Unit 09
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The document discusses various types of measuring instruments used in electrical work. It describes the operation of analog meters like the d'Arsonval movement and how to connect a voltmeter, ammeter, and ohmmeter. Digital multimeters and low-impedance voltage testers are also covered. Key points include how d'Arsonval meters work, the proper connections for voltmeters, ammeters, and ohmmeters, and the differences between analog and digital instruments.
Unit 09
- 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’Arsonval type 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 voltmeter is 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 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
- 8. Unit 9 Measuring Instruments A shunt is 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: The d’Arsonval movement is one of the common types of meters. The d’Arsonval movement operates only on DC currents or rectified AC current. Voltmeters have a high resistance and are designed to be connected directly across the power line.
- 21. Unit 9 Measuring Instruments Review: Ammeters have a low resistance and must be connected in series with a load. Shunts are used to change the value of DC ammeters. 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.