Ammeter&voltmeter

How to use PMMC as voltmeter an ammeter

1. Electronic Ammeter and Voltmeter

2. Content
•Basic Ammeter
•MultirangeAmmeter
•Numerical basedon range of Ammeter
•BasicVoltmeter
•MultirangeVoltmeter
•Numerical based on Voltmeter
•ElectronicVoltmeter(EVM)

3. Basic Ammeter
•ThebasicMovementofDCammeterisPMMCd’Arsonval
galvanometer.
•Thecoilwindingofbasicmovementissmall,lightandcan
carryverysmallcurrentssincetheconstructionofanaccurate
instrumentwithamovingcoiltocarrycurrentgreaterthan
100mAisimpracticableowingtothebulkandweightofcoil
toberequired.
•Whenheavycurrentismeasured,themajorpartofcurrentis
bypassthroughalowresistanceconnectedinparallelto
meter.
•Thislowresistanceisknownas“ShuntResistance”.

4. Basic Ammeter cont.
Where,
Rm=internal resistance of meter(coil)
Rsh=shunt resistance
Im=Ifs=full scale deflection current of movement
Ish= current through shunt resistance
I=Current to be measured
Since the shunt resistance and meter branch are in parallel or voltage
across both branch is equal
??????
??????ℎ??????
??????ℎ=??????
????????????
??????
??????
??????ℎ= Τ??????
????????????
????????????
??????ℎ
??????
??????ℎ=??????−??????
??????
??????
??????ℎ= Τ??????
????????????
????????????−??????
??????
??????
????????????=Τ??????
????????????−??????Where�=Τ????????????
??????

5. Multirange Ammeter
Let m1, m2, m3 and m4 be shunt multiplying powers for currents I1,I2,I3 and I4
??????
1=??????
??????/�
1−1
??????
2=??????
??????/�
2−1
??????
3=??????
??????/�
3−1

6. Multirange Ammeter cont.
•Amovingcoilmeterhavinginternalresistance??????
??????=12Ωand
fullscaledeflectioncurrentof20�??????.Designamultirange
ammeterandcalculateshuntresistanceforfollowingcurrent:
a.500mAmp b.1Amp c.5Amp
a.�
??????=
??????
????????????
=
??????
??????
????????????
=
500∗10
−3
20∗10
−3
=25
??????
??????ℎ=
??????
??????
�−1
=
??????
??????
�
??????−1
=
12
(25−1)
=0.5Ω
b.0.244Ω
c.0.048Ω

7. Numerical
•A2mAmetermovementwithaninternalresistanceof300Ωis
tobeconvertedinto0-100mAammeter.Calculatethevalue
ofshuntresistancerequired.
??????
??????=2�??????
??????
??????=300Ω
??????
??????ℎ=?and??????
??????ℎ=?
??????
??????ℎ=??????−??????
??????=100−2=98�??????
�=
??????
??????
??????
=
100
2
=50
??????
??????ℎ=
??????
??????
�−1
=
300
(50−1)
=6.122Ω
??????
??????ℎ∗??????
??????ℎ=??????
??????∗??????
??????
??????
??????ℎ=
??????
??????∗??????
??????
??????
??????ℎ
=
2∗10
−3
∗300
98∗10
−3
=6.122Ω

8. Numerical
•Amovingcoilmeterhavingaresistanceof8Ωandgivesafull
scaledeflectionwith25mA.Howwecanusetomeasurethe
currentupto2A.
??????
??????=25�??????
??????=2??????=2000�??????
??????
??????=8Ω
??????
??????ℎ=?and??????
??????ℎ=?
??????
??????ℎ=??????−??????
??????=2000−25=775�??????
�=
??????
??????
??????
=
2000
25
=80
??????
??????ℎ=
??????
??????
�−1
=
8
(80−1)
=0.101Ω
??????
??????ℎ∗??????
??????ℎ=??????
??????∗??????
??????
??????
??????ℎ=
??????
??????∗??????
??????
??????
??????ℎ
=
25∗10
−3
∗8
775∗10
−3
=0.101Ω

9. DC Voltmeter
•Theadditionofseriesresistance,ormultiplier,convertsthe
basicd’ArsonvalmovementintoVoltmeter.
•Themultiplierlimitsthecurrentthroughthemovementsoas
nottoexceedthevalueofthefullscaledeflectioncurrent
(Ifs).
•ADCvoltmetermeasurethepotentialdifferencebetween
twopointsinadccircuitandisthereforeconnectedacrossa
sourceofemforacircuitcomponent.
•Themeterterminalaregenerallymarked“Pos”and“Nes”
sincepolaritymustbemeasured.

