DC METER EFFECT
Voltmeter Loading Effect
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- When voltmeter resistance is not high enough, connecting it across a circuit can reduce the measured voltage.
This effect is called loading down the circuit, because the measured voltage decreases due to the additional load current for the meter.
(a) High-resistance series circuit without voltmeter.
(b) Connecting voltmeter across one of the series resistances.
(c) Reduced R and V between points 1 and 2 caused by the voltmeter as a parallel branch across R2. The R2V is the equivalent of R2 and RV in parallel.
(a) High-resistance series circuit without voltmeter.
(b) Same voltages in circuit with voltmeter connected, because RV is so high.
The loading effect is minimized by using a voltmeter with a resistance much greater than the resistance across which the voltage is measured.
The loading effect of a voltmeter causes too low a voltage reading because RV is too low as a parallel resistance.
The digital multimeter (DMM) has practically no loading effect as a voltmeter because its input is usually 10 to 20 MΩ on all ranges.
The following formula can be used to correct for loading:
V = VM + [R1R2/RV(R1 + R2)]VM
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