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UNIVERSAL HIGH-RESISTANCE VOLTMETER

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					                                                        C I R C U I T     I D E A S




                                                                                                              TABLE I

UNIVERSAL HIGH-RESISTANCE                                                                           Position 1 of Function Switch
                                                                                                 Edc input            Meter Current



VOLTMETER
                                                                                                 5.00V                   44 µA
                                                                                                 4.00V                   34 µA
                                                                           I                     3.00V                   24 µA
                                                                        VED
                                                                     DWI                         2.00V                   14 µA
                                                                S.C.                             1.00V                   4 µA
        YOGESH KATARIA
                                                                                                              TABLE II


T
       he full-scale deflection of the uni-       Calculate RI according to the applica-
       versal high-input-resistance volt-     tion from one of the following equations:             Position 2 of Function Switch
       meter circuit shown in the figure                                                         Erms input          Meter Current
                                                 (a) DC voltmeter: RIA = full-scale EDC/IFS
                                                                                                 5V                      46 µA
                                                          (b) RMS AC voltmeter (sine             4V                      36 µA
                                                       wave only): RIB = 0.9 full-scale          3V                      26 µA
                                                       ERMS/ IFS                                 2V                      18 µA
                                                          (c) Peak reading voltmeter             1V                      10 µA
                                                       (sine wave only): RIC = 0.636 full-
                                                       scale EPK/IFS                                         TABLE III
                                                            (d) Peak-to-peak AC voltme-             Position 3 of Function Switch
                                                       ter (sine wave only): RID = 0.318         EPk input             Meter Current
                                                       full-scale EPK-TO-PK / IFS                5V peak                  46 µA
                                                            The term IFS in the above            4V peak                  36 µA
                                                       equations refers to meter’s full-         3V peak                  26 µA
                                                                                                 2V peak                  16 µA
                                                       scale deflection current rating in        1V peak                  6 µA
                                                       amperes.
                                                            It must be noted that neither                     TABLE IV
                                                       meter resistance nor diode volt-
                                                                                                    Position 4 of Function Switch
                                                       age drops affects meter current.
                                                                                                 EPk-To-Pk            Meter Current
                                                            Note: The results obtained
                                                                                                 5V peak to peak          46 µA
                                                       during practical testing of the cir-      4V peak to peak          36 µA
depends on the function switch position       cuit in EFY lab are tabulated in Tables I          3V peak to peak          26 µA
as follows:                                   through IV.                                        2V peak to peak          16 µA
    (a) 5V DC on position 1                       A high-input-resistance op-amp, a              1V peak to peak          7 µA
    (b) 5V AC rms in position 2               bridge rectifier, a microammeter, and a
    (c) 5V peak AC in position 3              few other discrete components are all that      ing the value of the resistor connected be-
    (d) 5V AC peak-to-peak in position 4      are required to realise this versatile cir-     tween the inverting input terminal of the
    The circuit is basically a voltage-to-    cuit. This circuit can be used for mea-         op-amp and ground. The voltage to be
current converter. The design procedure       surement of DC, AC RMS, AC peak, or             measured is connected to non-inverting in-
is as follows:                                AC peak-to-peak voltage by simply chang-        put of the op-amp.




                                                    ELECTRONICS FOR YOU ❚  FEBRUARY 2000

				
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