An electromagnet coil 2 wound on a fixed iron core 6 is energized by d.c. current supplied from a full-wave rectifier 1 to attract a movable armature 7. A transistor 10 is connected in series with the coil, a bias resistor 8 is connected across the coil to the base of the transistor, and a capacitor is connected between the base and collector of the transistor. The capacitor absorbs inductive energy stored in the coil 2 when the a.c. supply circuit is opened to thereby reduce the armature release time.