DC Voltmeters

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DC Voltmeters

• Still in Chapter 02

2
Warm-ups

• Stand-up.
• Take a deep breath 3x.
• Sit down and relax.
• Enjoy a short movie.

3
Warm-ups
4
Objectives

• At the end of this lecture, students should be
  able to
• explain the purpose of multipliers put in series
  with a meter movements.
• calculate the multiplier resistance of a
  Voltmeter.
• explain and calculate the voltmeter loading
  effects.

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DC Voltmeter

• Most D'Arsonval meter movements are sensitive
  devices- have full-scale deflection current
  ratings as low as 50 µA, with an (internal) wire
  resistance of less than 1000 O.
• This makes for a voltmeter with a full-scale
  rating of only 50 mV (50 µA X 1000 O)!.

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DC Voltmeter

• In order to build voltmeters with practical
  scales from such sensitive movements, we need to
  reduce the voltage down to a level the movement
  can handle
• The basic dArsonval meter movement can be
  converted to a DC Voltmeter by connecting a
  Multiplier (Rs) with the meter movement.

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DC Voltmeter
Figure 1 The dArsonval meter movement used in a
DC voltmeter
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DC Voltmeter

• The purposes of the Multiplier (Rs)
• to extend the voltage range of the meter
  movements.
• to limit the current trough the dArsonval meter
  movement to a maximum full-scale deflection
  current.

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DC Voltmeter

• To find Rs value, we may first determine the
  Sensitivity, S of the meter movements.
• Sensitivity, S is the reciprocal? of the
  full-scale deflection current, as below

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DC Voltmeter

• The units of sensitivity is Ohms per Volts.

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DC Voltmeter

• Voltage measurements are made by placing the
  Voltmeter across the resistance of interest.

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DC Voltmeter

• This in effect places the total voltmeter
  resistances in parallel with the measured circuit
  resistance.
• Therefore, it is desirable to make the voltmeter
  resistance much-much more higher than the circuit
  resistance.

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DC Voltmeter

• Since the value of the multiplier is different
  for each range, total resistance is difficult to
  express.
• More meaningful info can be conveyed via
  Sensitivity rating of the instrument- telling the
  resistance of the instrument for a-one-volt
  range.

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DC Voltmeter

• To determine the total resistance (RsRm)? that a
  voltmeter present to a circuit, just multiply the
  Sensitivity by the Range.
• Thus
• Rs Rm S X Range
• Rs S X Range Rm

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DC Voltmeter

• Example 1.
• Calculate the sensitivity of a 100-mA meter
  movement which is to be used as a DC Voltmeter.
• Example 2.
• Calculate the value of the multiplier Rs on the
  50-V range of a DC Voltmeter that used 200-mA
  meter movements with an internal resistance of
  1.2k?.

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DC Voltmeter

• Example 3.
• Calculate the values of Rs for the multiple-
  range DC Voltmeter circuits as shown below

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DC Voltmeter

• Example 4.
• Calculate the values of Rs for the multiple-
  range DC Voltmeter circuits as shown below

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Voltmeter Loading Effects

• When a voltmeter is used to measure the voltage
  across a circuit component, the voltmeter circuit
  itself is in parallel with the circuit component.
• Since the parallel combination of two resistors
  is less than either resistor alone, the resistor
  seen by the source is less with the voltmeter
  connector than without.

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Voltmeter Loading Effects

• Therefore, the voltage across the component is
  less whenever the voltmeter is connected.
• The decrease in voltage maybe negligible or
  appreciable, depending on the Sensitivity of the
  voltmeter being used.
• This effect is called voltmeter loading and the
  resulting error is called loading error.

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Voltmeter Loading Effects

• Example 5
• Two different voltmeters are used to measure the
  voltage across RB in the circuit below. The
  meters areMeter A S 1k?/VRm0.2k? Range
  10VMeter B S20k?/VRm2.2k? Range 10V
• Calculate
• Voltage across RB without any meter.
• Voltage across RB when meter A is used.
• Voltage across RB when meter B is used.
• Loading Errors in both voltmeter readings.

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Voltmeter Loading Effects

• Example 6
• Find the voltage reading and the percentage of
  loading error of each reading obtained with a
  voltmeter on
• Its 5-V range.
• Its 10-V range
• Its 50-V range.
• The meter has a 20-k?/V sensitivity and connected
  across RA.

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Summary

• In this sub-topic, we have learned about
• the purpose of multipliers put in series with a
  meter movements.
• calculation of the multiplier resistance of a
  Voltmeter.
• voltmeter loading effects.

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Conclusion

• The basic dArsonval meter movement can be
  converted to a DC Voltmeter by connecting a
  Multiplier (Rs) with the meter movement.
• Sensitivity, S is the reciprocal of the
  full-scale deflection current.
• Therefore, it is desirable to make the voltmeter
  resistance much-much more higher than the circuit
  resistance.

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Evaluation

• Quiz 04