Lect No. Range Extension of PMMC Galvanometer

7/25/2019 Lect No. Range Extension of PMMC Galvanometer

http://slidepdf.com/reader/full/lect-no-range-extension-of-pmmc-galvanometer 1/33

Instrumentation &

Measurements

Extending range of PMMC Galvanometer



PMMC DC Ammeter

The basic movement of d.c. ammeter is a PMMC

D’Arsonval galvanometer.

Galvanometer are use to detect or measure very small

currents in µA

The Coil of basic movement is small, light eight ! carry

very small currents.

The construction of an accurate instrument ith a moving

coil to carry bul" current is im#racticable.



PMMC DC Ammeter

To construct an ammeter for measuring large current

some low resistance is to connected across the meter coil.



PMMC DC Ammeter

This resistance is called as $hunt resistance.

The ma%or #ortion of the current being measured flos

through the shunt resistance.

A small #ortion of current flos through the meter coil to

#roduce deflection.

Derivation to find the value of suitable shunt resistance

& sh.



PMMC DC Ammeter

• The ratio of total current to the current in the moving coil

is called multi#lying #oer of Shunt.

'

'

−

=⇒=

+⇒=

=

m

R R

R

R

I

I m

m

sh

sh

m

m



PMMC DC Ammeter



PMMC DC Ammeter



General requirements forshunt

The general re(uirements for shunts are,

'. )ery lo tem#erature co*efficient.

+. Their resistance should not vary ith time.

. They should carry current ithout e-cessive tem#erature

rise.



Multi-range Ammeter

Extension of Ammeter range by Using Multiple shunts



• The range of the basic d.c. ammeter can be e-tended by using

number of shunts and a selector sitch. $uch ammeter is

called multi*range ammeter. and is shon in the ig.

• & ' ,& + , & and & / are four shunts. 0hen connected in #arallel ith

the meter, they can give four different ranges 1' ,1+ , 1 and 1/. Theselector sitch $ is multi*#osition sitch, having lo contact

resistance and high current carrying ca#acity. The ma"e before

brea" ty#e sitch is used for the range changing.



.

• 1f the ordinary sitch is used, hile range changing the sitch

remains o#en for small amount of time and full current #asses

through the meter. The meter may get damaged due to such high

current. $o ma"e before brea" sitch is used.

• The design of such sitch is so that it ma"es contact ith ne-t

terminal before com#letely brea"ing the contact ith the

#revious terminal.



.

• The multi*range ammeters are used for the ranges u# to 23 A.

0hile using the multi*range ammeter, highest range should be

used first and the current range should be decreased till goodu#scale reading is obtained. All the shunts are very #recise

resistance and hence cost of such multi*range ammeter is high.

•The mathematical analysis of basic d.c. ammeter is e(uallya##licable to such multi*range ammeter. Thus,

• & ' 4 & m 5m' *'

• & + 4 & m 5m+ *' and so on,

here m' ,m+ ,m, .... are the shunt multi#lying #oers for thecurrents 1' ,1+ , 1, ......





Multi-range Ammeter

Multi*range ammeter ith Ayrton shunt.



.

• e have seen that in multi*range ammeter, a ma"e before brea" sitch

is must. The Ayrton shunt or universal shunt eliminates the #ossibilityof having a meter ithout a shunt.

• The selector sitch $, selects the a##ro#riate shunt re(uired to change

the range of the meter. 0hen the #osition of the sitch is 6'6 then theresistance & ' is in #arallel ith the series combination of & + , & and

&m. 7ence current through the shunt is more than the current through

the meter, thus #rotecting the basic meter.



.

• 0hen the sitch is in the #osition 6+6, then the series resistance

of &' and &+ is in #arallel ith the series combination of & , and

&m. The current through the meter is more than through the

shunt in this #osition.

• 1n the #osition 66, the resistances & ' , & + and & , are in series and

acts as the shunt. 1n this #osition, the ma-imum current flos

through the meter. This increases the sensitivity of the meter.



