Unit 09 Instruments

8/10/2019 Unit 09 Instruments

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Unit 9

Measuring InstrumentsObjectives:

Discuss the operation of a dArsonvalmeter movement.

Connect a voltmeter to a circuit. Read an analog multimeter.

Connect an ammeter.

Measure resistance using an ohmmeter.


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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 dArsonvaltype of meter movement.

Analog meters use a moving coil placedbetween the poles of a magnet.


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Basic dArsonval meter movement.

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Basic dArsonval meter movement with rectifier tochange AC voltage to DC voltage.

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The voltmeteris

designed to be

connected directlyacross the source of

power.

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Reading an analog meter is similar to

reading a speedometer or fuel gauge.

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The ammeteris 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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Unit 9

Measuring InstrumentsA shuntis used to set the value of the ammeter.


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Measuring InstrumentsMany electricians use the clamp-on type of ACammeter.

Please note the clamp-on ammeter reads only one

conductor at a time.


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Unit 9

Measuring InstrumentsOhmmeters

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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Unit 9

Measuring InstrumentsDigital Ohmmeters

Digital ohmmeters display the resistance in

figures instead of using a meter movement.

They operate by measuring the voltage dropacross 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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Digital Ohmmeter


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Digital Voltmeter


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Low-impedance Voltage Tester

The low-impedance voltage tester has a very

large current draw compared to other typesof voltmeters and should never be used to

test low-power circuits.


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This tester has an impedance of 5000 and

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


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This is also known as a solenoid type tester.

This tester is not susceptible to giving themisleading voltage readings caused by high-

impedance ground paths or feedback

voltages.


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Low-impedance Voltage TesterHigh-impedance ground paths can produce

misleading voltage readings.


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Unit 9

Measuring InstrumentsLow-impedance Voltage Tester


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Reading a Digital Meter

Many digital meters are auto-ranging, which

means that they select the range scaleautomatically. This type will display a

notation beside the numerical digits to

indicate the meter scale.


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Review:1. The dArsonval movement is one of the

common types of meters.

2. The dArsonval movement operates only onDC currents or rectified AC current.

3. Voltmeters have a high resistance and are

designed to be connected directly across

the power line.


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Unit 9


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 DCammeters.

6. Clamp-on ammeters read only one

conductor at a time.


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Unit 9

Measuring InstrumentsReview:

7. Ohmmeters are used to measure the

resistance in a circuit.

8. There are two basic types ofOhmmeters: analog and digital.

9. Ohmmeters must never be connected to

a circuit that has power applied to it.


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Review:10. Digital multimeters display their value in

digits instead of using a meter movement.

11. Digital ohmmeters measure resistance bymeasuring the voltage drop across an

unknown resistor when a known amount of

current flows through it.


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Unit 9


Review:

12. Low-impedance voltage testers (solenoid

type) are not susceptible to indicating a

negligible voltage caused by a high-

impedance ground or feedback.

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Unit 09 Instruments