2. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Electricity Principles & Applications Eighth Edition Chapter 15 Instruments and Measurements (student version) Richard J. Fowler McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 15 - 1

3. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. INTRODUCTION Digital Ohmmeter Basic Meter Movement Analog Meters Current Transformer Meter Loading Bridges 15 - 2

4. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Dear Student: This presentation is arranged in segments. Each segment is preceded by a Concept Preview slide and is followed by a Concept Review slide. When you reach a Concept Review slide, you can return to the beginning of that segment by clicking on the Repeat Segment button. This will allow you to view that segment again, if you want to. 15 - 3

5. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Concept Preview The voltmeter circuitry is used to measure resistance with the DMM. A meter movement has a I fs rating, an R i rating, and a V fs rating. Shunts and multipliers extend the range of ammeters and voltmeters respectively. Ohmmeters have internal power sources. Rectifiers convert ac to pulsating dc. 15 - 4

6. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Facts About Instruments and Measurements Many analog instruments use a d’Arsonval meter movement. Analog wattmeters use electrodynamometer meter movements. Rectifiers convert ac to pulsating dc. A DVM measures the time to charge a capacitor, from a constant- current source, to the value of the voltage being measured. Meter movements have voltage, current, and resistance ratings. Shunts and multipliers are used to extend the range of meter movements. High-frequency currents can be measured with thermocouple meters. Digital capacitance meters measure the time required to charge the capacitor. Digital inductance meters measure the inductor’s cemf. Wattmeters measure true power. Impedance can be measured by the equivalent-resistance method. 15 - 5

7. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 1 mA constant current source Voltmeter circuitry Ohmmeter Function of a DMM (Page 412) The ohmmeter function uses the voltmeter section of the DMM. A constant current sourcedevelops a voltage across the resistor. If the voltmeter measures 0 to 999.9 mV, and the current source is 1mA, then the displayed output will be in ohms. This resistance of this resistor is very close to its coded value. 15 - 6

8. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 50 µA 1 k  Basic Meter Movement (Page 413) The basic meter movement and an internal resistance ( R i ) rating. Using these two ratings, the full-scale voltage rating (V fs ) is: V fs = I fs x R i = 50 µA x 1 k  = 50 mV has a full-scale current (I fs ) rating 50 mV 15 - 7

9. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Analog Ammeters (Page 416) This meter movement is a 50 µA ammeter. Its range can be extended by adding a shunt resistor. 50 µA 1 k  50 mV The shunt resistance for a 1-mA range is calculated thus: R shunt = V fs / (I range - I fs ) =50 mV / (1 mA - 50 µA) = 52.63  15 - 8

10. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Analog Voltmeters (Page 420) This meter movement is a 50 mV voltmeter. Its range can be extended by adding a multiplier resistor. 50 µA 1 k  50 mV The multiplier resistance for a 20-V range is calculated thus: R mult = (V range - V fs ) / I fs = (20 V - 50 mV) / 50 µA = 399 k  15 - 9

11. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Analog Ohmmeters (Page 424) This meter movementcan be converted to an ohmmeter by adding a fixed and a variable resistor 50 µA 1 k  50 mV R T = R 1 + R 2(mid value) + R i = Make R 2(mid value) = 0.05R T = 1.5 k . Thus, R 1 = 30 k  k  k  =27.5 k  R1R1 R2R2 and a cell. 1.5 V / 50 µA = 30 k  15 - 10

12. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. 50 µA 1 k  50 mV  + Rectifier-Type Ac Ammeter (Page 416) Input ac Output pdc A polarized (dc) metercan respond to ac when a rectifier is used. The rectifier allows current to flow in only one direction. The converted meter can be used in extended-range ammeters and voltmeters. 15 - 11

13. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Analog-Meters Quiz The V fs rating a meter movement is ____ when I fs = 100  A and R i = 800 . 80 mV To extend the range of a 500-  A, 298-  meter movement to 100 mA requires a____ resistor of ____ . shunt 1.5 To extend the range of a 200-  A, 500-  meter movement to 100 V requires a____ resistor of ____ . multiplier 499.5 k An analog ohmmeter requires a(n) ____ as well as resistors and a meter movement. cell A ____ allows a polarized meter movement to measure ac. rectifier 15 - 12

14. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Concept Review Repeat Segment The voltmeter circuitry is used to measure resistance with the DMM. A meter movement has a I fs rating, an R i rating, and a V fs rating. Shunts and multipliers extend the range of ammeters and voltmeters respectively. Ohmmeters have internal power sources. Rectifiers convert ac to pulsating dc. 15 - 13

15. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Concept Preview A clamp-on meter uses a split-core transformer connected to an ac meter. A clamp-on meter can measure current without interrupting the circuit. Significant voltmeter loading occurs when meter resistance is < 20 X load resistance. A capacitor bridge is adjusted for null by varying a resistance in the bridge. 15 - 14

16. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Current Transformer Principle (Page 418) AMPERES The current transformer, with a split core, is used in the clamp-on meter. 15 - 15

17. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Current Transformer Principle AMPERES The current transformer, with a split core, is used in the clamp-on meter. To measure current in a conductor, open the coreand insert the conductor. 15 - 16

18. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Current Transformer Principle The current transformer, with a split core, is used in the clamp-on meter. The conductor is now a single-turn transformer primary. The current induced in the secondary is measured by the digital meter. AMPERES To measure current in a conductor, open the coreand insert the conductor. 15 - 17

19. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. R1R1 R2R2 B 1 90 V 2 k  45 V V V Voltmeter Loading (Page 422) Circuit analysis tells us that each resister should drop 45 V. Using a 20-k  /V meter on the 50-V range, the voltages are as expected. The 1 M  of R meter in parallel with 2 k  does not significantly change the resistance in the circuit. Thus, there is no meter loading! (Case 1) 15 - 18

20. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. R1R1 R2R2 B 1 60 V 1 M  20 V V V Voltmeter Loading Circuit analysis tells us that each resister should drop 30 V. Using a 20-k  /V meter on the 50-V range, the voltages are much lower than expected. The 1 M  of R meter in parallel with 1 M  gives an equivalent resistance of 0.5 M  in series with 1 M . The meter-resister combination drops only 20 V. There is meter loading! (Case 2) 15 - 19

21. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Capacitance Bridge (Page 432) Bridge null occurs when R 1 is adjusted to 2.2 k . Therefore, the measured capacitance is: C u = (C 1 / R 2 ) R 1 = (0.002 / 1.6) x 2.2 =.00275 µF 2.2 k  R1R1 R2R2 1.6 k  V CuCu C1C1 0.002 µF.00275 µF 15 - 20

22. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Meter-Use Quiz A ____ ammeter can measure current without physically interrupting the circuit. clamp-on A clamp-on ammeter uses a ____ ____ connected to another ammeter. current transformer Voltmeter ____ occurs when the meter’s R i is not many times larger than the resistance to which it is connected. loading A DMM with 10 M  of R i should indicate ____ V when connected to a 10-M  resistor that is series connected with a 5-M  resistor to a 40-V source. 20 A capacitor bridge uses ____ capacitor(s) and ____ resistor(s) in the bridge. two 15 - 21

23. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. Concept Review Repeat Segment A clamp-on meter uses a split-core transformer connected to an ac meter. A clamp-on meter can measure current without interrupting the circuit. Significant voltmeter loading occurs when meter resistance is < 20 X load resistance. A capacitor bridge is adjusted for null by varying a resistance in the bridge. 15 - 22

24. McGraw-Hill © 2013 The McGraw-Hill Companies Inc. All rights reserved. REVIEW Digital Ohmmeter Basic Meter Movement Analog Meters Current Transformer Meter Loading Bridges 15 - 23