lecture notes 08.ppt
TRANSCRIPT
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AC Meters
continued
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Warm-ups
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Outlines
• d’Arsonval MM with Full Wave Rectification.
• Electro-Dynamometer Movement• Iron-vane meter movement.• Loading effects of AC Voltmeters.
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d’Arsonval 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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d’Arsonval MM withFull Wave Rectification.
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d’Arsonval MM withFull Wave Rectification.
D1 D2
D3
D4
Rs
Vin
Rm
IfsVout
Figure 2: Full-Wave Bridge Rectifier used in an AC voltmeter
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d’Arsonval 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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d’Arsonval 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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d’Arsonval MM withFull Wave Rectification.
D1 D2
D3
D4
Rs
Vin
Rm
IfsVout
Example 01.
For the circuit below, if given that Vin = 10Vrms, Rs= 10kΩ and Ifs = 1mA. Calculate: a) Vp
b) Vave
c) Sac
d) Sdc
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d’Arsonval MM withFull Wave Rectification.
D1 D2
D3
D4
Rs
Vin
Rm
IfsVout
Example 02:
By using the same circuit, with Vin = 10Vrms, Rm=500Ω and Ifs = 1mA, calculate: a) Sdc
b) Sac
c) Rs
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d’Arsonval MM withFull Wave Rectification.
D1 D2
D3
D4
Rs
Vin
Rm
IfsRsh
Example 03
For the circuit below, given that Vin = 10Vrms, Rm=500Ω, Ifs = 1mA, Rsh=500Ω, average forward resistance of 50Ω and infinite reverse resistance of each diode, calculate the following:a) the multiplier, Rs
b) the ac sensitivity, Sac
c) 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 D’Arsonval MM, which uses essentially the same principle of movement.
• Remember that the D’Arsonval 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 D’Arsonval 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 d’Arsonval could handle.
• However, it has much less Sensitivity rating compared to d’Arsonval MM, which ranges about 20Ω/V up to only 100Ω/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 = 50Ω.
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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 40Ω, 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 06a) Determine the reading obtained with a
DC voltmeter at RB when the switch is set at point A.
b) 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-A and set at 10-V dc or rms range.
RB
E = 20V
RA 10kΩ
5.5kΩ
AC20Vrms
A B
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Conclusions
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Summary
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Q&A