AC_bridges मैक्सवेल ब्रिज मैक्सवेलब्रिज इंडसइंड ब्रिज

AC Bridge

1. 1.ACbridges,
2.MaxwellBridge,
3.Maxwell-WienBridge,
4.AndersonBridge,
5.Hay’sBridge,
6.OwenBridge,
7.DeSautyBridge
8.Sheringbridge,
9.WienSeriesBridge.
ACBridges

2. ACbridges
A.C.Bridgesarethosecircuitswhichareusedtomeasuredtheunknownresistances,
capacitance,inductance,frequencyandmutualinductance.
Anybridgecircuitisbalancedwhenthecurrentthroughbranchbetweentwoarmsiszeroas
wellasphase.
Inthesimplestformanacbridgeconsistsoffourimpedances,anacsourceandnulldetector.
Insomecasesacamplifierwithanoutputmeterisusedasnulldetector.
Thenullindicationisobtainedwhenthevoltageatpointbisequaltothevoltageatpointd.
Thesetwovoltagesshouldbeequalintermsofbothamplitudeandphase.Thatmeansto
obtainthiscondition;thevoltagedropacrossabshouldbeequaltothevoltagedropacrossad.
Intermsofcurrentandimpedancewecanwrite,
I1Z1=I2Z2.and I3Z3=I4Z4.

3. ExampleThefourimpedancesofanacbridgeasshowninthefigureareZ1=500<
40
o
Ω,Z2=100<90
o
Ω,Z3=45<20
o
Ω,Z4=30<30
o
Ω.Findoutwhetherthebridgeis
balancedornot.
Solution:Asweknowinpolarco-ordinatesystemrepresentation,thebridgebalance
conditionsare
Thus,
bridgeisunbalanced.
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4. Maxwell’sBridge
MaxwellInductanceBridge
Inthisbridgearrangementthevalueofunknowninductanceismeasuredbycomparisonwith
avariablestandardself-inductance.FigureshowsthecircuitarrangementforMaxwell’s
inductancebridgeunderbalancecondition.Twobranchesbcandcdconsistofnon-inductive
resistanceR3andR4.OneofthearmsadconsistsofvariableinductanceL2connectedin
serieswithvariableresistanceR2.TheremainingarmabconsistsofunknowninductanceL1
andresistanceR1.Asourceofcurrentisappliedtotwooppositejunctionsacrossacanda
nulldetectorisconnectedtotheothertwojunctionsbandd.

5. Maxwell-WienBridge
MaxwellInductanceCapacitanceBridge
Inthisbridgearrangement,thevalueofunknowninductanceismeasuredbycomparisonwith
avariablestandardcapacitor.Figureshowsitscircuitarrangement.Twoarmsbcandad
consistofnon-inductiveresistanceR2andR3.
OneofthearmsacconsistsofvariablestandardcapacitorCconnectedinparalleltoanon-
inductiveresistanceR4.
TheremainingarmabconsistsofunknowninductanceL1ofeffectiveresistanceR1.Asource
ofcurrentisappliedtotwooppositejunctionsacrossacandanulldetectorisconnectedtothe
othertwojunctionsbandd.Hence,theimpedancesoffourarmsare
Atbalanceweget
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6. Hence,fromtheaboveconditionfor theunknown
inductancevaluecanbedeterminedbycomparisonwithvariable
standardcapacitor.
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7. •Thescaleofresistancecanbe
calibratedtoreadinductancedirectly.
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8. Example
AMaxqell-WienbridgeusesastandardcapacitorofC=0.IμFandoperatesatasupply
frequencyof100Hz.BalanceisachievedwhenR2=1.26kΩ,R3=500Ω,andR4=410Ω.
Calculatetheinductanceandresistanceofthemeasuredinductor,
=1.26kΩx500Ωx0.IμF
=63mH
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9. AndersonBridge
•Thisbridge,infact,isamodificationoftheMaxwell-Wienbridge.
•Inthismethod,theself-inductanceismeasuredintermsofastandard
capacitor.
•Figureshowstheconnectionsofthebridgeforbalancedconditions.
Fromloopabd
Fromloopbce
Fromloopdce
I1=I2
Atbalancebetweenbandd Ic=Ir
fromthetheruleatanyjunctiontheinputcurrentequalthe
output
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10. from
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and
theequationsof
balance

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12. Hay’sBridge
ItisalsoamodificationoftheMaxwell’sWienBridgeandisparticularlyusefulif
thephaseangleoftheinductiveimpedanceislarge.
ThelimitationofMaxwell’sbridgeisthatitcanbeusedforhighQvalues.TheHay’s
bridgeissuitableforthecoilshavinghighQvalues.
ThedifferentinMaxwell’sWienbridgeandHay’sbridgeisthattheHay’sbridge
consistsofresistanceR2inserieswiththestandardcapacitorCinoneoftheratio
arms.HenceforlargerphaseanglesR2neededisverylow,whichispracticable.
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13. 2
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222
●
42
1●
3
124

14. •Itisbestsuitableforthe
measurementofinductance
withhighQ,typicallygreater
than.10
•Itgivesverysimpleexpression
forQfactorintermsof
elementsinthebridge.
•Itrequiresverylowvalue
resister
R1tomeasurehighQ
inductance.
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•Itisonlysuitable
formeasurementof
highQinductance.

15. Thisbridgeused
variable
resistors
tomeasuretheself-inductionLofthecoilwithcapacitorand
two
fromthisequation
measureL
fromthisequation
measureC2
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OwenBridge,

16. DeSautyBridge
Thisisthesimplestmethodforcapacitancemeasurementfig.
showsthebasiccircuitarrangement.
Fromthebalance
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17. ScheringBridge
Scheringbridgeusedforthemeasurementof
capacitanceanddielectriclossofacapacitor.
Itisadeviceforcomparinganimperfectcapacitor
C1intermsofaloss-freestandardcapacitorC2.
Asshowninfig.
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18. WienSeriesBridge
Itisabridgeusedforthemeasurementoffrequency.
Apartfromthisithasavarietyofapplicationsintheharmonic
distortionanalyzerwhereitisusedasanotchfilter,discriminating
againstaspecificfrequencyintheaudioandHFoscillatorsas
frequencydeterminingagainstaspecificfrequencyintheaudio
andHFoscillatorsasfrequencydeterminingelement.
Hence,theimpedancesoffourarmsare
Z3
=R3
Z4
=R4
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19. ExampleThearmsofafour-armbridge,,andsuppliedwith
sinusoidalvoltagehavethefollowingvalues.
Determinethevalueof2andfrequencyatwhichthebridgewill
balance?
frombalanceequation
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20. MultipleChoice
Questions
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1.Maxwellinductancecapacitancebridgeisusedformeasurementofinductance
of
(a)lowcoils(b)mediumcoils
(c)highcoils(d)lowandmediumcoils
2.Frequencycanbemeasuredbyusing
(a)Wienbridge(b)Scheringbridge
(c)Maxwellbridge(d)HeavisideCampbellbridge
3.Theeffectivereactanceofaninductivecoil
(a)increasesbecauseofstraycapacitanceasfrequencyincreases
(b)remainsunchanged
(c)decreasesbecauseofstraycapacitanceasfrequencyincreases
4.Inductancecanbemeasuredby
(a)Maxwellbridge
(c)Schering
bridge
b)Haybridge
(d)Wienbridge