Unit 09

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Delmar\'s STE Chapter on Meters

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Unit 09

  1. 1. Unit 9 Measuring Instruments <ul><li>Objectives: </li></ul><ul><li>Discuss the operation of a d’Arsonval meter movement. </li></ul><ul><li>Connect a voltmeter to a circuit. </li></ul><ul><li>Read an analog multimeter. </li></ul><ul><li>Connect an ammeter. </li></ul><ul><li>Measure resistance using an ohmmeter. </li></ul>
  2. 2. Unit 9 Measuring Instruments <ul><li>Analog meters are characterized by the fact that they use a pointer and scale to indicate their value. </li></ul><ul><li>One of the common types of meters uses the d’Arsonval type of meter movement. </li></ul><ul><li>Analog meters use a moving coil placed between the poles of a magnet. </li></ul>
  3. 3. <ul><li>Basic d’Arsonval meter movement. </li></ul>Unit 9 Measuring Instruments
  4. 4. <ul><li>Basic d’Arsonval meter movement with rectifier to change AC voltage to DC voltage. </li></ul>Unit 9 Measuring Instruments
  5. 5. <ul><li>The voltmeter is designed to be connected directly across the source of power. </li></ul>Unit 9 Measuring Instruments
  6. 6. <ul><li>Reading an analog meter is similar to reading a speedometer or fuel gauge. </li></ul>Unit 9 Measuring Instruments
  7. 7. <ul><li>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. </li></ul>Unit 9 Measuring Instruments
  8. 8. Unit 9 Measuring Instruments A shunt is used to set the value of the ammeter.
  9. 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. 10. Unit 9 Measuring Instruments <ul><li>Ohmmeters </li></ul><ul><li>The ohmmeter is used to measure resistance. </li></ul><ul><li>There are two basic types: analog or digital. </li></ul><ul><li>The common VOM (volt-ohm-milliammeter) contains an ohmmeter. </li></ul><ul><li>An ohmmeter should always be readjusted to zero when the scale is changed. </li></ul>
  11. 11. Unit 9 Measuring Instruments <ul><li>Digital Ohmmeters </li></ul><ul><li>Digital ohmmeters display the resistance in figures instead of using a meter movement. </li></ul><ul><li>They operate by measuring the voltage drop across a resistor. </li></ul><ul><li>The ohmmeter, whether digital or analog, must never be connected to a circuit when the power is turned on! </li></ul>
  12. 12. Unit 9 Measuring Instruments Digital Ohmmeter
  13. 13. Unit 9 Measuring Instruments Digital Voltmeter
  14. 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. 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. 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. 17. Unit 9 Measuring Instruments Low-impedance Voltage Tester High-impedance ground paths can produce misleading voltage readings.
  18. 18. Unit 9 Measuring Instruments Low-impedance Voltage Tester
  19. 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. 20. Unit 9 Measuring Instruments <ul><li>Review: </li></ul><ul><li>The d’Arsonval movement is one of the common types of meters. </li></ul><ul><li>The d’Arsonval movement operates only on DC currents or rectified AC current. </li></ul><ul><li>Voltmeters have a high resistance and are designed to be connected directly across the power line. </li></ul>
  21. 21. Unit 9 Measuring Instruments <ul><li>Review: </li></ul><ul><li>Ammeters have a low resistance and must be connected in series with a load. </li></ul><ul><li>Shunts are used to change the value of DC ammeters. </li></ul><ul><li>Clamp-on ammeters read only one conductor at a time. </li></ul>
  22. 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. 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. 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.

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