Electronic Electrical Devices | Electronic Electrical Components
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
V = ImRs + ImRm
ImRs = V - ImRm
Rs = V/Im - Rm
V = Voltage range desired
Im = Meter current
Rm = Meter resistance
= 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
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
Im=Vo/(Ro = Rin)
= [ (VoRin)/(Ro + Rin)
Kv = Rin/(Ro+ Rin)
Im = Meter Current
Vo = True voltage
Ro = Circuit resistance
Rin = Input resisitance of the voltmeter
Kw = Indicated voltage
Kv = Meter accuracy