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Electrical Measurement
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Learning Outcomes• Explain how a moving-coil movement
measures current.
•Calculate the internal resistance of a moving-
coil voltmeter.
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Learning Outcomes• Calculate the loading effect of a voltmeter.
• Design an ohmmeter using a moving-coil
movement.
• Use a Wheatstone bridge for accurate
resistance measurement.
•
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11-1 Moving-Coil Meters• Used for measuring current, voltage,
resistance.
• Depends on moving-coil movement ord’Arsonval movement!, which uses the
interaction of a current-carr"ing coil and a
permanent magnet.
• #perates on the motor principle, which
states that a magnetic field exerts a force on
a current flowing perpendicular to it.
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11-1 Moving-Coil MetersWhen current flows through the coil, it becomes
an electromagnet. $s current increases, the
electromagnetic
force increases,
moving the
pointer acrossthe scale.
Moving coil —— —— Soft-iron core
Figure 11-1 Moving-coil meter
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11-1 Moving-Coil Meters%wo &e" characteristics'
sensitivity
the value of current that causes full-scaledeflection of a d($rsonval meter movement)
determined b" the strength of the permanent
magnet and number of turns on the moving coil
resistance
comes from the resistance of the wire in the
coil) more turns on the moving coil means
higher resistance
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11-2 The Ammeter • Used for measuring the current flowing in a
branch of a circuit.
• %o measure larger currents, it must beconnected in parallel to a shunt resistor.
shunt resistor
a resistor connected in parallel with anammeter movement so that the full-scale
reading corresponds to a current greater than
the current capabilit" of the movement
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11-2 The Ammeter • *or a moving-coil movement with a sensitivit"
of +. m$ and a resistance of Ω,
• %herefore the calibrating
resistance is
Ω
5050
1.0mA
mVT
I
V R
Ω
232750C R
Figure 11-2 Moving-coil ammeter with
shunt resistor
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11-2 The Ammeter • /ensitive 0-µ $ movements are used for
measurements in electronic circuits.
• 0.-m$ movements are used with electric
power circuits.
• $ calibrating or swamping! resistor is
connected in series with the moving coil to
bring the total resistance of the movement
up to some specific value.
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11-2 The Ammeter %o use the +.-m$ movement to measure
current up to + $, we arrange the meter circuit
as shown in *ig ++-1,forming two parallel
branches with 222
times as much current
flowing through theshunt resistor as
through the meter
movement.Figure 11-3 Measuring current
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11-3 The Voltmeter • $ useful instrument for chec&ing the operation
of a circuit.
• With suitable clips on the test leads, we canconnect a voltmeter across an" part of a circuit
without having to disconnect an" components.
Figure 11-5 Simple voltmeter
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Key Termsvoltmeter sensitivity
a parameter indicating the load that a
given voltmeter adds to a circuit,
usuall" expressed as resistance per
volt of full-scale reading. When used
as a voltmeter, a 0-m$ movement
has a voltmeter sensitivit" of &Ω34.
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11-4 Voltmeter Loading !!ect• $n ideal voltmeter should draw 5ero current
and have an infinitel" high internal resistance.
• 6owever, a moving-coil voltmeter shoulddraw some current in order to deflect the
pointer.
• %he smaller the current re7uired for full-scaledeflection, the more sensitive the voltmeter and
the smaller the loading effect it has on the
circuit tested.
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11-" #esistance Measurement• 4er" low resistances ma" be measured b"
appl"ing #hm(s law to readings obtained
from a voltmeter and ammeter.
• 8uic& chec&s of circuit resistance can be
accomplished with an ohmmeter, whichmeasures resistance directl".
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11-" #esistance Measurement• %he nonlinear scale of an ohmmeter ma&eslow resistances easier to read but high
resistances harder to read.• With a 0-µ $ move-
ment the total internal
resistance of the ohm-meter in *ig ++-2a! is
Ωk 50μ0
VT 90
5.4
I
E R
Figure 11- Simple ohmmeter
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Key Termsgalvanometer
a sensitive moving-coil meter in which
the pointer is at the center of its range
when the current through the coil is
5ero) essentiall", a sensitive ammeter
that can measure current flowing
through it in either direction
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.
Figure 11-13 !lectrodynamometermovement
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•
Figure 11-1" !lectrodynamometer movementas a wattmeter
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• /ince the deflection of the pointer in a
moving-coil movement is proportional to the
current through the coil, the movement canbe used as the measuring device in an
ammeter.
• $ voltmeter with a number of voltageranges can be constructed from a moving-
coil movement b" placing multiplier
resistors in series with it.
$ummary
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• $n analog voltmeter can reduce the
voltage being measured.
• $n ohmmeter with a number of resistance
ranges can be constructed using a moving-
coil movement, a batter", shunt resistors,
and series resistors.• %he Wheatstone bridge can be used for
accurate measurements of resistances.
$ummary
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•END