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Electricity
Principles & Applications
Eighth Edition
Richard J. Fowler
Chapter 15
Instruments and Measurements
(student version)
McGraw-Hill
© 2013 The McGraw-Hill Companies Inc. All rights reserved.
15 - 2
INTRODUCTION
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McGraw-Hill
Digital Ohmmeter
Basic Meter Movement
Analog Meters
Current Transformer
Meter Loading
Bridges
© 2013 The McGraw-Hill Companies Inc. All rights reserved.
15 - 3
Dear Student:
This presentation is arranged in segments. Each segment
is preceded by a Concept Preview slide and is followed by a
Concept Review slide. When you reach a Concept Review
slide, you can return to the beginning of that segment by
clicking on the Repeat Segment button. This will allow you
to view that segment again, if you want to.
McGraw-Hill
© 2013 The McGraw-Hill Companies Inc. All rights reserved.
15 - 4
Concept Preview
• The voltmeter circuitry is used to
measure resistance with the DMM.
• A meter movement has a Ifs rating, an
Ri rating, and a Vfs rating.
• Shunts and multipliers extend the range
of ammeters and voltmeters respectively.
• Ohmmeters have internal power sources.
• Rectifiers convert ac to pulsating dc.
McGraw-Hill
© 2013 The McGraw-Hill Companies Inc. All rights reserved.
15 - 5
Facts About
Instruments and Measurements
• Many analog instruments use a d’Arsonval meter movement.
• Analog wattmeters use electrodynamometer meter movements.
• Rectifiers convert ac to pulsating dc.
• A DVM measures the time to charge a capacitor, from a constantcurrent source, to the value of the voltage being measured.
• Meter movements have voltage, current, and resistance ratings.
• 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 inductor’s cemf.
• Wattmeters measure true power.
• Impedance can be measured by the equivalent-resistance method.
McGraw-Hill
© 2013 The McGraw-Hill Companies Inc. All rights reserved.
15 - 6
Ohmmeter Function of a DMM (Page 412)
1 mA
constant
current
source
Voltmeter
circuitry
The ohmmeter function uses the voltmeter section of the DMM.
A constant current source develops a voltage across the resistor.
If the voltmeter measures 0 to 999.9 mV, and the current
source is 1mA, then the displayed output will be in ohms.
This resistance of this resistor is very close to its coded value.
McGraw-Hill
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15 - 7
Basic Meter Movement (Page 413)
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:
Vfs = Ifs x Ri = 50 µA x 1 kW = 50 mV
McGraw-Hill
© 2013 The McGraw-Hill Companies Inc. All rights reserved.
15 - 8
Analog Ammeters
(Page 416)
50 µA
1 kW
50 mV
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:
Rshunt = Vfs / (Irange - Ifs) = 50 mV / (1 mA - 50 µA) = 52.63 W
McGraw-Hill
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15 - 9
Analog Voltmeters
(Page 420)
50 µA
1 kW
50 mV
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:
Rmult = (Vrange - Vfs) / Ifs = (20 V - 50 mV) / 50 µA = 399 kW
McGraw-Hill
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15 - 10
Analog Ohmmeters
(Page 424)
50 µA
1 kW
50 mV
R1
R2
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
McGraw-Hill
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15 - 11
Rectifier-Type Ac Ammeter
(Page 416)
Output pdc
-
50 µA
1 kW
50 mV
+
Input ac
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.
McGraw-Hill
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15 - 12
Analog-Meters Quiz
The Vfs rating a meter movement is ____
when Ifs = 100 mA and Ri = 800 W.
80 mV
To extend the range of a 500-mA, 298-W
meter movement to 100 mA requires a____
resistor of ____ W.
shunt
1.5
To extend the range of a 200-mA, 500-W
meter movement to 100 V requires a____
resistor of ____ W.
multiplier
499.5 k
An analog ohmmeter requires a(n) ____ as
well as resistors and a meter movement.
cell
A ____ allows a polarized meter movement
to measure ac.
rectifier
McGraw-Hill
© 2013 The McGraw-Hill Companies Inc. All rights reserved.
15 - 13
Concept Review
• The voltmeter circuitry is used to
measure resistance with the DMM.
• A meter movement has a Ifs rating, an
Ri rating, and a Vfs rating.
• Shunts and multipliers extend the range
of ammeters and voltmeters respectively.
• Ohmmeters have internal power sources.
• Rectifiers convert ac to pulsating dc.
Repeat Segment
McGraw-Hill
© 2013 The McGraw-Hill Companies Inc. All rights reserved.
15 - 14
Concept Preview
• A clamp-on meter uses a split-core
transformer connected to an ac meter.
• A clamp-on meter can measure current
without interrupting the circuit.
• Significant voltmeter loading occurs when
meter resistance is < 20 X load resistance.
• A capacitor bridge is adjusted for null
by varying a resistance in the bridge.
McGraw-Hill
© 2013 The McGraw-Hill Companies Inc. All rights reserved.
15 - 15
Current Transformer Principle
(Page 418)
AMPERES
The current transformer, with a split core, is used in the clamp-on meter.
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15 - 16
Current Transformer Principle
AMPERES
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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15 - 17
Current Transformer Principle
AMPERES
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.
McGraw-Hill
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15 - 18
Voltmeter Loading
(Case 1)
R1
2 kW
V 45 V
R2
2 kW
V 45 V
(Page 422)
B1
90 V
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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15 - 19
Voltmeter Loading
(Case 2)
R1
1 MW
V 20 V
R2
1 MW
V 20 V
B1
60 V
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!
McGraw-Hill
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15 - 20
Capacitance Bridge
R1
2.2 kW
(Page 432)
R2
1.6 kW
V
C1
0.002 µF
Cu
.00275 µF
Bridge null occurs when R1 is adjusted to 2.2 kW.
Therefore, the measured capacitance is: Cu = (C1 / R2) R1 =
(0.002 / 1.6) x 2.2 = .00275 µF
McGraw-Hill
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15 - 21
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 meter’s
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.
A capacitor bridge uses ____ capacitor(s)
and ____ resistor(s) in the bridge.
McGraw-Hill
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two
two
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Concept Review
• A clamp-on meter uses a split-core
transformer connected to an ac meter.
• A clamp-on meter can measure current
without interrupting the circuit.
• Significant voltmeter loading occurs when
meter resistance is < 20 X load resistance.
• A capacitor bridge is adjusted for null
by varying a resistance in the bridge.
Repeat Segment
McGraw-Hill
© 2013 The McGraw-Hill Companies Inc. All rights reserved.
15 - 23
REVIEW
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•
McGraw-Hill
Digital Ohmmeter
Basic Meter Movement
Analog Meters
Current Transformer
Meter Loading
Bridges
© 2013 The McGraw-Hill Companies Inc. All rights reserved.