PURPOSE: To constitute the secondary side of an insulated circuit only with passive elements by providing a constant current circuit at the primary side of the insulated circuit using a transformer and modulating the output current of the current circuit into an alternating current to transmit it to the secondary side of the transformer.

CONSTITUTION: The switching elements 10 and 11 are driven in a double phase waveform, i.e., waveforms (b) and (c) and therefore repeat ON and OFF alternately. The output current of a constant current circuit 2 is modulated into an alternating current which flows in the opposite direction alternately from a center tap of a transformer 12. The secondary side current flowing to the transformer 12 is proportional to the output current of the circuit 2, i.e., proportional to an input signal since the transformer 12 is equal to a current transformer. Then the output current of the circuit 2 is rectified by rectifying elements 13 and 14 and turned into a secondary current waveform like (d). The waveform (d) is smoothed by an LPF15 and turned into a current waveform (e). Thus a DC output current proportional to the input signal is obtained.