10. DC Voltmeter cont.
Multiplier
The Value of multiplier, required to extend the voltage range, is
calculated from shown Diagram Where,
??????
�=�������??????������������������������??????
��
??????
�=??????�����??????�??????��??????��??????������������������
??????
�=����??????��??????��??????��??????��??????���
??????=�����??????�������??????����??????���������
??????=??????
�??????
�+??????
�

11. Multi Range Voltmeter
•Theadditionofanumberofmultiplier,togetherwiththe
rangeswitch,providestogetherWithaworkableranges.
•Amultirangevoltmeterusingfourpositionswitchandfour
multipliers:R1,R2,R3andR4forthevoltagerangeV1,V2,V3
andV4respectively.
•Themultiplierresistancecanbecalculatedbyfollowing
method:
•??????
??????=??????
??????�+1,Where
•�=
??????
??????
??????
,Where
•??????
??????=??????
??????∗??????
??????

12. Numerical
•ThebasicPMMCinstrumentwithaninternalresistanceof
100Ωandfullscalecurrentof1mAistobeconvertedinto
0-10VVoltmeter.
??????
??????=1�??????=1∗10
−3
??????
??????
??????=100Ω
??????
??????=??????
??????∗??????
??????=100∗1∗10
−3
=0.1
�=
??????
??????
??????
=
10
0.1
=100
??????
??????=??????
??????�−1=100∗100−1=9900=9.9??????

13. Electronic Voltmeter
•Theelectronicvoltmeteruseamplifier,rectifiersandother
circuitstogenerateacurrentproportionaltovoltagebeing
measured.
•ThecurrentsoproducebeingmeasuredbyPMMCinstrument
havingatautbandsuspensioninsteadofpivot-jewel
mechanics.
•Advantages:
1.Lowpowerconsumption
2.DetectionofLowSignals
3.HighFrequencyRange(0-1000MHz)

14. Electronic Voltmeter cont.
•Vacuum tube Voltmeter
•TransistorVoltmeter
•FET input voltmeter
•Differential FETVoltmeter
•ChopperAmplifierVoltmeter
•Solid-state Voltmeter

15. Vacuum tube Voltmeter
•InVacuumtubevoltmeterthereisatriodesvalvestooperate
cathodefollower.
•Thegridoffirstvalveisfedbyinputsignalanditscathode
voltagechangesinsympathywithinputsignalgivean
amplificationofroughlyunit.
•Thesecondtriodesisfedfromfixedviasvoltageandits
cathodeprovidedafixedreferenceagainstwhichtheinput
signalismeasured.
•Themovingcoilinstrumentanditsassociatedmultiplier
resistorisconnectedbetweentheanodesandwillprovidea
voltagereadingcorrespondingtotheinputvoltage.
•Now,thecurrentrequiredtodrivethemovingcoilisgivenby
currentflowfromanodetocathode.

16. Vacuum tube Voltmeter cont.

17. Vacuum tube Voltmeter cont.
•Advantages:
1.Theeffectiveinputimpedanceofacathodefollower
circuitisveryhigh.
2.Thebiaslevelappliedtothegridsecondvalveismade
variablesothatmeterreadingcanbesettozero,when
inputtoterminalareshorttogether.
•Disadvantages:
1.ItrequiredaACpowersupply.
2.ThevalvesofVacuumtubegeneratesalotofheat.So,it
takelottimetosettledowntostablebeforeanyother
measurementdone

18. Transistor Voltmeter
•Transistor Differential amplifier circuit is used as Transistor
voltmeter.
•Ithasveryhighinput resistance.
•Thetemperatureisalso compensated through emitter
resistance Re.

19. Transistor Voltmeter cont.

20. Transistor Voltmeter cont.
•Advantages:
1.It do not required any warn up time.
2.Itiscompact, light weight and well suitable for field work.
3.It canalsobedesignasabatteryoperated.
•Disadvantages:
1.Sensitivity islowascompare to VTEM.
2.Accuracy of instrument is only for ambient temperature
only.(Drift Current)