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Electricity
Principles & Applications
Sixth Edition
Richard J. Fowler
Chapter 15
Instruments and Measurements
©2003
Glencoe/McGraw-Hill
INTRODUCTION
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Digital Ohmmeter
Basic Meter Movement
Analog Meters
Current Transformer
Meter Loading
Bridges
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.
• 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.
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
inductor’s cemf.
• Wattmeters measure true power.
• Impedance can be measured by the equivalentresistance method.
Ohmmeter Function of a DMM
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.
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:
Vfs = Ifs x Ri = 50 µA x 1 kW = 50 mV
Analog Ammeters
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
Analog Voltmeters
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
Analog Ohmmeters
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
Rectifier-Type Ac Ammeter
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.
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
Current Transformer Principle
AMPERES
The current transformer, with a split core, is used in the clamp-on meter.
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.
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.
Voltmeter Loading
(Case 1)
R1
2 kW
V 45 V
R2
2 kW
V 45 V
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!
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!
Capacitance Bridge
R1
2.2 kW
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
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.
20
two
two
REVIEW
• Digital Ohmmeter
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Basic Meter Movement
Analog Meters
Current Transformer
Meter Loading
Bridges