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Chapter 2

Chapter 2. Basic Instruments and Measurements. Objectives. Explain the correct procedure for using an ammeter, a voltmeter, and an ohmmeter. Interpret a linear scale. Compute shunt resistor values. Compute multiplier resistor values. Objectives. Interpret a nonlinear scale.

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Chapter 2

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  1. Chapter 2 Basic Instruments and Measurements

  2. Objectives • Explain the correct procedure for using an ammeter, a voltmeter, and an ohmmeter. • Interpret a linear scale. • Compute shunt resistor values. • Compute multiplier resistor values. Permission granted to reproduce for educational use only.

  3. Objectives • Interpret a nonlinear scale. • Discuss the concept of meter sensitivity. • Understand basic electrical diagrams. • State and explain Ohm’s law. Permission granted to reproduce for educational use only.

  4. Basic Analog Meter Movement • D’Arsonval movement Permission granted to reproduce for educational use only.

  5. D’Arsonval Movement • Permanent magnet and rotating coil • Current passes through rotating coil • Magnetic field reacts with stationary field • Rotation, or deflection, moves meter’s needle • Current is necessary to move needle Permission granted to reproduce for educational use only.

  6. D’Arsonval Movement (Cont.) • Proper polarity is necessary for meter to function • Polarity is marked with plus sign (+) and negative sign (-) • Some meters use COM, abbreviation for common, for negative polarity marking • Coils will deflect in opposite direction if polarity is not observed Permission granted to reproduce for educational use only.

  7. Iron Vane Meter Movement • May indicate voltage or current • Best used for high-power circuit measurements • Measures ac or dc circuits • Always deflects in same direction Permission granted to reproduce for educational use only.

  8. Iron Vane Meter Movement (Cont.) • Two pieces of iron in a coil • Magnetized with same polarity by current • Iron pieces repel each other – one fixed, the other moving • Moving piece of iron controls the needle Permission granted to reproduce for educational use only.

  9. Linear Meter Scale • Evenly spaced marks • Scale factor = Value of major division / Number of spaces Permission granted to reproduce for educational use only.

  10. Review What is the common type of meter movement used to measure current and voltage? • D’Arsonval movement Permission granted to reproduce for educational use only.

  11. Review What does the abbreviation COM show on a meter? Negative polarity Permission granted to reproduce for educational use only.

  12. Review What can happen if proper polarity is not observed when connecting a meter to an electrical circuit? • The meter may be damaged Permission granted to reproduce for educational use only.

  13. Review What kind of scale is divided into even spaces? • Linear meter scale Permission granted to reproduce for educational use only.

  14. Ammeter • May measure in amperes, milliamperes, or microamperes • Coil wound with several turns of fine wire • A shunt stops ammeters from burning out • Must be connected in series with the circuit Permission granted to reproduce for educational use only.

  15. Ammeter (Cont.) • First, voltage is computed • Shunt carries 90% of current Permission granted to reproduce for educational use only.

  16. Ammeter (Cont.) • Shunt carries 98% of current • Shunt carries 99% of current Permission granted to reproduce for educational use only.

  17. Ammeter (Cont.) • Example of ammeter with three shunt resistors Permission granted to reproduce for educational use only.

  18. Ammeter (Cont.) • Proper connection to circuit Permission granted to reproduce for educational use only.

  19. Review What does an ammeter measure? • A circuit’s electrical current Permission granted to reproduce for educational use only.

  20. Review What is a shunt used for? • An alternate path for current Permission granted to reproduce for educational use only.

  21. Review How is a shunt connected with the meter coil? • In parallel Permission granted to reproduce for educational use only.

  22. Review What could happen to an ammeter if a large current flows through the meter coil? • The ammeter will burn out Permission granted to reproduce for educational use only.

  23. Review An ammeter should be connected in _____ with the circuit or device you are measuring. • series Permission granted to reproduce for educational use only.

  24. Voltmeter • Measures voltage • Multiplier resistors are used • Voltage across coil should never exceed 0.1 volt • Always connect in parallel or across the circuit Permission granted to reproduce for educational use only.

