1. Electrical Measurements ─ Meters
Chapter 6
Electrical Measurements ─ Meters

2. Objectives After completing this chapter, you will be able to:
Identify the two types of meter movements available
Describe how a voltmeter is used in a circuit
Describe how an ammeter is used in a circuit
Describe how an ohmmeter is used for measuring resistance

3. Objectives (cont’d.) Identify the functions of a multimeter
Identify the advantages/disadvantages of DMMs and VOMs
Describe how to use a multimeter to measure voltage, current, and resistance
Describe how to measure current using an ammeter
Describe how to connect an ammeter into a circuit

4. Objectives (cont’d.) List safety precautions for using an ammeter
Describe how to connect a voltmeter to an electrical circuit
List safety precautions for connecting a voltmeter to a circuit
Describe how resistance values are measured using an ohmmeter
Define continuity check

5. Objectives (cont’d.)
Describe how an ohmmeter is used to check open, short, or closed circuits

6. Introduction to Meters
Analog meter
Uses a graduated scale with a pointer
Provides better graphic display of rapid changes in current/voltage
Digital meter
Provides a reading in numbers
More accurate than analog meters

7. Types of Meters
Figure 6-5. The placement of an ammeter in a circuit.

8. Types of Meters (cont’d.)
Figure 6-6. (A) Schematic symbol for a voltmeter.
(B) A voltmeter is connected in parallel in a circuit.

9. Types of Meters (cont’d.)
Figure 6-7. (A) Schematic symbol for an ohmmeter. (B) An ohmmeter applies
a voltage across the component being measured and monitors the current
flowing through it.

10. Multimeters Multimeter VOM (volt-ohm-milliammeter) DMM
Combines a voltmeter, ammeter, and ohmmeter into one package
VOM (volt-ohm-milliammeter)
An analog multimeter that measures volts, ohms, and milliamperes
DMM
A digital multimeter

11. 5 Voltage positions 4 current positions & 3 resistance positions
Function switch
+DC, -DC, AC
Figure 6-8. An analog multimeter.

12. DMM

13. Multimeters (cont’d.) Advantages of DMMs over VOMs:
Easier readability, resulting in a higher accuracy with smaller voltage
Autoranging capabilities (does not need to identify range of signal to be tested)
Autozeroing for resistance reading
Autolock of a displayed value

14. Multimeters (cont’d.) Disadvantages of a DMM:
Requirement for batteries or other source of electricity
Voltage limitation; if exceeded could damage the meter.
Inability to measure an instantaneous change in a signal faster than the sampling time

15. Measuring Current Red: + Connect - To - & + to + Black: -
Figure 6-9. An ammeter is connected in series to a circuit.
Red: +
Black: -
Connect - To - & + to +
Pointer from left to right.
Reverse leads if needle moves in opp. direction

16. Figure 6-10. An ammeter is one part of this VOM
Figure An ammeter is one part of this VOM. A black negative lead plugs into the
common or negative hack. A red positive lead plugs into the jack with the plus symbol.

17. Measuring Current (cont’d.)
Caution:
Always turn off power before connecting an ammeter to a circuit
An analog ammeter must never be connected in parallel with any ckt component (s/c; fuse will blow & damage to meter)
Never connect an ammeter directly to a voltage source (s/c)
Internal resis of Ammeter adds to ckt resis. So measured current reduces.

18. Measuring Voltage
Figure When connecting a voltmeter into a circuit, be sure to observe polarity.

19. Polarity is imp. Connect + to +
Polarity is imp. Connect + to +. Reverse the leads if pointer goes from right to left.
Voltmeter’s internal resis. is in parallel with resis of component to be measured. So eq resis reduces. So V measured reduces.s

20. Measuring Resistance
Figure When using an ohmmeter to measure resistance, the component being measured must be removed from the circuit.

21. Measuring Resistance (cont’d.)
Continuity test (or check)
Determines if current path is continuous
Open circuit
Indicates high resistance
Short circuit
Zero ohms of resistance
Closed circuit
Indicates low resistance

22. Advantages of DMM:
higher accuracy
Digital readout
Repeatability of reading
Uses very little current to test resis.
uses audio signal for continuity test

23. Reading Meter Scales Full scale value Ammeters and voltmeters
Maximum value indicated by a meter
Ammeters and voltmeters
Read from left to right
Linear scales
Ohmmeters
Read from right to left
Nonlinear scales

24. Summary Analog meters use a graduated scale with a pointer
Digital meters provide a direct readout
A multimeter combines a voltmeter, ammeter, and ohmmeter into one package
A VOM is an analog multimeter
A DMM is a digital multimeter

25. Summary
On a multimeter, the range selector switch selects the function to be used
An ammeter must be connected in series with a circuit
A voltmeter is connected in parallel with a circuit
An ohmmeter measures resistance by applying a known voltage