Simple DC Voltmeter Review

A Simple DC Voltmeter can be constructed by placing a resistor (RS), called a multiplier, in series with the ammeter meter movement, and marking the meter face to read voltage . Voltmeters are connected in parallel with the load (RL) being measured.

When constructing a voltmeter, the resistance of the multiplier must be determined to measure the desired voltage. Equation below is a mathematical representation of the voltmeter’s multiplier resistance.

V = ImRs + ImRm

ImRs = V - ImRm

Rs = V/Im - Rm

Where:

V = Voltage range desired
Im = Meter current
Rm = Meter resistance
Rs = Multiplier resistance or series resistance

When a voltmeter is connected in a circuit, the voltmeter will draw current from that circuit. This current causes a voltage drop across the resistance of the meter, which is subtracted from the voltage being measured by the meter. This reduction in voltage is known as the loading effect and can have a serious effect on measurement accuracy, especially for low current circuits.

The accuracy of a voltmeter (Kv) is defined as the ratio of measured voltage when the meter is in the circuit (Vw) to the voltage measured with the meter out of the circuit. The equation below is a mathematical representation of the accuracy of a voltmeter, or true voltage (Vo).

Kv = Vw/Vo

Meter accuracy can also be determined by comparing the relationship between the input and circuit resistances using Ohm’s Law as described below.

Kv = Vw/Vo
and
Vw-ImRin

ImRm/Vo
and
Im=Vo/(Ro = Rin)

= [ (VoRin)/(Ro + Rin) ]/Vo

Kv = Rin/(Ro+ Rin)

Where:

Im = Meter Current
Vo = True voltage
Ro = Circuit resistance
Rin = Input resisitance of the voltmeter
Kw = Indicated voltage
Kv = Meter accuracy