MEASUREMENTS AND INSTRUMENTATION

ELE 4101

LECTURE III

Lecturer: Jomo N. Gill

DETERMINING Rm

The meter resistance Rm can be determined using the following methods: - Meter calibrator method - Full-scale deflection method

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THE METER CALIBRATOR METHOD

The meter calibrator places a controlled and calibrated current to the meter under test.

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When the current is adjusted for full-scale deflection, the voltage across the meter terminals is measured, and the resistance is determined by applying Ohm's Law.

THE FULL-SCALE DEFLECTION METHOD

This process is illustrated via the Figures below.

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- Vs is increased until the meter reaches full scale. Vs is noted.

- RA is a current limiter

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- A pot is then placed across the meter and adjusted until the meter current is just less than half full-scale. - Vs is doubled, and the pot is adjusted until the current in the meter approaches full-scale.

- The meter resistance and RB are now the same.

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- Vs is turned off and RB is measured to determine the internal resistance of the meter.

Worked example on Rshunt.

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CLASSWORK

A multiple range ammeter is shown in the Figure.

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MULTI-RANGE AMMETERS

Drawback:

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- When the rotary switch is between ranges, the resulting open circuit in the shunt path causes all the current to flow through the meter, which could be disastrous. Practical solution: - Use an Ayrton arrangement

- Ammeters are placed in series with other circuit elements.

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IN-CIRCUIT SETUP

- Ammeters should have negligible resistance, in order to minimise loading effects.

- We should always start measurements with the highest range.

Slide Number 1DETERMINING RmTHE METER CALIBRATOR METHODTHE FULL-SCALE DEFLECTION METHOD- Vs is increased until the meter reaches full scale. Vs is noted.Slide Number 6Slide Number 7Slide Number 8Slide Number 9Slide Number 10