching element connected in series with a coil with a voltage sent from a circuit consisting of a diode, a capacitor and a resistor.

CONSTITUTION: When an AC voltage is applied across terminals (a) and (c), a current flows through a diode D → capacitor C → resistor R2 since resistors R1, R2 are set as R1R2. Moreover, when a gate voltage of SCR exceeds the predetermined voltage, the SCR turns ON. Thereby, a current of positive half cycle flows into a coil L1 in the set side. Meanwhile, when voltage and current are inverted, a current does not flow into the coil L1 and a charging voltage of capacitor C is gradually discharged. When the power supply voltage becomes positive again, a capacitor C is charged again. When a voltage of capacitor C increases gradually, a gate voltage of SCR drops and the SCR turns OFF and thereby the coil L1 becomes non-conductive. For the set operation, a voltage must be applied to the terminals (b) and (c). In this case, the same operation as described above is also carried out.