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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
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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
–
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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