ch5 dc meter

8/13/2019 Ch5 DC Meter

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INSTRUMENTATION &

MEASUREMENT

Chapter 4:

Introduction to Oscilloscope


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For many decades the basic dc meter movement has

been used as the readout device in electronic

instruments, and even in many instruments that

measures as values. The two most common dc meter movement

configurations are the dArsonvaland the taut band

designs.

Both are examples of the Permanent Magnet Moving

Coil (PPMC) galvanometer.

They are based on the same fundamental principle

as the dc motor.


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Moving Coil Instrument

Moving coil instrument or generally referred to as

the dArsonval meter or a permanent magnet moving

coil (PMMC) meter is mostly used in direct current

measurement. The PMMC instrument is essentially a low-level dc

ammeter.

However, with the use of parallel-connected resistor,

it can be employed to measure a wide range of direct

current levels.


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Moving Coil Instrument

The instrument may also be made to function as a dc

voltmeter by connecting appropriate value resistor in

series with the coil.

AC ammeter and voltmeter can be constructed byusing rectifier circuits with the PMMC instrument.

Multimeters are combinations of multirange

ammeter, voltmeter and ohmmeter functions in one

instrument.


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Basic construction

The basic construction of this instrument is as shown

in figure 5.1, figure 5.2, and figure 5.3.

It is a horseshoe permanent magnet with soft iron

pole pieces attached to it.

Between the pole pieces is a cylindrical shaped soft

iron core about which a coil of fine wire is wound.

The wire is wound on a very light metal frame andmounted in such a setting so that it can rotate freely.

A pointer attached to the moving coil deflects up the

scale as the moving coil rotates.


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Basic operation principle

Current from a circuit in which measurement are

made with the meter passes through the winding of

the moving coil.

This current causes the moving coil to behave as anelectromagnet with its own north and south poles.

These poles interact with the poles of the permanent

magnet producing a force which causes the coil to

rotate.


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Basic operation principle

The pointer deflects up the scale whenever current

flows in the proper direction in the coil.

For this reason, all dc meters show polarity markings.

It should be emphasized that dArsonval meter

movement is a current responding device.

Regardless of the unit for which the scale is

calibrated, the moving coil responds to the amountof current through its windings.


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Advantages of Moving Coil

Instrument

Low power consumption

Scales are uniform

No hysterisis loss Cheap and robust


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Disadvantages of Moving Coil

Instrument

Cam measure dc only

Errors are introduced due

Spring elasticity may deteriorate


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DArsonval meter as a DC

Ammeter Since the windings of the moving coil are of very fine

wires, the basic dArsonval meter has limited

usefulness without modification.

One desirable modification is to increase the range ofcurrent that can be measured by the basic meter.

This is done by placing a resistor of low resistance in

parallel with the moving coil whose resistance is .

This low resistance resistor is called a shunt ( ).


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Its function is to provide an alternative path for the

total metered current I around the meter movement.

This basic dc ammeter circuit is shown in figure 4.

In most circuits, is greater than , which flows in themoving coil itself.

ch5 dc meter

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