Electrical Measurements:

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Unit 9

• Electrical Measurements
• Meters

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• 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.

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• Identify the functions of a multimeter.
• Identify the advantages and 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.
• List safety precautions for using an ammeter.

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• 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.

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• METERS
• Detect and measure the invisible action of
  electrons.
• Are indispensable in examining the operation of a
  circuit.
• Two types are available.

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• Analog meter
• Graduated scale with a pointer.
• Provides a better graphic display of rapid
  changes.
• Must be adjusted to zero before use.

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• Digital meter
• Provides a reading in numbers.
• Easier to read.
• Provides a more accurate reading.

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• Polarity of the terminals
• Red is positive.
• Black is negative.

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• 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

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• Voltmeter
• Measures the voltage between two points in a
  circuit.
• Can be considered a pressure gauge.

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• Ohmmeter
• Measures resistance by placing a voltage across
  the device to be measured, inducing a current
  flow through the device.

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• 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.

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• Advantages of DMMs
• Easier to read.
• Auto-ranging capabilities.
• Auto-zeroing for resistance reading.
• Auto lock capability.

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• Disadvantages of DMMs
• Requires external power source.
• Excessive voltage can damage meter.
• Limitations due to the sampling rate.

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• 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.

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• Never connect an ammeter directly to a voltage
  source.
• Set the meter to its highest scale before turning
  on the power.

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• Measuring voltage
• Remove the power source.
• Connect the voltmeter.
• Pay close attention to polarity.
• Set meter to its highest scale.
• Reapply power.

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• Measuring resistance
• Remove power supply.
• Disconnect one end of the component from the
  circuit.
• Connect the ohmmeter leads across the device.

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• 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.

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• 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.

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• Reading meter scales
• Voltmeter and ammeter scales.
• Read from left to right.
• Value of voltage or current is read under the
  pointer.

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• Ohmmeter scale
• Read from right to left.
• Nonlinear.
• Must be calibrated before use.

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• In Summary
• Meters
• Analog
• Digital
• Ammeter
• Voltmeter
• Ohmmeter

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• Multimeters
• VOM
• DMM