Unit 9 Measuring Instruments Objectives: Discuss the operation of a d’Arsonval meter movement. Connect a voltmeter to a circuit. Read an analog multimeter.

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.


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.


Basic d’Arsonval meter movement with rectifier to change AC voltage to DC voltage.


The voltmeter is designed to be connected directly across the source of power.


Reading an analog meter is similar to reading a speedometer or fuel gauge.


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.



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.


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.


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!


Digital Ohmmeter


Digital Voltmeter


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.


Low-impedance Voltage TesterThis tester has an impedance of 5000 Ω and

can generally be used to measure voltages as high as 600 V.


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.


Low-impedance Voltage TesterHigh-impedance ground paths can produce

misleading voltage readings.


Low-impedance Voltage Tester


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.


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.


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.


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.


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.


Review:12. Low-impedance voltage testers (solenoid

type) are not susceptible to indicating a negligible voltage caused by a high-impedance ground or feedback.

Unit 9 Measuring Instruments Objectives: Discuss the operation of a d’Arsonval meter movement. Connect a voltmeter to a circuit. Read an analog multimeter.

Documents

measuring instruments

measuring instruments

ac voltage

dc voltage

digital meters

meter scale

basic darsonval meter

darsonval movement