Patent ID: 7667452

Claim:
A detector circuit for measuring current including a main current transformer ( 1 ) and two substantially identical, serially coupled auxiliary current transformers ( 2 , 3 ), where a main current (I 1 ) induces magnetomotive forces in said main current transformer ( 1 ), said magnetomotive forces being counteracted by magnetomotive forces induced by a compensating current (i 1 ) while at the same time the two serially coupled auxiliary current transformers ( 2 , 3 ) are magnetised in antiphase by means of a modulation signal of a predetermined frequency, where the circuit includes a synchronous rectifier influenced by two times the modulation frequency for generating an adjusting signal to be added to the compensating current (i 1 ) to produce an adjusted compensating current, said adjusted compensating current being used for measuring the current in a first common winding (L 3 ) surrounding the main transformer as well as the serially coupled auxiliary transformers ( 2 , 3 ) while at the same time a possible error signal between the serially coupled auxiliary transformers ( 2 , 3 ) is used for adjusting the compensating current (i 1 ), a negative feedback loop being provided, which automatically tries to establish an equilibrium, wherein the detector circuit further comprises a second common winding (L 5 ) surrounding the main transformer as well as the serially coupled auxiliary transformers ( 2 , 3 ) which detects the possible error signal as alternating voltage, wherein the second common winding compensates for said alternating voltage by means of the negative feedback loop.