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Chapter 6 Electrical Measurements ─ Meters 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 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 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 Objectives (cont’d.) – Describe how an ohmmeter is used to check open, short, or closed circuits 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 Types of Meters Figure 6-5. The placement of an ammeter in a circuit. Types of Meters (cont’d.) Figure 6-6. (A) Schematic symbol for a voltmeter. (B) A voltmeter is connected in parallel in a circuit. 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. Multimeters • Multimeter – 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 5 Voltage positions 4 current positions & 3 resistance positions Function switch +DC, -DC, AC Figure 6-8. An analog multimeter. DMM 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 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 Measuring Current 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 Figure 6-10. 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. 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. Measuring Voltage Figure 6-13. When connecting a voltmeter into a circuit, be sure to observe polarity. • 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 Measuring Resistance Figure 6-14. When using an ohmmeter to measure resistance, the component being measured must be removed from the circuit. 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 • Advantages of DMM: 1. higher accuracy 2. Digital readout 3. Repeatability of reading 4.Uses very little current to test resis. 5. uses audio signal for continuity test Reading Meter Scales • Full scale value – Maximum value indicated by a meter • Ammeters and voltmeters – Read from left to right – Linear scales • Ohmmeters – Read from right to left – Nonlinear scales 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 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