GB1345869A - Electrical current measuring instruments - Google Patents

Abstract

1345869 Measuring direct current DEFENCE SECRETARY OF STATE FOR 30 Nov 1971 [30 Nov 1970] 56771/70 Heading G1U In order to measure the direct current i 3 , Fig. 5, through a line L 3 , a magnetic core M is cyclically driven around its hysterisis loop by a multivibrator comprising transistors Q 1 , Q 2 , and oppositely wound coils L 1 , L 2 , and the difference in the average currents through the two transistors Q 1 , Q 2 , is measured, this being a measure of the unknown current i 3 . This arises because in the absence of current i 3 in the line L 3 , a potentiometer RV is adjusted so that the conduction pattern of the transistors Q 1 , Q 2 is symmetrical, and therefore a voltmeter R5, A, reads zero, whereas once a current i 3 is present the conduction pattern becomes asymmetrical to an extent which is proportional to i 3 . In a modification, rather than the asymmetry being determined by an averaging-volt meter R5, A, a voltage representing the asymmetry is derived by a filter F and an amplifier Da, Fig. 7, and used to apply a compensating current i 4 to the core in a direction which opposes i 3 . In a further modification Fig. 6, (not shown), an additional core (M') is coupled to the line L 3 and windings on it are energized in such a way as to cancel signals induced in the line by the main core. Range changing may be effected either by taps on the coils L 1 , L 2 , Fig. 5, Fig. 7, (the taps not being shown) or by taps on the compensating winding.