Electronic Electrical Devices  Electronic Electrical Components
A Simple DC Voltmeter can be constructed by placing a resistor
(R_{S}), 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 (R_{L}) 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 = I_{m}R_{s} + I_{m}R_{m}
I_{m}R_{s} = V  I_{m}R_{m}
R_{s} = V/I_{m}  R_{m} Where:
V = Voltage range desired
I_{m} = Meter current
R_{m} = Meter resistance
R_{s}
= 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 (K_{v}) is defined as the ratio of measured voltage when the meter is
in the circuit (V_{w}) 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
(V_{o}).
K_{v} = V_{w}/V_{o}
Meter accuracy can also be determined by comparing the relationship between the input and
circuit resistances using Ohm’s Law as described below.
K_{v} = V_{w}/V_{o}
and
V_{w}I_{m}R_{in}
I_{m}R_{m}/V_{o}
and
I_{m}=Vo/(Ro = Rin)
= [ (V_{o}R_{in})/(R_{o} + R_{in})
]/V_{o}
K_{v} = R_{in}/(R_{o}+ R_{in})
Where:
I_{m} = Meter Current
V_{o} = True voltage
R_{o} = Circuit resistance
R_{in} = Input resisitance of the voltmeter
K_{w} = Indicated voltage
K_{v} = Meter accuracy
