High Resistance Voltmeter

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.
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
1 of Function Switch
Edc input    Meter Current

5.00V            44 ?A

4.00V            34 ?A

3.00V            24 ?A

2.00V            14 ?A

4 ?A


Position 2 of Function Switch

Erms input    Meter Current

46 ?A

36 ?A

26 ?A

18 ?A

10 ?A


Position 3 of Function Switch

EPk input      Meter Current

5V peak
46 ?A

4V peak
36 ?A

3V peak
26 ?A

2V peak
16 ?A

1V peak
6 ?A


Position 4 of Function Switch
Meter Current

5V peak to peak
46 ?A
4V peak to peak
36 ?A
3V peak to peak
26 ?A
2V peak to peak
16 ?A
1V peak to peak
7 ?A

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