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True rms voltmeter

Abstract:

The mirror of a galvanometer G reflects a beam of light from a filament lamp to a split photocell P1P2. The bridge circuit RIPlP2 is arranged to be balanced for “zero” input to the galvanometer, and meter M, connected in the circuit externally, indicates zero. Any small deflexion of the galvanometer by an input signal causes the illumination of onehalf of the split photocell to be increased at the expense of the other. The bridge then becomes unbalanced and a resultant current flows through meter M. Negative feedback, controlling the sensitivity and stability, the speed and the linearity of response, is derived across the feedback resistor R, which is connected so that the output current tends to oppose the input current through the galvanometer. The system depends inherently on the sensitivity of the photocell to differential changes of illumination of its two halves. The large current gain obtainable in this way through a …