..Sorry i meant, an "op amp", not comparator. It appeared to be using the principle of "v on one terminal = v on the other". The nxt question, if i may, was what kind of frequency woudl you expect the shown fluxgate to be good up to? (frequency of the DC in the mains)...and what is the expected frequency of DC in the mains?
Yes, sorry what i meant is, the error amp's aux coil has to cancel out the field in the torroid.....so the whole aux coil driving circuit has to be able to repond quick enough to do the cancellation.......AYK, the "DC in the mains" will always be fluctuating.....since it is caused by fault connection of primary side HV DC bus to earth.m AYK, the "ground" output of the mains diode bridge is fluctuating in voltage about earth....thus causes the fluctuation in the fault DC current.