Instrumentation

Analog and Digital Meters

Topics:-

PMMC- Construction and working DC Ammeters Calculation of shunt Resistance, Ayrton multirange ammeters and numeric Problem

2

To understand the basic contruction and working principle of D’Arsonval meter movement.

Construction of Ammeters by using PMMC for measurment currentin differant ranges.

Designing of Multirange ammeter.

To study universal shunt Multirange ammeter.

To study precauton while using ammeter.

Numerics problem

PMMC- Construction and working:-

Objective :-

PMMC- Mechanism

Construction:- The D’Arsonval movement also

called Permanent-Magnet Moving Coil (PMMC).

Based on the moving-coil galvanometer constructed by Jacques D’ Arsonval in 1881.

PMMC has two Horse-Shoe permanent magnet.

The coil can rotate freely inpermanent magnetic field B

A pointer is attached to the coil.Motor action is produced by the flow

of small current through moving coil.When current flow in the coil, the

torque is produced. This torque is counterbalanced by

mechanical torque of the control spring attached to the coil.

The balance torque produces angulardisplacement.

Constructional Detail:-

Torque developed by coil is given by :-Ԏ =B.A. I. NWhere B = magnetic Flux dencity [ wb/m2 ]A= Area of coil [m2]I= Current flowing through the coil.N = No of turn of coil.,

Torque due to control springԎ = C ѲBoth Torque are equalB.A.I.N = C.Ѳ

PMMC :- Advantage and Disadvantage

PMMC can be modified with the help of connecting shunt and seriesresistances to cover wide range of current , voltages and resistance measurement.

It is most accurate for measurement of DC voltages and current. It is not affected by stray magnetic field Some error may be possible due to ageing of control spring , permanent magnet and

friction due to Jewel-pivot suspension.

Rsh

+

_

_

+

Rm

D’Arsonval Movement

DC Ammeters :-

DC ammeter can be obtained by connecting PMMC with shunt resistance.The coil winding of a basic movement is small and light, so it can carry only very small currents.A low value resistor (shunt resistor) is used in DC ammeter to measure large current.When large current are to be measured, it is necessary to by pass a major part of current through shunt resistance.

I Ish Im

Basic DC Ammeter

Where

Rm = internal resistance of the movement

Rsh = shunt resistance

Ish =shunt current

Im = full scale deflection current of the movement

I = full scale current of the ammeter + shunt (i.e total current)

m

mm

sh

msh

mmshsh

II

RIR

III

RIRI

V Shunt = V Movement

Multirange ammeter :-

More than one different ranges of current are available for measurement .

The different ranges are obtained by connecting desired value of shunt resistance to

the movement.

The range of the dc ammeter is extended by connecting a number of shunts to the

movement.

The selector Switch [S] select the desired range for current measurement.

S (multiposition switch) protects the meter movement from being damage during

range changing.

R1 R2 R3 R4

+

_

+

_Rm

D’ArsonvalMovement

Figure :-Multirange Ammeter

S

I1 I4

I2I3

Im

Fig shows multirange ammeter having four ranges I1,I2,I3 and L4 and four shunt resistance R1,R2,R3 and R4 respectively . The switch S (multiposition switch) protects the meter movement from being damage during range changing.Desiredrange can be selected by make before break switch S. For I1 range,R1 is shunt resistance ,for I2range, R2 is shunt resistance and so on. Multirange ammeter are used for ranges up to 50 A. While using multirange ammeter, First select highest current range, then decreases the range until good upscale reading is obtained.

Multirange ammeter :- Working Principle

Aryton shunt OR Universal shunt multirange ammeter:-

When multirange ammeter is connected in a certain circuit for the measurement of current,

while changing the switches from one current range to another, meter movement remain in the

circuits without any shunt resistance. During this time ,excessive current flows through the

meter movement coil and may damage its. To avoid this situation Ayton shunt arrangement is

used along with meter movement.

Ayton shunt protect the meter movement and eliminates the possibility of having the meter in

the circuit without a shunt resistance.

Reduce cost

Rc

Rb

Ra

RmD’Arsonval Meter

+

_+

_

Fig: -Aryton Shunt

I3

I2

I1Im

Aryton shunt OR Universal shunt multirange ammeter:-

When switch is at I1 range ,( Ra+Rb +Rc ) is parallel with meter movement resistance RmWhen switch is at I2 range ,( Ra+Rb) is parallel with meter movement resistance [Rm + Rc ]When switch is at I3 range ,( Ra) is parallel with meter movement resistance [Rm +Rb + Rc ]For I1 range

Ra+ Rb+ Rc =

For I2 range

Ra+ Rb =

Ra =

For I3 range

Precaution while using ammeter for measurement of current:-

Never connect the ammeter across the source of emf. (Battery) Because it has low resistance it would draw a high current so that movement coil break and destroy the movement.Always connect ammeter in series with load resistance ,capable of current limiting so to protect the movement.Observe the correct polarity.While using multirange, first use highest current range, then decreases current range until substantial deflection is obtained.To increase the accuracy use the proper range that will give reading near to the actual value.