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 fullscale 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.
TABLE I
Position 1 of Function Switch
Edc input Meter Current
5.00V 44 μA
4.00V 34 μA
3.00V 24 μA
2.00V 14 μA
1.00V 4 μA
TABLE II
Position 2 of Function Switch
Erms input Meter Current
5V 46 μA
4V 36 μA
3V 26 μA
2V 18 μA
1V 10 μA
TABLE III
Position 3 of Function Switch
EPk input Meter Current
5V peak 46 μA
4V peak 36 μA
3V peak 26 μA
2V peak 16 μA
1V peak 6 μA
TABLE IV
Position 4 of Function Switch
EPk-To-Pk Meter Current
5V peak to peak 46 μA
4V peak to peak 36 μA
3V peak to peak 26 μA
2V peak to peak 16 μA
1V peak to peak 7 μA
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