Download Unit 09 Instruments

Unit 9
Measuring Instruments
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Objectives:
Discuss the operation of a d’Arsonval
meter movement.
Connect a voltmeter to a circuit.
Read an analog multimeter.
Connect an ammeter.
Measure resistance using an ohmmeter.
Unit 9
Measuring Instruments
Analog meters are characterized by the
fact that they use a pointer and scale to
indicate their value.
• One of the common types of meters uses
the d’Arsonval type of meter movement.
• Analog meters use a moving coil placed
between the poles of a magnet.
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Measuring Instruments
Basic d’Arsonval meter movement.
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Measuring Instruments
Basic d’Arsonval meter movement with rectifier to
change AC voltage to DC voltage.
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Measuring Instruments
The voltmeter is
designed to be
connected directly
across the source of
power.
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Measuring Instruments
Reading an analog meter is similar to
reading a speedometer or fuel gauge.
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Measuring Instruments
The ammeter is used to measure current and
must be connected in series with the load to
permit the load to limit the current flow.
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Measuring Instruments
A shunt is used to set the value of the ammeter.
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Measuring Instruments
Many electricians use the clamp-on type of AC
ammeter.
Please note the clamp-on ammeter reads only one
conductor at a time.
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Measuring Instruments
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Ohmmeters
The ohmmeter is used to measure
resistance.
There are two basic types: analog or digital.
The common VOM (volt-ohm-milliammeter)
contains an ohmmeter.
An ohmmeter should always be readjusted
to zero when the scale is changed.
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Measuring Instruments
Digital Ohmmeters
• Digital ohmmeters display the resistance in
figures instead of using a meter movement.
• They operate by measuring the voltage drop
across a resistor.
• The ohmmeter, whether digital or analog,
must never be connected to a circuit when
the power is turned on!
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Measuring Instruments
Digital Ohmmeter
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Measuring Instruments
Digital Voltmeter
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Measuring Instruments
Low-impedance Voltage Tester
The low-impedance voltage tester has a very
large current draw compared to other types
of voltmeters and should never be used to
test low-power circuits.
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Measuring Instruments
Low-impedance Voltage Tester
This tester has an impedance of 5000 Ω and
can generally be used to measure voltages
as high as 600 V.
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Measuring Instruments
Low-impedance Voltage Tester
This is also known as a solenoid type tester.
This tester is not susceptible to giving the
misleading voltage readings caused by highimpedance ground paths or feedback
voltages.
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Measuring Instruments
Low-impedance Voltage Tester
High-impedance ground paths can produce
misleading voltage readings.
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Measuring Instruments
Low-impedance Voltage Tester
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Measuring Instruments
Reading a Digital Meter
Many digital meters are auto-ranging, which
means that they select the range scale
automatically. This type will display a
notation beside the numerical digits to
indicate the meter scale.
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Measuring Instruments
Review:
1. The d’Arsonval movement is one of the
common types of meters.
2. The d’Arsonval movement operates only on
DC currents or rectified AC current.
3. Voltmeters have a high resistance and are
designed to be connected directly across
the power line.
Unit 9
Measuring Instruments
Review:
4. Ammeters have a low resistance and must
be connected in series with a load.
5. Shunts are used to change the value of DC
ammeters.
6. Clamp-on ammeters read only one
conductor at a time.
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Measuring Instruments
Review:
7. Ohmmeters are used to measure the
resistance in a circuit.
8. There are two basic types of
Ohmmeters: analog and digital.
9. Ohmmeters must never be connected to
a circuit that has power applied to it.
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Measuring Instruments
Review:
10. Digital multimeters display their value in
digits instead of using a meter movement.
11. Digital ohmmeters measure resistance by
measuring the voltage drop across an
unknown resistor when a known amount of
current flows through it.
Unit 9
Measuring Instruments
Review:
12. Low-impedance voltage testers (solenoid
type) are not susceptible to indicating a
negligible voltage caused by a highimpedance ground or feedback.