Voltmeter circuit

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.
Voltmeter circuit - schematic

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-to-current 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 full-scale 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.

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