GB1488262A - Measuring direct current - Google Patents

Abstract

1488262 Measuring D C current T I (GROUP SERVICES) Ltd 3 April 1975 [11 June 1974] 25752/74 Heading G1U A D.C. current to be measured flowing in conductor 6 linking the core of a saturable transformer 1 induces a constant flux, which sets the mean level on which a varying flux due to an alternating current in a primary winding 2 is superimposed. Since the magnitude of the A.C. is enough to saturate the core, voltage pulses are induced in a secondary winding 3 at the zero crossing points only. The spacing of the pulses in each cycle is determined by the mean level of the flux, i.e. by the D.C. current in conductor 6. An amplifier 7 connected as a bi-stable flip-flop converts the pulse pairs into a pulse train whose width is proportional to the D.C. current. This is averaged 8, amplified 9 and fed to a compensating winding 4 on the transformer to oppose the flux due to the current being measured. The amplifier 9 output current is thus automatically adjusted to cancel the measured current, resulting in evenly spaced output pulses from winding 3. A network 11 operates such that if no pulses are produced e.g. if the mean level is so high as to result in a unipolar flux, the amplifier self-oscillates, thereby "hunting" to bring the flux to a detectable position. In a modified embodiment , Fig. 3 (not shown) the primary, secondary and compensating windings are combined into one. The feedback current is fed to one input of a summing amplifier whose other input receives the A.C. signal and whose output feeds the transformer winding, via a power amplifier stage. The device may be used for measuring automobile battery current, in which case mu-metal screening may be necessary for the transformer. Alternatively, the bi-stable input can be monitored to ignore spurious 'spiky' signals. The alternating current signal should have a substantially linear rate of charge around the zero-crossing points, e.g. a triangular waveform or a pure sinusoid.