AC Meters

1
AC Meters

• continued

2
Warm-ups
3
Outlines

• dArsonval MM with Full Wave Rectification.
• Electro-Dynamometer Movement
• Iron-vane meter movement.
• Loading effects of AC Voltmeters.

4
dArsonval MM withFull Wave Rectification.

• Frequently, it is more desirable to use a
  full-wave rectifier in AC voltmeters because it
  shows higher sensitivity rating compared to ½
  wave rectifier.
• The most frequently used circuit for full-wave
  rectification is the bridge-type rectifier, as
  shown in next figure.

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dArsonval MM withFull Wave Rectification.
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dArsonval MM withFull Wave Rectification.
Figure 2 Full-Wave Bridge Rectifier used in an
AC voltmeter
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dArsonval MM withFull Wave Rectification.

• Positive ½ cycle
• Current flows from Vin trough D2, trough the MM
  from positive to negative, then trough D3.
• Negative ½ cycle
• Current flows from Vin trough D4, trough the MM
  from positive to negative, then trough D1.

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dArsonval MM withFull Wave Rectification.

• Even though the polarity of Vin changes in every
  ½ cycle, the direction of current that flows
  through MM does not change.
• Since current flows through MM on both ½ cycles,
  then the deflection of MM will be greater
  compared to ½ wave rectification.

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dArsonval MM withFull Wave Rectification.
Example 01. For the circuit below, if given that
Vin 10Vrms, Rs 10k? and Ifs 1mA. Calculate
a) Vpb) Vavec) Sacd) Sdc
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dArsonval MM withFull Wave Rectification.
Example 02 By using the same circuit, with Vin
10Vrms, Rm500? and Ifs 1mA, calculate a)
Sdcb) Sacc) Rs
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dArsonval MM withFull Wave Rectification.
Example 03 For the circuit below, given that Vin
10Vrms, Rm500?, Ifs 1mA, Rsh500?, average
forward resistance of 50? and infinite reverse
resistance of each diode, calculate the
followinga) the multiplier, Rsb) the ac
sensitivity, Sacc) the dc sensitivity Sdc.
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The Electro-Dynamometer Movement

• It is one of the most fundamental meters that is
  widely used today.
• It is a current sensitive device similar to the
  DArsonval MM, which  uses essentially the same
  principle of movement.
• Remember  that  the DArsonval movement is a DC
  device and can only measure DC current or AC
  current rectified to DC.

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The Electro-Dynamometer Movement
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The Electro-Dynamometer Movement
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The Electro-Dynamometer Movement

• The Electrodynamometer movement has the same
  basic operating principle as the DArsonval
  meter movement, except that the permanent magnet
  is replaced by fixed coils.
• The moving coil and pointer, which are attached
  to the coil, are suspended between and connected
  in series with the two field coils.

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The Electro-Dynamometer Movement

• The two field coils and moving coil are connected
  in series such that the same current flows
  through each coil.
• Current flow through the three coils in either
  direction causes a magnetic field to be produced
  between the field coils.
• The same current flow through the moving
  coil causes it to act as a magnet exerting
  a force against the spring.

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The Electro-Dynamometer Movement

• If the current is reversed, the field polarity
  and the polarity of the moving coil reverse, and
  the force continues in the same direction.
• Due to this characteristic of the
  Electro-dynamometer movement, it can be used in
  both AC and DC systems to measure current.
• Some voltmeters and ammeters use the
  electrodynamometer.
• However, its most important use is in the
  wattmeter.

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The Electro-Dynamometer Movement

• The basic EDMM is capable of handling much more
  current than a dArsonval could handle.
• However, it has much less Sensitivity rating
  compared to dArsonval MM, which ranges about
  20?/V up to only 100?/V.

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The Electro-Dynamometer Movement

• Example 04
• An EDMM that has a full scale deflection current
  rating of 10mA is to be used in a voltmeter
  circuit. Calculate the value of the multiplier
  for a 10-V range if Rm 50?.

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The Electro-Dynamometer Movement

• Example 05
• An EDMM with a full scale deflection rating of
  10mA is to be used as a 1-mA ammeter. If the
  resistance of the moving coil is 40?, what is the
  value of the shunt resistor, Rsh?

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Loading Effects of AC Voltmeters

• As already being discussed, the sensitivity of AC
  Voltmeters, using either ½ wave or Full wave
  rectification, is always less than the
  sensitivity of the DC Voltmeters.
• Therefore, the loading effect of an AC Voltmeter
  is always greater than that of a DC Voltmeter.

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Loading Effects of AC Voltmeters

• Example 06
• Determine the reading obtained with a DC
  voltmeter at RB when the switch is set at point
  A.
• Determine the reading at the same RB using ½
  wave and Full wave rectifier AC meter
  respectively when the switch is set at point B.
• Given that Ifs 100-mA and set at 10-V dc or
  rms range.

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Conclusions
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Summary
25
QA