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ASSIGNMENT OFMEASUREMENTS ANDINSTRUMENTATION

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ASSIGNMENT TOPIC:

1.DISCUSS IN DETAIL ELECTRONIC AC VOLTMETER

2.ADVANTAGES OF ANALOG AND DIGITAL VOLTMETER

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ELECTRONIC AC VOLTMETER

INTRODUCTION:

ELECTRONIC AC VOLTMETERS differ from their dc counterparts only in

that, the AC voltage must be converted to dc before being applied to

the meter movement.

DEFINITION:

A meter circuit built around an active device, such as vacuum tube or

a transistor, and having high input impedance.

AC electromechanical meter movements come in two basic

arrangements:

Those based on DC movement designs.

Those engineered specifically for AC use.

CONDITIONING:

Permanent-magnet moving coil (PMMC) meter movements will not

work correctly if directly connected to alternating current, because the

direction of needle movement will change with each half-cycle of theAC. From diagram, Permanent-magnet meter movements, like

permanent-magnet motors, are devices whose motion depends on the

polarity of the applied voltage (or, as in terms of the direction of the

current). The alternating current must be rectified into DC. This is

most easily accomplished through the use of devices called diodes.

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Diagram of PMMC movement when it is applied AC input

WORKING PRINCIPLE:

In electronic ac voltmeters input signal is firstly rectified and thensupplied to the dc amplifier. Sometimes signal is firstly amplified by acamplifier and then rectified before supplying it to dc meter.

A simple electronic voltmeter circuit using an operational amplifires,

If the combination of voltage to current converterand precision diode

rectifire.by putting the diode or diodes inside the feedback loop of the

operational amplifire the effect of diode voltage drop is minimized.

Then if here use half wave rectifire the current waveform through

meter is a half rectifired sinewave even for input voltages less than

0.7v peak and current through meter is Im is given by

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Im = Vin/R

Since average current through meter is

Im (average) = 0.318Im (Peak)

Im (average) = 0.45IrmsOn other hand a full wave bridge rectifire can be used in op- amps

feed back loop as shown below .

Two diodes are forward biased on positive half cycle and current flows

from + to of PMMC meter. During negative half cycle yhe other two

diodes are forward biased and current flowing through the meter as

during the positive half cycle.

instantaneous current through meter is;

o Im = Vin/R2

For a sinewave the average current through the meter is

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Im (average) = 0.637Im (Peak)

Im (average) = 0.9 Irms

In case, the circuit shown in figure making use of rectifying diode,series resistance R, dc amplifier and PMMC is employed, the averagevoltage across R will indicate the peak value of applied voltage. Thisalternative is preferred, as explained earlier; the power consumptioncan be reduced by making series resistance R high. By making seriesresistance R high a less sensitive type of PMMC instrument can also be

used. The high value input resistance also gives more linearrelationship between peak applied voltage and the instrumentindication.

ADVANTAGES OF ANALOG VOLTMETER

AND DIGITAL VOLTMETER:

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Advantages of analog voltmeter:

As costs have fallen, analog meters much cheaper than digital

voltmeter.

Analog, if you know the meter scale, you will know approximatereadings peripherally or with just a glance to see that the needleis in the right neighborhood.

Analog meter resistance should be ten times the circuitresistance to ensure accurate readings.

A high sensitivity is necessary to prevent upsetting the testcircuit.

Analog meters have several different options for displaying data:commonly a galvanometer registers movement.

Advantages of digital voltmeter:

The digital readout of a digital meter circumvents all of theseconcerns by displaying a precise readout in digital numeric form.

This feature makes them potentially more portable than theiranalog counterparts, which are limited in scale by their need to

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house mechanical components. Increased portability necessitatesincreased durability.

A digital meter is calibrated by exposing it to a neutral, or zero-level, signal and then setting the readout to zero. This is

typically accomplished with the push of a button, whichsends a signal to the meter to reset the readout.

Electronics-related digital meters have built-in overloadprotection that is uncommon in analog meters. The componentsof an analog meter will burn out if exposed to too strong anelectrical load. A protected digital meter is protected from thesame load.