An apparatus for measuring the dc current in a power supply circuit includes a transformer having a primary winding, a core and a secondary winding, a current generator, a pulse generator, and a measurement circuit. The secondary winding of the transformer is coupled to the power supply circuit, wherein the power supply circuit supplies dc current to the secondary winding generating a second field of magnetic flux. The current generator, coupled to the primary winding of the transformer, supplies an ac current to the primary winding generating a first field of magnetic flux. The pulse generator, coupled to the primary winding, generates a pulse when the first field of magnetic flux is substantially equal to the second field of magnetic flux. The measuring circuit, coupled to the current generator and responsive to the pulse generator, measures the ac current from the current generator in response to a pulse received from the pulse generator, wherein the ac current is proportional to the dc current.