Control circuit for moving iron instrument - has capacitor in parallel with coil to generate demagnetising current

摘要

The control winding of a ferromagnetic device (1) is fed from a frequency/current converter. The output from the converter is coupled to an emitter follower transistor amplifier (2) whose collector circuit is connected by way of a capacitor (3) to the positive power supply. A second capacitor (7) forms a parallel resonant circuit with the control winding (1). An RC input circuit (8) feeds the base electrode of a switching transistor (6). The value of the resistance and capacitance are chosen to provide a delay corresponding to a predetermined fraction of the input pulse duration. The resonant effect of the second capacitor (7) in parallel with the control winding (1) ensures that a demagnetising current for the moving iron element is passed each cycle.