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DC voltmeter is used to measure the DC voltage across any two points of electric circuit. If we place a resistor in series with the Permanent Magnet Moving Coil (PMMC) galvanometer, then the entire combination together acts as DC voltmeter.
The series resistance, which is used in DC voltmeter is also called series multiplier resistance or multiplier. It basically limits the amount of current that flows through galvanometer in order to prevent the meter current from exceeding the full scale deflection value.
Figure 2.4.1
Rs -Series multiplier resistance (used to determine the sensitivity (S) of the meter movement: π = 1/ πΌππ π (Ξ©/V)
Rm -Internal resistance of the movement
Im -Full scale deflection current of the movement (Ifsd)
V -Full range voltage of the instrument
Equations related to DC Voltmeter
Current in series;
Is = Im (Eq. 1)
From Ohm Law:
V = Im (Rm + Rm)
= ImRs + ImRm (Eq. 2)
Rs = V/Im - Rm (Eq. 3)
If using S;
Rs = π Γ π ππππ β π π (Eq. 4)
EXAMPLE 2.4.1
A basic Dβ Arsonval movement with a full-scale deflection of 50 Β΅A and internal resistance of 500β¦ is used as a DC voltmeter. Determine the value of the multiplier resistance needed to measure a voltage range of 0-10V.
SOLUTION
Rs = V/Im - Rm
Rs = 10V/50 Β΅A - 500β¦
= 199.5 ππΊ
Figure 2.4.2
DC Voltmeter: Multirange
A DC voltmeter can be converted into a multirange voltmeter by connecting a number of resistors (multipliers) in series with the meter movement.
Figure 2.4.3
EXAMPLE 2.4.2
Convert a basic Dβ Arsonval movement with an internal resistance of 100β¦ and a full scale deflection current of 1mA into a multirange dc voltmeter with voltage ranges of 0-15V and 0- 50V.
SOLUTION
Range 0 β 15V
RT1= V/IM R1 = RT1 - Rm
= 15/1Ma R1 = 15k β 100
= 15kβ¦ R1 = 14.9kβ¦
Range 0 β 50V
RT2= V/IM R2 = RT2 β Rm-R1
= 50/1mA R1 = 50k β 100-14.9k
= 50kβ¦ R1 = 35kβ¦
How to use a voltmeter:
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