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 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
