A novel circuit for electronic power-factor meters and wattmeters, and a novel division device

Abstract

Three novel electronic measuring instruments are presented, which may be used to measure power, power factor (pf), voltage, current, or impedance. They use two tubes, operated as cathode followers, in a push-pull fashion, so that the inherent property of a wide-range linear characteristic is employed to advantage. The new principle underlying the operation of the tubes involves the commutation of their currents in alternate half-cycles by means of a square-wave generator. The resulting average plate current is a direct measure of the product I cos ϕ. A rectifier gives a direct current proportional to the alternating voltage, and an electrodynamic instrument multiplies the two direct currents, thus giving a direct reading of the power. For the direct reading of pf, two direct currents are obtained, one being proportional to the product V cos ϕ and the other proportional to V. Their quotient is a direct measure of cos ϕ. The division is effected by means of a twin logarithmic d-c instrument having a movable scale. The instruments are characterized by their simplicity, reliability, high accuracy, and stability. They have exceptionally good features and predictable performance over a frequency range extending from the lowest power frequencies to a predicted value of about 30 megacycles (mc) when using high-frequency tubes; the main limiting factor being the stray capacities across the output of the square-wave generator. There are, however, some other factors which tend to lower this operating limit, and 10 mc is considered a reasonable higher limit.