• The voltage dro# across the to #arallel branches is alays e(ual.

Thus, 1sh & sh 4 1m & m

In position 1, & ' is in #arallel ith & + 8 & 8 & m

here 1' is the first range re(uired.

• In position 2, & ' 8 & + is in #arallel ith & 8 & m .

here 1+ is the second range re(uired.



• In position 3, & ' 8 & + 8 & is in #arallel ith & m . here 1 is

the third range re(uired.

• The current range 1 is the minimum hile 1' is ma-imumrange #ossible.

• $olving the e(uations 9':, 9+: and 9: the re(uired Ayrtonshunt can be designed.







Precautions to be TakenWhile Using an Ammeter

The folloing #recautions must be ta"en hile using an ammeter.

• ': As the ammeter resistance is very lo, it should never be

connected across any source of e.m.f. Alays connect an ammeter in

series ith the load.

• +: The #olarities must be observed correctly. The o##osite #olarities

deflect the #ointer in o##osite direction against the mechanical sto#

and this may damage the #ointer.

• : 0hile using multi*range ammeter, first use the highest current

range and then decrease the current range until sufficient deflection

is obtained. $o to increase the accuracy, finally select the range

hich ill give the reading near full scale deflection.



DC Voltmeter

• $im#le D’Arsonval movement can be converted to a

voltmeter by connecting suitable resistance in series ith

the galvanometer coil.

• This resistance is called to be as Multi#lier.



•The main function of the multi#lier is to limit the current

through the basic meter so that the meter current does not e-ceed

the full scale deflection value. The voltmeter measures the

voltage across the to #oints of a circuit or a voltage across

circuit com#onent.

•The multi#lier resistance can be calculated as;

<et & m 4 1nternal resistance of coil i.e. meter

& s 4 series multi#lier resistance

1m 4 full scale deflection current

) 4 full range voltage to be measured rom ig ', ... ) 4 1m 9& m 8 & s :

... ) 4 1m& m 8 1m & s... 1m& s 4 ) * 1m & m



•The multi#lying factor for multi#lier is the ratio of full range

voltage to be measured and the dro# across the basic meter.

<et v 4 dro# across the basic meter 4 1m& mm 4 multi#lying factor 4 )5v







Multi-range oltmeter

• The range of the basic d.c. voltmeter can be e-tended by

using number of multi#liers and a selector sitch. $uch a

meter is called multi*range voltmeter and is shon in the ig.'.



Multi-range oltmeter

• The & ', & +, & and & / are the four series multi#liers. 0hen

connected in series ith the meter, they can give four

different voltage ranges as )', )+, ) and )/. The selector

sitch $ is multi*#osition sitch by hich the re(uired

multi#lier can be selected in the circuit.

• The mathematical analysis of basic d.c. voltmeter is e(ually

a##licable for such multi*range voltmeter. Thus,



Practical Multi-range Voltmeter

• More #ractical arrangement of multi#lier resistances is

shon in the ig.+.



1n this arrangement, the multi#liers are connected in series

string. The connections are brought out from the %unctions of the

resistances. The selector sitch is used to select the re(uired voltage

range.

0hen the sitch $ is at #osition )', & ' 8 & + 8 & 8 & / acts as a

multi#lier resistance. 0hile hen the sitch $ is at #osition )/ then

the resistance & / only acts as multi#lier resistance. The )/ is

the loest voltage range hile )' is the ma-imum voltage range.



•The multi#lier resistances can be calculated as ;

•1n #osition )/, the multi#lier is & / only. The total resistance of the

circuit is say & T .

•1n #osition ), the multi#lier is & 8 & /.



1n #osition )', the multi#lier is & ' 8 & + 8 & 8 & /.

1n #osition )+, the multi#lier is & + 8 & 8 & /.



Difference Between Ammeter and Voltmeter

Lect No. Range Extension of PMMC Galvanometer

Documents