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