Electricity

1
Electricity Principles Applications Sixth
Edition
Richard J. Fowler
Chapter 15 Instruments and Measurements
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INTRODUCTION

• Digital Ohmmeter
• Basic Meter Movement
• Analog Meters
• Current Transformer
• Meter Loading
• Bridges

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Facts About Instruments and Measurements

• Many analog instruments use a dArsonval meter
  movement.

• Analog wattmeters use electrodynamometer meter
  movements.

• Rectifiers convert ac to pulsating dc.

• Meter movements have voltage, current, and
  resistance ratings.

• A DVM measures the time to charge a capacitor,
  from a constant-current source, to the value of
  the voltage being measured.

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More Facts About Instruments and Measurements

• Shunts and multipliers are used to extend the
  range of meter movements.

• High-frequency currents can be measured with
  thermocouple meters.

• Digital capacitance meters measure the time
  required to charge the capacitor.

• Digital inductance meters measure the inductors
  cemf.

• Wattmeters measure true power.

• Impedance can be measured by the
  equivalent-resistance method.

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Ohmmeter Function of a DMM
The ohmmeter function uses the voltmeter section
of the DMM.
A constant current source
develops a voltage across the resistor.

and the current source is
1mA,
If the voltmeter measures 0 to 999.9 mV,
then the displayed output will be in ohms.
This resistance of this resistor is very close to
its coded value.
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Basic Meter Movement
50 µA
1 kW
50 mV
The basic meter movement
has a full-scale current (Ifs) rating
and an internal resistance (Ri) rating.
Using these two ratings, the full-scale voltage
rating (Vfs) is
50 µA x 1 kW
Vfs Ifs x Ri
50 mV
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Analog Ammeters
This meter movement
is a 50 µA ammeter.
Its range can be extended by adding a shunt
resistor.
The shunt resistance for a 1-mA range is
calculated thus
52.63 W
Rshunt Vfs / (Irange - Ifs)
50 mV / (1 mA - 50 µA)
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Analog Voltmeters
This meter movement
is a 50 mV voltmeter.
Its range can be extended by adding a multiplier
resistor.
The multiplier resistance for a 20-V range is
calculated thus
399 kW
Rmult (Vrange - Vfs) / Ifs
(20 V - 50 mV) / 50 µA
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Analog Ohmmeters
This meter movement
can be converted to an ohmmeter
by adding a fixed and a variable resistor
and a cell.
RT R1 R2(mid value) R i
1.5 V / 50 µA
30 kW
Make R2(mid value) 0.05RT 1.5 kW.
Thus, R1 30 kW - 1.5 kW - 1 kW
27.5 kW
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Rectifier-Type Ac Ammeter
A polarized (dc) meter
can respond to ac when a rectifier is used.
The rectifier allows current to flow in only one
direction.
The converted meter can be used in extended-range
ammeters and voltmeters.
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Analog-Meters Quiz
80 mV
The Vfs rating a meter movement is ____ when Ifs
100 mA and Ri 800 W.
shunt 1.5
To extend the range of a 500-mA, 298-W meter
movement to 100 mA requires a____ resistor of
____ W.
multiplier 499.5 k
To extend the range of a 200-mA, 500-W meter
movement to 100 V requires a____ resistor of ____
W.
cell
An analog ohmmeter requires a(n) ____ as well as
resistors and a meter movement.
rectifier
A ____ allows a polarized meter movement to
measure ac.
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Current Transformer Principle
The current transformer, with a split core, is
used in the clamp-on meter.
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Current Transformer Principle
The current transformer, with a split core, is
used in the clamp-on meter.
To measure current in a conductor, open the core
and insert the conductor.
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Current Transformer Principle
The current transformer, with a split core, is
used in the clamp-on meter.
To measure current in a conductor, open the core
and insert the conductor.
The conductor is now a single-turn transformer
primary.
The current induced in the secondary is measured
by the digital meter.
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Voltmeter Loading
(Case 1)
Circuit analysis tells us that each resister
should drop 45 V.
Using a 20-kW/V meter on the 50-V range, the
voltages are as expected.
The 1 MW of Rmeter in parallel with 2 kW does not
significantly change the resistance in the
circuit.
Thus, there is no meter loading!
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Voltmeter Loading
(Case 2)
Circuit analysis tells us that each resister
should drop 30 V.
Using a 20-kW/V meter on the 50-V range, the
voltages are much lower than expected.
The 1 MW of Rmeter in parallel with 1 MW gives an
equivalent resistance of 0.5 MW in series with 1
MW.
The meter-resister combination drops only 20 V.
There is meter loading!
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Capacitance Bridge
2.2 kW
.00275 µF
Bridge null occurs when R1 is adjusted to 2.2 kW.
Therefore, the measured capacitance is
(0.002 / 1.6) x 2.2
.00275 µF
Cu (C1 / R2) R1
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Meter-Use Quiz
A ____ ammeter can measure current without
physically interrupting the circuit.
clamp-on
A clamp-on ammeter uses a ____ ____ connected to
another ammeter.
current transformer
Voltmeter ____ occurs when the meters Ri is not
many times larger than the resistance to which it
is connected.
loading
A DMM with 10 MW of Ri should indicate ____ V
when connected to a 10-MW resistor that is series
connected with a 5-MW resistor to a 40-V source.
20
A capacitor bridge uses ____ capacitor(s) and
____ resistor(s) in the bridge.
two two
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REVIEW

• Digital Ohmmeter
• Basic Meter Movement
• Analog Meters
• Current Transformer
• Meter Loading
• Bridges