  25. Voltmeter (Cont.) • The multiplier induces an IR drop of 0.9 V • Then, IR drop of 9.9 V Permission granted to reproduce for educational use only.

  26. Voltmeter (Cont.) • IR drop of 99.9 V • IR drop of 499.9 V Permission granted to reproduce for educational use only.

  27. Voltmeter (Cont.) • Example of voltmeter setup with a switch added as a range selector Permission granted to reproduce for educational use only.

  28. Voltmeter (Cont.) • Proper connection in a circuit Permission granted to reproduce for educational use only.

  29. Voltmeter Sensitivity • Measured in units of ohms-per-volt • Sensitivity gauges meter quality • From the previous example • Take RM, add to meter resistance • Divide that by meter range • Sensitivity = 1 V / coil resistance Permission granted to reproduce for educational use only.

  30. Loading a Circuit • Error in voltage reading Permission granted to reproduce for educational use only.

  31. Loading a Circuit (Cont.) • More accurate readings from sensitive meters Permission granted to reproduce for educational use only.

  32. Review What is the highest allowable impressed voltage across the coil in a voltmeter? • 0.1 V Permission granted to reproduce for educational use only.

  33. Review What must be used if the voltage exceeds 0.1 V? • Multiplier resistors Permission granted to reproduce for educational use only.

  34. Review How are multiplier resistors placed in series with the movement coil? • With a switch Permission granted to reproduce for educational use only.

  35. Review How must a voltmeter be connected in a circuit? • In parallel Permission granted to reproduce for educational use only.

  36. Review What units are used to measure meter sensitivity? • Ohms-per-volt Permission granted to reproduce for educational use only.

  37. Uses same meter movement as in voltmeter and ammeter Measures unknown resistance in a circuit Connected in series Ohmmeter Permission granted to reproduce for educational use only.

  38. Ohmmeter (Cont.) • 0.1 volt for current of 0.001 amps for full-scale deflection • Temperature changes or weak batteries affect total resistance of circuit • Ohmmeters must be calibrated to 0 resistance before use • To use ohmmeter, first short test leads together, applying 0 ohms across meter Permission granted to reproduce for educational use only.

  39. Ohmmeter (Cont.) • Adjust ohms adjustment knob until needle points at zero • Needle should deflect from position at rest on the left to the zero resistance indication position on the right • Before connecting an ohmmeter to any electrical circuit to read an unknown value, make sure circuit is not energized Permission granted to reproduce for educational use only.

  40. Shunt Ohmmeter • Connected in parallel • Lower resistance causes lower currents in meter, higher resistance causes higher currents • Reads in opposite direction of other ohmmeters Permission granted to reproduce for educational use only.

  41. Ohmmeter Scales • Ohms scale is a nonlinear scale Permission granted to reproduce for educational use only.

  42. Review What does an ohmmeter measure in an electrical circuit? • Resistance Permission granted to reproduce for educational use only.

  43. Review How is a typical ohmmeter connected? • In series Permission granted to reproduce for educational use only.

  44. Review Why must ohmmeters be calibrated before use? • Temperature and batteries can affect total circuit resistance Permission granted to reproduce for educational use only.

  45. Review How can an ohmmeter be calibrated? • By shorting the test leads together Permission granted to reproduce for educational use only.

  46. Review What kind of ohmmeter is connected in parallel? • Shunt ohmmeter Permission granted to reproduce for educational use only.

  47. Review What kind of scale does an ohmmeter use? • Nonlinear scale Permission granted to reproduce for educational use only.

  48. The Volt-Ohm-Milliammeter (VOM) • Inexpensive and portable digital meter • Low input resistance causes accuracy problems • Field-effect transistor-VOM (FET-VOM)was designed to overcome low input impedance problem Permission granted to reproduce for educational use only.

  49. Digital Multimeters (DMMs) • Rapidly replacing analog meters • Takes measurements and display readings using electronic circuitry (Knight Electronics) Permission granted to reproduce for educational use only.

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