Because voltmeters are always hooked to a circuit in parallel, they become part of the circuit and reduce the total resistance of the circuit. If a voltmeter has a resistance that is too low in comparison to the circuit, it will give a false measurement. The false reading is due to the meter changing the circuit by lowering the resistance, which increases the current flow in the circuit. The effect a voltmeter has on the circuit to which it is attached is sometimes referred to as "loading effect" of the meter. The loading effect a voltmeter has on a circuit is determined by the total resistance of the circuit in relation to the impedance of the voltmeter.
Every voltmeter has an impedance, which is the meter's internal resistance. The impedance of a conventional analog voltmeter is expressed in "ohms per volt." The amount of resistance an analog voltmeter represents to the circuit changes in relation to the scale on which it is placed. Digital voltmeters, on the other hand, have a fixed impedance which does not change from scale to scale and is usually 10 M ohms or more.
Impedance is the biggest difference between analog and digital voltmeters. Since most digital voltmeters have 50 times more impedance than analog voltmeters, digital meters are more accurate when measuring voltage in high resistance circuits.
For example, if you are using a low impedance (20,000 ohms per volt) analog meter on the 20 volt scale (the voltmeter represents 400,000 ohms resistance to the circuit) to measure voltage drop across a 1,000,000 ohm component in a circuit, two and a half times as much current is flowing through the meter than through the component. You are no longer measuring just that component, but the component plus your meter, giving you a false reading of the actual voltage drop across the component. This situation might lead you to believe the voltage at the component is low or that there is high resistance somewhere in the circuit or that the component is defective when it is just the meter you are using.
If you use a digital meter with 10 million ohms of impedance to test the same component, only 1/10 of the current will flow through the meter, which means it has very little effect on the circuit being measured.
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