Unit 9 Measuring Instruments Objectives: Discuss the operation of a d’Arsonval meter movement. Connect a voltmeter to a circuit. Read an analog multimeter. Connect an ammeter. Measure resistance using an ohmmeter.

Unit 9 Measuring Instruments Analog meters are characterized by the fact that they use a pointer and scale to indicate their value. One of the common types of meters uses the d’Arsonval type of meter movement. Analog meters use a moving coil placed between the poles of a magnet.

Unit 9 Measuring Instruments Basic d’Arsonval meter movement.

Unit 9 Measuring Instruments Basic d’Arsonval meter movement with rectifier to change AC voltage to DC voltage.

Unit 9 Measuring Instruments The voltmeter is designed to be connected directly across the source of power.

Unit 9 Measuring Instruments Reading an analog meter is similar to reading a speedometer or fuel gauge.

Unit 9 Measuring Instruments The ammeter is used to measure current and must be connected in series with the load to permit the load to limit the current flow.

Unit 9 Measuring Instruments A shunt is used to set the value of the ammeter.

Unit 9 Measuring Instruments Many electricians use the clamp-on type of AC ammeter. Please note the clamp-on ammeter reads only one conductor at a time.

Unit 9 Measuring Instruments Ohmmeters The ohmmeter is used to measure resistance. There are two basic types: analog or digital. The common VOM (volt-ohm-milliammeter) contains an ohmmeter. An ohmmeter should always be readjusted to zero when the scale is changed.

Unit 9 Measuring Instruments Digital Ohmmeters Digital ohmmeters display the resistance in figures instead of using a meter movement. They operate by measuring the voltage drop across a resistor. The ohmmeter, whether digital or analog, must never be connected to a circuit when the power is turned on!

Unit 9 Measuring Instruments Digital Ohmmeter

Unit 9 Measuring Instruments Digital Voltmeter

Unit 9 Measuring Instruments Low-impedance Voltage Tester The low-impedance voltage tester has a very large current draw compared to other types of voltmeters and should never be used to test low-power circuits.

Unit 9 Measuring Instruments Low-impedance Voltage Tester This tester has an impedance of 5000 Ω and can generally be used to measure voltages as high as 600 V.

Unit 9 Measuring Instruments Low-impedance Voltage Tester This is also known as a solenoid type tester. This tester is not susceptible to giving the misleading voltage readings caused by high-impedance ground paths or feedback voltages.

Unit 9 Measuring Instruments Low-impedance Voltage Tester High-impedance ground paths can produce misleading voltage readings.

Unit 9 Measuring Instruments Low-impedance Voltage Tester

Unit 9 Measuring Instruments Reading a Digital Meter Many digital meters are auto-ranging, which means that they select the range scale automatically. This type will display a notation beside the numerical digits to indicate the meter scale.

Unit 9 Measuring Instruments Review: The d’Arsonval movement is one of the common types of meters. The d’Arsonval movement operates only on DC currents or rectified AC current. Voltmeters have a high resistance and are designed to be connected directly across the power line.

Unit 9 Measuring Instruments Review: Ammeters have a low resistance and must be connected in series with a load. Shunts are used to change the value of DC ammeters. Clamp-on ammeters read only one conductor at a time.

Unit 9 Measuring Instruments Review: 7. Ohmmeters are used to measure the resistance in a circuit. 8. There are two basic types of Ohmmeters: analog and digital. 9. Ohmmeters must never be connected to a circuit that has power applied to it.

Unit 9 Measuring Instruments Review: 10. Digital multimeters display their value in digits instead of using a meter movement. 11. Digital ohmmeters measure resistance by measuring the voltage drop across an unknown resistor when a known amount of current flows through it.

Unit 9 Measuring Instruments Review: 12. Low-impedance voltage testers (solenoid type) are not susceptible to indicating a negligible voltage caused by a high-impedance ground or feedback.