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Unit 9 Measuring Instruments
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.
Basic d’Arsonval meter movement.
Unit 9 Measuring Instruments
Basic d’Arsonval meter movement with rectifier to change AC voltage to DC voltage.
Unit 9 Measuring Instruments
The voltmeter is designed to be connected directly across the source of power.
Unit 9 Measuring Instruments
Reading an analog meter is similar to reading a speedometer or fuel gauge.
Unit 9 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.
Unit 9 Measuring Instruments
Unit 9 Measuring Instruments
A shunt is used to set the value of the ammeter.
Unit 9 Measuring InstrumentsMany electricians use the clamp-on type of AC ammeter.
Please note the clamp-on ammeter reads only one conductor at a time.
Unit 9 Measuring Instruments
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.
Unit 9 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!
Unit 9 Measuring Instruments
Digital Ohmmeter
Unit 9 Measuring Instruments
Digital Voltmeter
Unit 9 Measuring Instruments
Low-impedance Voltage TesterThe 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.
Unit 9 Measuring Instruments
Low-impedance Voltage TesterThis tester has an impedance of 5000 Ω and
can generally be used to measure voltages as high as 600 V.
Unit 9 Measuring Instruments
Low-impedance Voltage TesterThis is also known as a solenoid type tester.
This tester is not susceptible to giving the misleading voltage readings caused by high-impedance ground paths or feedback voltages.
Unit 9 Measuring Instruments
Low-impedance Voltage TesterHigh-impedance ground paths can produce
misleading voltage readings.
Unit 9 Measuring Instruments
Low-impedance Voltage Tester
Unit 9 Measuring Instruments
Reading a Digital MeterMany 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.
Unit 9 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.
Unit 9 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.
Unit 9 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 high-impedance ground or feedback.
