PURPOSE: To contrive stable control of constant-current and improvement of power efficiency by making current flow to a required direction in a magnetic flux generating coil with two power transistors connected to the diagonal positions of a bridge circuit.

CONSTITUTION: Between the emitters of a pair of transistors 21, 24 in a drive circuit, the first coil 25 for generating magnetic flux, a resistance 26 for detecting current and the second coil 27 for generating magnetic flux are connected in series in this sequence. The coils 25, 27 are set to have equal impedance with equal number of ampere-turns. The voltages generated on both the ends of the resistance 26 for detecting current are fed back to each inversion input terminal of amplifiers 15, 16 through a resistance R4 or resistances R5, R6 respectively and a feedback loop for controlling constant-current is made.