JPH05244772

PURPOSE:To reduce a size and a cost by setting a DC voltage across a rectified voltage smoothing capacitor obtained when an AC voltage is applied from a commercial power source to both ends of a series connection circuit of a capacitor and a constant-voltage diode to a predetermined control DC voltage. CONSTITUTION:The control power source circuit comprises a series circuit of a capacitor Cd1 for dividing an AC voltage Vac from a commercial power source l to predetermined values and a constant-voltage diode ZD1' a protective shunt capacitor C6 connected in parallel with the diode; a rectifying diode D connected to an intermediate connection point M of the series circuit; and a smoothing capacitor C1. A dc voltage Vc across the smoothing capacitor is used as a predetermined control DC voltage. As a result, a resistance loss of a voltage divider of the commercial AC power source voltage for constituting an input unit and a quantity of heat generated therefrom are remarkably reduced, components can be reduced in size, and simultaneously mounted on a printed board. A control circuit can be reduced in size and cost.