GB1325533A - Apparatus for measuring the rms value of current or voltage - Google Patents

Abstract

1325533 R. M. S. Current and voltage measurement WAYNE KERR CO Ltd 2 Sept 1970 [2 June 1969] 27708/69 Heading G1U In a system for measuring R. M. S. values of currents Fig. 1 or voltages Fig. 2 (not shown) both the signal to be measured and a regulating signal applied to a power dissipating device so arranged that the total power dissipated is equal to the arithmetic sum of the powers that would be dissipated by the unknown and regulating signals if applied separately, the regulating signal is adjusted so that the total power dissipated is constant and the change in the regulating signal necessary to achieve constant power is measured. The powers dissipated by the two signals can be made independent of each other either by arranging that the unknown and regulating signals are non-coherent (e.g. direct and alternating signals) or by arranging that the power dissipating device includes two separate resistive loads. In a first embodiment Fig. 1 for measuring current, the unknown current Ia (this being its R.M.S. value) is passed from an input terminal 10 to an isolating injection unit 12, a filament lamp 14 constituting the power dissipating device, and a filter 16 to earth 11. A regulating current Ir from a source 19 is also passed through the lamp 14 to the filter 16 where it is separated from the current to be measured Ia and passed through a second filament lamp 21 to earth 20. The regulating current Ir is controlled by a photo-resistive cell 17 adjacent to the lamp 14, in such a way that the resistance of the cell remains constant and hence the total power dissipated remains constant. Thus Ir<SP>2</SP> = k - Ia<SP>2</SP> where k is a constant, and Ir will change with Ia. Ir is not, however, a linear function of Ia but a further circuit similar to that already described is used to produce a current Im which is a linear function of Ia. To effect this, a current Im from a source 24 is also passed through the second lamp 21, and by means of a photo-resistive cell 23 the current Im is adjusted to maintain the power dissipated by the second lamp 21 constant. It then follows that if the system is initially set up so that when Ia = O, Im = O, then Im = Ia. Thus providing Ia and Ir, and Ir and Im are non- coherent, then Im = Ia irrespective of the waveform of Ia. Im can be indicated in any appropriate manner, e.g. on a meter 25. In a second embodiment Fig. 2 (not shown) a voltage Vm is produced equal to the R.M.S value Va of an input voltage irrespective of its waveform.