Unit 09 instruments

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1. 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.

2. 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’Arsonvaltype of meter movement.
•Analog meters use a moving coil placed
between the poles of a magnet.

3. Basic d’Arsonval meter movement.
Unit 9
Measuring Instruments

4. Basic d’Arsonval meter movement with rectifier to
change AC voltage to DC voltage.
Unit 9
Measuring Instruments

5. The voltmeteris
designed to be
connected directly
across the source of
power.
Unit 9
Measuring Instruments

6. Reading an analog meter is similar to
reading a speedometer or fuel gauge.
Unit 9
Measuring Instruments

7. The ammeteris 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

8. Unit 9
Measuring Instruments
A shuntis used to set the value of the ammeter.

9. Unit 9
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.

10. 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.

11. 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!

12. Unit 9
Measuring Instruments
Digital Ohmmeter

13. Unit 9
Measuring Instruments
Digital Voltmeter

14. Unit 9
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.

15. Unit 9
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.

16. Unit 9
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 high-
impedance ground paths or feedback
voltages.

17. Unit 9
Measuring Instruments
Low-impedance Voltage Tester
High-impedance ground paths can produce
misleading voltage readings.

18. Unit 9
Measuring Instruments
Low-impedance Voltage Tester

19. Unit 9
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.

20. 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.

21. 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.

22. 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.

23. 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.

24. 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.