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Dr. Nirmalya GhoshElectrical Engineering, IIT Kharagpur
Figures Courtesy:"Electronic Instrumentation and Measurements 3rd
Edition" book by a!id # $ell
Measurement and ElectronicInstruments (EE 21004)
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2
Electrical Bridge ircuits
Measurement o! "esistance# Inductance# and
a$acitance
Measurement and Electronic Instruments(EE 21004)
2
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%
"esistance Measurement&oltmeter and 'mmeter
Methodso connectionsase 1* &oltmeter across
load
Error in current measurement'$$lica+le* " ,, " &
ase 2* 'mmeter in seriesith load
Error in -oltage measurement'$$lica+le* " " '
/u+stitution Method
Measurement and Electronic Instruments(EE 21004)
%
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4
Bridge ircuits* BasicsNull detection method't +alanced condition
or resistance measurement/u$$ly and detector* dcMagnitudes are only +alanced
or inductance andca$acitance measurement/u$$ly and detector* acMagnitude and $hase +oth
+alanced
3241 ZZZZVV ba ==
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4
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heatstone Bridge* !orresistanceGal-anometer initially shunted
ith a lo resistance't +alance#Bridge 'ccuracy*
3error o! "* sum 3 errors o! (/# #5).
/ensiti-ity o! G.Bridge recision *
Bridge sensiti-ity de$ends on*/u$$ly -oltage* EGal-anometer resistance* rGurrent sensiti-ity o! Gal-anometer
'$$lica+le* 6 to 10126
Q
PSR=
Q
PSR =
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7
Bridge /ensiti-ity
he-enin e8ui-alent circuitat G terminals
9" that $roduces smallest9IG to shi!t G $ointer +y 1 mmon scale (sensiti-ity o! G)
G
SR
G
BSR
rr
VV
I
SQRPr
QS
S
PR
REVV
+
=
+=
+
+=
||||
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7
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:
heatstone Bridge* limits;o resistance measurement
onnecting lead resistances4igh resistance measurementInsulation resistances&oltmeterigh -oltage D su$$ly&oltmeter across su$$ly (" "&)
Modi?ed heatstone Bridgeo se$arate out -olume current
and sur!ace currentGuard ires# guard rings
Measurement and Electronic Instruments(EE 21004):
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@
=el-in Bridge* lo resistance;ead resistance A is remo-ed
i!
E8uate -oltages o! across G
'$$lica+ility* 10 6 to 1 6hermo
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heatstone Bridge !or high "Insulations
Guard ire# guard rings/ur!ace resistances* +# cModi?ed heatstone
+ridgea+le !ault detection
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10
Inductora$acitor* Basics
Measurement and Electronic Instruments(EE 21004)10
E
V
E
VQ
CRR
L
Q
R
EIVV
CL
CL
==
==
==
1
and
:resonanceAt
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' Bridge* BalancingBoth magnitude and
$hase to +e +alancedo (or more) elements
are adFusted in turn anditerati-ely
Bridge sensiti-ityDe?ned and com$uted
similarly# +ut ithim$edances
De$ends on* E and9IG
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12
/eries " Bridges/im$le a$acitance Bridge
De /auty +ridge
/eries
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14
/chering Bridge' series " +ridge
/tandard lo D airmica 1o +alance# adFusta+le %
'$$lica+le*
/mall Hat lo E>igh & ca$acitance
Guard ring to isolate leaCagecurrent
Grounded screens* eliminate
stray Insulationdielectric material/u+stitution method
33
4
31
1
34
RCRCD
R
RCC
C
CRR
XX
XX
==
==
)1()2( SSX CCC =
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1
ien Bridge
om+ination o! seriesal! Immeans IDislarge
Diode +iased +eyondCneein linear &
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4
>al!
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0
ull
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1
Electrodynamometer*constructionermanent magnet is
su+stituted +ystationary ?eld coils/ame or diQerent currents
in mo-ing and stationarycoils
Interaction +eteenmagnetic ?elds
'ir dam$ing* Mo-ing-ane
'ir
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2
Electrodynamometer*$eration
Magnetic ?eld $roduced +y current So* re$ulsi-e eQectluH direction* rightmeters.Measurement and Electronic Instruments (EE
21004)
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%
Electrodynamic -oltmeter Kammeter &oltmeter
Need high resistance*'ll coils in series>igher /D current than in MM;oer coil resistance* ;oading eQect.
