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CHARACTERISTICS OF MOVING COIL METER MOVEMENT

Following are few characteristics of Moving Coil Meter Movement.

(1) Full-scale deflection current (Im),
(2) Internal resistance of the coil (Rm),
(3) Sensitivity (S).

1. FULL-SCALE DEFLECTION CURRENT (IM)
It is the current needed to deflect the pointer all the way to the right to the last mark on the calibrated scale. Typical values of Im for D’ Arsonval movement vary from 2 μA to 30 mA.

It should be noted that for smaller currents, the number of turns in the moving coil has to be more so that the magnetic field produced by the coil is strong enough to react with the field of the permanent magnet for producing reasonable deflection of the pointer.


Fine wire has to be used for reducing the weight of the moving coil but it increases its resistance. Heavy currents need thick wire but lesser number of turns so that resistance of the moving coil is comparatively less. The schematic symbol is shown in Figure.
2. INTERNAL RESISTANCE (RM)
It is the dc ohmic resistance of the wire of the moving coil. A movement with smaller Im has higher Rm and vice versa. Typical values of Rm range from 1.2 Ω for a 30 mA movement to 2 kΩ for a 50 μA movement.

3. SENSITIVITY (S)
It is also known as current sensitivity or sensitivity factor. It is given by the reciprocal of full scale deflection current Im.
S=1/ Im  ohms/volt

The sensitivity of a meter movement depends on the strength of the permanent magnet and number of turns in the coil. Larger the number of turns, smaller the amount of current required to produce full-scale deflection and, hence, higher the sensitivity. A high current sensitivity means a high quality meter movement. It also determines the lowest range that can be covered when the meter movement is modified as an ammeter or voltmeter.

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