Electric Meters

• Upon completion of this course, you will be able to:
– Describe the use of the volt-ohm meter and clamp-on ammeter in the heating, cooling, and refrigeration industry
– Explain the operation of the basic electric analog meter

– Explain how analog electric meters transfer a known value in an electrical circuit to the meter movement
– Describe the operation of an analog voltmeter
– Describe the operation of an analog and digital clamp-on ammeter
– Describe the operation of an analog ohmmeter

– Explain the operation of a digital volt-ohm meter
– Give the advantages and disadvantages of the analog and digital meters
– Describe the conditions of resistance that can exist in an electrical circuit in reference to continuity

– Describe the source of energy for the operation of the analog voltmeter, ammeter, and ohmmeter

• Ammeter
• Analog meter
• Clamp-on ammeter
• Continuity
• Ohmmeter
• Open
• Short
• Voltmeter
• Digital meter • Volt-ohm-
• Magnetic field
• Measurable resistance milliammeter

• Most important characteristics in a circuit
– Volts, amperes, and ohms
• Popular electrical meters are built around these characteristics
– Volt-ohm-milliammeter
– Clamp-on ammeter

• Devices used to measure some electrical characteristic of a circuit
– Common types
• Voltmeter
• Ammeter
• Ohmmeter
• Caution: use the proper electrical test equipment for the job performed

• Electric measuring instruments
– Most make use of the magnetic effect of electric current
• Electrons flow through a conductor and a magnetic field is created around it
• Magnetic field is used to move the needle of a meter a certain distance, which represents the amount of the characteristic measured

Figure 4.1 A magnetic field is produced around a conductor when current is flowing through the conductor.
(Delmar/Cengage Learning)

Figure 4.2 When there is no current flow, the compass reacts only to the magnetic field of the earth.
(Delmar/Cengage Learning)
Figure 4.3 When a current flows through a conductor, the compass needle swings in line with the conductor’s magnetic field.
(Delmar/Cengage Learning)

• Not in the meter movements
– Except for digital meters
• In the internal circuits and how magnetic fields are created
– Voltmeter: uses a resistor as a load to produce a magnetic field
– Ohmmeter: uses device being checked as the load to produce a magnetic field

• Measure current flow
– Strength of magnetic field determines the distance that the needle moves
Figure 4.11 Scale of ammeter showing current flow.
(Delmar/Cengage Learning)

Figure 4.14 Using a clamp-on ammeter to check the amp draw of a compressor.
(Delmar/Cengage Learning)

• Measure amount of electromotive force available
– Resistor is added to prevent a direct short and allow electrons to flow in the meter
Figure 4.10 Drawing showing a voltmeter being used to check the voltage supplied to the compressor.
(Delmar/Cengage Learning)

• Determine operating condition of a component
– Open circuits
– Open components
– Direct shorts
– Actual resistance
• Continuity
– Circuit or component has a complete path

Figure 4.22 Three conditions of a circuit as read on an ohmmeter.
(Delmar/Cengage Learning)

Figure 4.23 Using an ohmmeter to check the resistance of a compressor.
(Delmar/Cengage Learning)

Figure 4.25 The internal wiring of an ohmmeter.
(Delmar/Cengage Learning)