Multi$lier resistance'mmeter
ield coil current is measured Ihermal eQect not im$ortant
/hunt and /am$ing resistorhermal eQect im$ortant as in MM
DeSection tor8ue
Energy ()* 0. I!2;!P 0. Im2;mP I!ImMDJ I! ImdMd(theta) * heta J
deSection (in radians)Nonlinear scale
Measurement and Electronic Instruments (EE
21004)
i ( )
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4
Mo-ing Iron (MI) Meters*construction
'ttraction ty$e
"e$ulsion ty$e
oncentric
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MI instruments* or8ue# ros#ons
J 0. I2;DJ 0. I2d;d(theta) * heta J deSection (in radians)'d-antage
erminal non
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Electrodynamic attmeterield (current) coils
single coil sym+olMo-ing (-oltage) coil
Multi$lier resistor to restrictcurrent
"e-ersed $olarity* still $ositi-e
rans!er instrumentali+rated in D* J & I (in atts)In '* actual J &rms Irmscos X
Measurement and Electronic Instruments (EE
21004)
E
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attmeter* Errors#com$ensation
Burdens o! the coilsom$ensated attmeter
ield coil* hicC co$$er ireom$ensating coil* hin# re-erseold
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rigger le-el control andBlanCing
rigger control'dFusted +y "
a-e!orm cancommence atany le-el
BlanCingly
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Dual trace "
Multi$le signals com$aredo sets o! -ertical deSection systemsb one
set !or horiontal
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21004)@
DeSection system in dual trace
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DeSection system in dual trace" Dual
+eam/$lit
+eam
Measurement and Electronic Instruments (EE
21004)@7
/itched hannel Method
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/itched hannel Method
/ingle
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$ ymode Both signal
ith same
$eriod D shi!t addedintentionally
'lternatelyand re$eatedlydis$layed
>igh sitching!re8uency*dis$lays seem
simultaneous.EHce$t at lo
!re8uency
Measurement and Electronic Instruments (EE21004)@@
Dual trace dis$lay in ho$
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Dual trace dis$lay in ho$mode 'lternate dis$lay
time slots'$$ear
simultaneous!or lo and mid
!re8uency
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scillosco$e controls
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scillosco$e controls
/ingle +eam dual trace analog oscillosco$erigger
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" +ased measurements
eaC
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ulse measurements
ulse am$litude# $ulse idth# s$ace idth
"ise
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scillosco$e ro+esy$ically " in$ut
im$edance* (i# "i) J1M6 %0 $
't normally o$erating!re8uencies*a$acitor reactance
-ery high&i J &s "i ("i P "s)
o a-oid signi?cantattenuation or $haseshi!t# signal!re8uency , (%dBcut
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'ttenuator $ro+es
Generally 10*1 $ro+es ith "1 J M6/cre
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ro+e cali+ration
E-ery $ro+e to adFusted hen ?rst use in a$articular "ront $anel control'cti-e $ro+es
E -oltage !olloers* Electronic am$li?ers toincrease $ro+e in$ut resistance and minimiein$ut ca$acitance.
Measurement and Electronic Instruments (EE21004)
Dis$lay o! de-ice
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Dis$lay o! de-icecharacteristics
'B
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A Dis$lays# L Dis$lays
;issaFou ?gures ne signal
/ame !re8uency# diQerent $hase DiQerent !re8uencies (ith ratio J rationalnum+er)
L
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scillosco$e s$eci?cations/ensiti-ities
>oriontal-ertical2m&di- V 10&di-0. sdi- V 200 nsdi-
ro+e ratio (1*1# 10*1)ime
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$ $
re8uency res$onse* 2 > V 100 M>'ttenuation !or in$ut circuit (am$li?ers)# $ro+e and oscillosco$eDeSection am$li?er cutoQs/ignal !re8uency* 10 !lo 'ND , !hi10hen i$ and o$ +oth dis$layed# " attenuations and $hase