PURPOSE: To minimize power loss, and to remove wasteful power consumption by short-circuiting the DC output side of a diode bridge for a gate of a bidirectional thyristor.

CONSTITUTION: A diode bridge 5 is set up between a gate of a bidirectional thyristor 3 ON-OFF controlling a vacuum cleaner and a power supply. A switch 10 switching an output full-wave rectified from the diode bridge 5 is fitted among collectors and bases for transistors 6, 7 by Darlington connection connected on the output side of the diode bridge. When the switch 10 is turned ON, the DC output side of the diode bridge 5 is short-circuited so that a sufficient amplification factor can be ensured even in a low voltage section; enough voltage is output to the gate of the bidirectional thyristor 3 and the bidirectional thyristor 3 is turned ON, and 100% supply voltage is applied to a motor 2.