Principles & Applications Instruments and Measurements Electricity Principles & Applications Sixth Edition Richard J. Fowler Chapter 15 Instruments and Measurements ©2003 Glencoe/McGraw-Hill

INTRODUCTION Digital Ohmmeter Basic Meter Movement Analog Meters Current Transformer Meter Loading Bridges

Facts About Instruments and Measurements Many analog instruments use a d’Arsonval meter movement. Analog wattmeters use electrodynamometer meter movements. Rectifiers convert ac to pulsating dc. Meter movements have voltage, current, and resistance ratings. A DVM measures the time to charge a capacitor, from a constant-current source, to the value of the voltage being measured.

Instruments and Measurements More Facts About Instruments and Measurements Shunts and multipliers are used to extend the range of meter movements. High-frequency currents can be measured with thermocouple meters. Digital capacitance meters measure the time required to charge the capacitor. Digital inductance meters measure the inductor’s cemf. Wattmeters measure true power. Impedance can be measured by the equivalent-resistance method.

Ohmmeter Function of a DMM Voltmeter circuitry 1 mA constant current source The ohmmeter function uses the voltmeter section of the DMM. A constant current source develops a voltage across the resistor. If the voltmeter measures 0 to 999.9 mV, and the current source is 1mA, then the displayed output will be in ohms. This resistance of this resistor is very close to its coded value.

Basic Meter Movement The basic meter movement 1 kW 50 mV The basic meter movement has a full-scale current (Ifs) rating and an internal resistance (Ri) rating. Using these two ratings, the full-scale voltage rating (Vfs) is: Vfs = Ifs x Ri = 50 µA x 1 kW = 50 mV

Analog Ammeters This meter movement is a 50 µA ammeter. 1 kW 50 mV This meter movement is a 50 µA ammeter. Its range can be extended by adding a shunt resistor. The shunt resistance for a 1-mA range is calculated thus: Rshunt = Vfs / (Irange - Ifs) = 50 mV / (1 mA - 50 µA) = 52.63 W

Analog Voltmeters This meter movement is a 50 mV voltmeter. 1 kW 50 mV This meter movement is a 50 mV voltmeter. Its range can be extended by adding a multiplier resistor. The multiplier resistance for a 20-V range is calculated thus: Rmult = (Vrange - Vfs) / Ifs = (20 V - 50 mV) / 50 µA = 399 kW

Analog Ohmmeters R1 R2 This meter movement 1 kW 50 mV R1 R2 This meter movement can be converted to an ohmmeter by adding a fixed and a variable resistor and a cell. RT = R1 + R2(mid value) + R i = 1.5 V / 50 µA = 30 kW Make R2(mid value) = 0.05RT = 1.5 kW. Thus, R1 = 30 kW - 1.5 kW - 1 kW = 27.5 kW

Rectifier-Type Ac Ammeter Output pdc 50 µA 1 kW 50 mV - + Input ac A polarized (dc) meter can respond to ac when a rectifier is used. The rectifier allows current to flow in only one direction. The converted meter can be used in extended-range ammeters and voltmeters.

Analog-Meters Quiz The Vfs rating a meter movement is ____ 80 mV when Ifs = 100 mA and Ri = 800 W. 80 mV To extend the range of a 500-mA, 298-W meter movement to 100 mA requires a____ resistor of ____ W. shunt 1.5 To extend the range of a 200-mA, 500-W meter movement to 100 V requires a____ resistor of ____ W. multiplier 499.5 k An analog ohmmeter requires a(n) ____ as well as resistors and a meter movement. cell A ____ allows a polarized meter movement to measure ac. rectifier

Current Transformer Principle AMPERES The current transformer, with a split core, is used in the clamp-on meter.

Current Transformer Principle AMPERES The current transformer, with a split core, is used in the clamp-on meter. To measure current in a conductor, open the core and insert the conductor.

Current Transformer Principle AMPERES The current transformer, with a split core, is used in the clamp-on meter. To measure current in a conductor, open the core and insert the conductor. The conductor is now a single-turn transformer primary. The current induced in the secondary is measured by the digital meter.

Voltmeter Loading R1 V 45 V B1 90 V R2 V 45 V 2 kW (Case 1) R1 R2 B1 90 V 2 kW 45 V V 45 V V Circuit analysis tells us that each resister should drop 45 V. Using a 20-kW/V meter on the 50-V range, the voltages are as expected. The 1 MW of Rmeter in parallel with 2 kW does not significantly change the resistance in the circuit. Thus, there is no meter loading!

Voltmeter Loading R1 V 20 V B1 60 V R2 V 20 V 1 MW (Case 2) R1 R2 B1 60 V 1 MW 20 V V 20 V V Circuit analysis tells us that each resister should drop 30 V. Using a 20-kW/V meter on the 50-V range, the voltages are much lower than expected. The 1 MW of Rmeter in parallel with 1 MW gives an equivalent resistance of 0.5 MW in series with 1 MW. The meter-resister combination drops only 20 V. There is meter loading!

Capacitance Bridge R1 R2 1.6 kW V Cu C1 0.002 µF 2.2 kW .00275 µF Bridge null occurs when R1 is adjusted to 2.2 kW. Therefore, the measured capacitance is: Cu = (C1 / R2) R1 = (0.002 / 1.6) x 2.2 = .00275 µF

Meter-Use Quiz A ____ ammeter can measure current without physically interrupting the circuit. clamp-on A clamp-on ammeter uses a ____ ____ connected to another ammeter. current transformer Voltmeter ____ occurs when the meter’s Ri is not many times larger than the resistance to which it is connected. loading A DMM with 10 MW of Ri should indicate ____ V when connected to a 10-MW resistor that is series connected with a 5-MW resistor to a 40-V source. 20 A capacitor bridge uses ____ capacitor(s) and ____ resistor(s) in the bridge. two

REVIEW Basic Meter Movement Analog Meters Current Transformer Digital Ohmmeter Basic Meter Movement Analog Meters Current Transformer Meter Loading Bridges