Patent ID: 8975890
Filing Date: 2015-03-10
Classification: G01R

Abstract:
1. A circuit for detecting a DC current in at least one conductor, the circuit including: a current transformer having a ferromagnetic core, a primary winding comprising the conductor and at least one secondary winding, an impedance in series with the secondary winding, an oscillator for supplying an oscillating signal across the impedance and the secondary winding and means for detecting a dc offset in the current flowing in the impedance and the secondary winding, wherein: the impedance comprises a first capacitor, the current transformer has two secondary windings and the first capacitor is connected in series between the secondary windings, and the detecting means is arranged to detect a non-zero voltage across the first capacitor above a certain level, the non-zero voltage corresponding to a dc offset greater than a predetermined magnitude, and wherein the detecting means comprises: a pair of transistors each of which is turned on in response to a detected non-zero voltage in a respective one of the two opposite directions across the first capacitor, a resistance and a second capacitor mutually connected in parallel, the parallel combination of the resistance and second capacitor being connected in series with each transistor so that the second capacitor is charged up when either transistor is turned on, and means for monitoring the voltage on the second capacitor.