Rectifier-Type Meters

Author: E.E. Kimberly

Alternating-current voltmeters of any particular type require a definite volt-ampere input for a given pointer deflection, regardless of the range of voltage of the instrument. Hence,, a low-voltage meter must draw from the circuit more current than a high-voltage meter. If the current is small through the device of low impedance across which the voltage is to be measured, the low impedance of the low-reading voltmeter may cause sufficient current to be by-passed around the device to seriously disturb the measurement.

The rectifier-type voltmeter consists of a sensitive D'Arsonval movement in conjunction with a rectifier unit of copper oxide. The impedance of such an instrument may be made much higher than that of an iron-vane or dynamometer-type instrument, but the readings cannot be relied on within 5 per cent of full-scale value. Such voltmeters are calibrated for sinusoidal wave shape only. Low-range low-impedance a-c ammeters are also available in this type. A circuit for a rectifier-type instrument is shown in Fig. 16-12.

Fig. 16-12. Rectifier-Type Instrument Circuit