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Unit 9 Electrical Measurements: Meters • Objectives – After completing this chapter, the student should 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. • Identify the functions of a multimeter. • Identify the advantages and disadvantages of DMM’s and VOM’s. • 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. • 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. • Describe how an ohmmeter is used to check open, short, or closed circuits. • METERS – Detect and measure the invisible action of electrons. – Are indispensable in examining the operation of a circuit. – Two types are available. • Analog meter – Graduated scale with a pointer. – Provides a better graphic display of rapid changes. – Must be adjusted to zero before use. • Digital meter – Provides a reading in numbers. – Easier to read. – Provides a more accurate reading. • Polarity of the terminals – Red is positive. – Black is negative. • Ammeter – Measures current in a circuit. – Can be considered a flow meter. – Measures the number of electrons flowing past a given point in a circuit. – Must be connected in series with the load – Has a very low impedance • Voltmeter – Measures the voltage between two points in a circuit. – Can be considered a pressure gauge. • Ohmmeter – Measures resistance by placing a voltage across the device to be measured, inducing a current flow through the device. • Multimeter – Combines the voltmeter, ammeter, and the ohmmeter into a single instrument. – An analog multimeter is also referred to as a volt-ohm-milliammeter (VOM). – A digital multimeter is also referred to as a DMM. • Advantages of DMM’s – – – – Easier to read. Auto-ranging capabilities. Auto-zeroing for resistance reading. Auto lock capability. • Disadvantages of DMM’s – Requires external power source. – Excessive voltage can damage meter. – Limitations due to the sampling rate. • Measuring current – – – – Turn off the power. Open the circuit. Insert the ammeter into the circuit series. When correctly connected, the needle will move from left to right. • Never connect in parallel with any circuit component. • Never connect an ammeter directly to a voltage source. – Set the meter to its highest scale before turning on the power. • Measuring voltage – Remove the power source. – Connect the voltmeter. • Pay close attention to polarity. – Set meter to its highest scale. – Reapply power. • Measuring resistance – Remove power supply. – Disconnect one end of the component from the circuit. – Connect the ohmmeter leads across the device. • Use an ohmmeter to determine whether a circuit is: – Open • Has infinite resistance because no current flows through it. – Shorted • Has zero ohms of resistance because current flows through it without developing a voltage drop. – Closed • A complete path for current where resistance varies depending on the components in the circuit. • Continuity check – The testing for an open, short, or closed circuit. • Reading meter scales – Voltmeter and ammeter scales. • Read from left to right. • Value of voltage or current is read under the pointer. – Ohmmeter scale • Read from right to left. • Nonlinear. • Must be calibrated before use. • In Summary – Meters • Analog • Digital – Ammeter – Voltmeter – Ohmmeter – Multimeters • VOM • DMM