lect no. range extension of pmmc galvanometer
TRANSCRIPT
7/25/2019 Lect No. Range Extension of PMMC Galvanometer
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Instrumentation &
Measurements
Extending range of PMMC Galvanometer
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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.
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PMMC DC Ammeter
To construct an ammeter for measuring large current
some low resistance is to connected across the meter coil.
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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.
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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
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PMMC DC Ammeter
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PMMC DC Ammeter
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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.
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Multi-range Ammeter
Extension of Ammeter range by Using Multiple shunts
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• 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.
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.
• 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.
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.
• 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, ......
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Multi-range Ammeter
Multi*range ammeter ith Ayrton shunt.
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.
• 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.
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.
• 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.
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• 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.
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• 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.
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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.
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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.
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•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
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•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
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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.'.
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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,
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Practical Multi-range Voltmeter
• More #ractical arrangement of multi#lier resistances is
shon in the ig.+.
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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.
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•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 & /.
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1n #osition )', the multi#lier is & ' 8 & + 8 & 8 & /.
1n #osition )+, the multi#lier is & + 8 & 8 & /.
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Difference Between Ammeter and Voltmeter