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Chapter 5: DC Voltmeter. Voltmeter Circuit Extremely high resistance Always connected across or in parallel with the points in a circuit at which the voltage is to be measured

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chapter 5 dc voltmeter
Chapter 5: DC Voltmeter
  • Voltmeter Circuit
    • Extremely high resistance
    • Always connected across or in parallel with the points in a circuit at which the voltage is to be measured
    • The voltmeter range is increased by connecting a multiplier resistance with the instrument (single or individual type of extension of range).
Example 5.1: A PMMC instrument with FSD of 100 A and a coil resistance of 1k is to be converted into a voltmeter. Determine the required multiplier resistance if the voltmeter is to measure 50V at full scale (Figure 3-15). Also calculate the applied voltage when the instrument indicate 0.8, 0.5, and 0.2 of FSD.


For V=50V FSD

At 0.8 FSD:

At 0.5 FSD:

At 0.2 FSD:

  • The voltmeter designed in Example 5.1 has a total resistance of Rv = Rs+Rm = 500k . Since the instrument measures 50V at full scale, its resistance per volt or sensitivity is 500k / 50V =10 k / V.
  • The sensitivity of a voltmeter is always specified by the manufacturer, and it is frequently printed on the scale of the instrument.
Swamping Resistance
    • The change in coil resistance (Rm) with temperature change can introduce errors in a PMMC voltmeter.
The presence of the voltmeter multiplier resistance (Rs) tends to swamp coil resistance changes, except for low voltage ranges where Rx is not very much larger than Rm.
  • Multi-range Voltmeter
    • In Figure 3.16(a), only one of the three multiplier resistors is connected in series with the meter at any time.
    • The range of this voltmeter is

V = Im(Rm+R)

where R can be R1, R2, or R3

In Figure 3.16(b), the multiplier resistors are connected in series, and each junction is connected to one of the switch terminals.
  • The range of this voltmeter can also be calculated from the equation

V = Im(Rm+R)

where R can now be R1, R1+R2, or R1+R2 +R3.

  • Of the two circuits, the on in Figure 3.16(b) is the least expensive to construct. This is because all of the multiplier resistors in Figure 3.16(a) must be special (nonstandard) values, while in Figure 3.16(b) only R1 is a special resistor.
Example 5.2: A PMMC instrument with FSD = 50A and Rm = 1700 is to be employed as a voltmeter with ranges of 10V, 50V, and 100V. Calculate the required values of multiplier resistors for the circuits of Figure 3.16(a) and (b).


Example 5.3: From Example 5.2, Calculate Rin for each range


Find sensitivity

Range V1 = 10V

Range V2 = 50V

Range V3 = 100V

Example 5.4 A voltmeter with sensitivity of 20kΩ/V is used for measuring a voltage across R2 with range of 50V as shown in figure below. Calculate

a) reading voltage

b) accuracy of measurement

c) error of measurement