Voltmeter Circuit

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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
• The voltmeter range is increased by connecting a
  multiplier resistance with the instrument (single
  or individual type of extension of range).

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• 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.
• Solution
• For V50V FSD

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• 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 RsRm 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.

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• Swamping Resistance
• The change in coil resistance (Rm) with
  temperature change can introduce errors in a PMMC
  voltmeter.

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• 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(RmR)
• where R can be R1, R2, or R3

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• 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(RmR)
• where R can now be R1, R1R2, or R1R2 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.

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• 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).
• Solution

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• Voltmeter Internal Resistance Rin

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• Example 5.3 From Example 5.2, Calculate Rin for
  each range
• Solution
• Find sensitivity
• Range V1 10V
• Range V2 50V
• Range V3 100V

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• Voltmeter Loading Effect

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• Example 5.4 A voltmeter with sensitivity of
  20k?/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
• Solution

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