The full-scale deflection of the universal high-input-resistance voltmeter circuit shown in the figure depends on the function switch position as follows:

(a) 5V DC on position 1
(b) 5V AC rms in position 2
(c) 5V peak AC in position 3
(d) 5V AC peak-to-peak in position 4

The circuit is basically a voltage-tocurrent converter. The design procedure is as follows:

Calculate RI according to the application from one of the following equations:
(a) DC voltmeter: RIA = full-scale EDC/IFS
(b) RMS AC voltmeter (sine wave only): RIB = 0.9 full-scale ERMS/ IFS
(c) Peak reading voltmeter (sine wave only): RIC = 0.636 fullscale EPK/IFS
(d) Peak-to-peak AC voltmeter (sine wave only): RID = 0.318 full-scale EPK-TO-PK / IFS

The term IFS in the above equations refers to meter’s full scale deflection current rating in amperes. It must be noted that neither meter resistance nor diode voltage drops affects meter current.

Note: The results obtained during practical testing of the circuit in EFY lab are tabulated in Tables I through IV. A high-input-resistance op-amp, a bridge rectifier, a microammeter, and a few other discrete components are all that are required to realise this versatile circuit.

This circuit can be used for measurement of DC, AC RMS, AC peak, or AC peak-to-peak voltage by simply changing the value of the resistor connected between the inverting input terminal of the op-amp and ground. The voltage to be measured is connected to non-inverting input of the op-amp.

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