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Measurement of R,L and C

Measurement of R,L and C. Unit-2. Bridges. Bridges are widely used in the measurement of components such as resistor, capacitors and inductors. They are design to have four arms with a very sensitive galvanometer connected between two point and the supply at the other two ends.

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Measurement of R,L and C

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  1. Measurement of R,L and C Unit-2

  2. Bridges • Bridges are widely used in the measurement of components such as resistor, capacitors and inductors. • They are design to have four arms with a very sensitive galvanometer connected between two point and the supply at the other two ends. • Bridges owe their popularity to high accuracy, high sensitivity, and the ability to measure a wide range of quantities. • Bridges may be d.c and a.c, depend on the component that make up the bridges arms and the kind of current used to energize them.

  3. In the case of a d.c source the bridges can be used in the measurement of resistance only while in the case of a.c source, it could be used in the measurement of both resistance and reactance due to capacitors or inductor connected in any of the arms. • Typical example of d.c bridge is the wheat stone bridge. While example of a.c bridges are the Maxwell, owen, wien, De santy, Hay’s and Schering bridges .

  4. Classification of resistance • Low resistance:- >1Ω • Medium resistance:- from 1Ωupwords to about 0.1MΩ • High resistance:- from 0.1MΩ to high resistance.

  5. Measurement of medium resistance: Medium resistance Wheatstone bridge method Ohmmeter method Substitution method Ammeter –voltmeter method

  6. Measurement of low resistance: low resistance potentiometer Kelvin’s double bridge Ammeter voltmeter method

  7. Measurement of low resistance: high resistance Meggar Megohm bridge Direct deflection method

  8. Meggers • The megger is portable instrument used for testing the insulation resistance of a circuit, and for measuring the resistance of the order of mega ohms in which the measured value of resistance is directly indicated on a scale. • The indication of the megger is independent of voltage. Meggers use high voltages to measure very high resistance. • Applications of MeggerMeggers are used in industries for observing the following tests:- Open circuit tests Short circuit tests Continuity tests Ground tests Earth resistance tests

  9. Ac bridges: • Measurement of inductance, capacitance may be made conveniently and accurately by employing a.c bridge networks. • An a.c bridge, in its basic form, consists of four arms, a source of excitation, and a balance detector. • In an a.c bridge each of the four arms, is an impedance, and the battery and the galvanometer of wheatstone bridge are replaced respectively by an a.c source and a detector sensitive to small alternating potential differences . • A. C bridge circuits are commonly used for phase shifting, providing feedback paths for oscillators and amplifiers, filtering out undesirable signals and measuring the frequency of audio signals.

  10. Anderson Bridge • An Anderson Bridge is used to measure the self inductance of a coil.

  11. DC potentiometer • A potentiometer is an instrument designed to measure an voltage by comparing it with a known voltage. • The known voltage may be supplied by a standard cell or any other known voltage----reference source. • A resistor having a continuously adjusted sliding contact that is generally mounted on a rotating shaft; used chiefly as a voltage divider. Also known as pot (slang). • A device for the measurement of an electromotive force by comparison with a known potential difference. • An instrument that precisely measures an electromotive force (emf) or a voltage by opposing to it a known potential drop established by passing a definite current through a resistor of known characteristics. (A three-terminal resistive voltage divider is sometimes also called a potentiometer.)

  12. advantages • the potentiometers is that since a potentiometers make use of balance or null condition,no current flows and hence no power is consumed in the ckt containing the unknown emf when the instrument is balanced. • A potentiometer measure voltage , it can also be used to determine current simply by measure the voltage drop produced by the unknown current passing through a known standard resistance. • The potentiometer is extensively used for a calibration of voltmeter and ammeter and has in fact become the standard for the calibration of these instruments.

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