Patent ID: 7176673
Filing Date: 2007-02-13
Classification: G01R,H02H

Abstract:
1. A direct current detection circuit comprising: a zero-phase current transformer having source lines inserted therethrough and being adapted to magnetically detect current differences among said source lines and to vary self-impedance according to changes in magnetic field generated by said current difference; a voltage divider resistor connected in series to said zero-phase current transformer; a comparison voltage generating circuit for generating a comparison voltage value based on a divided voltage value obtained between said zero-phase current transformer and said voltage divider resistor according to a change in said self-impedance of said zero-phase current transformer; a control circuit for detecting a present direct current value based on said comparison voltage value generated by said comparison voltage generating circuit; an offset current line which is inserted through said zero-phase current transformer; and an offset current generating circuit provided separate from said source lines for constantly generating a constant offset current through said offset current line to make it possible to detect on the basis of said comparison voltage value a direct current value in a range which is difficult to detect accurately on the basis of said comparison voltage value without said offset current because of influence of hysteresis characteristic of said zero-phase current transformer; wherein said control circuit is adapted to detect a current value on the basis of said comparison voltage value generated by said comparison voltage generating circuit and to calculate said present direct current value on the basis of said detected value and the value of said offset current.