Simple DC Voltmeter Review

Electronic Electrical Devices

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_mR_s + I_mR_m

I_mR_s = V - I_mR_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_mR_in

I_mR_m/V_o
and
I_m=Vo/(Ro = Rin)

= [ (V_oR_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