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MultimetersandCROs
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AV Meter(Multimeter)
.
2. Pointer
3. Zero positionadjuster
4. Zero-ohmposition adjuster
2
.
6. Full-scaledeflection value
7. Input terminals
Takingareading
Selectasuitablerangetoproduceamaximumdeflection.
Rangecorrespondstothefullscaledeflection(f.s.d.)
of
the
meter.
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If f.s.d. is 50 V,then the readingis 47 V
Aswingcoilmeter
1. The pointer moves
swinging coil
2. An iron core whichconverges themagnetic field lines
3. Pole piece of the
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permanent magnet
4. The swinging coil
5. Returning hairspring.
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Characteristics
Inputimpedance(internalresistance)inthecoildeterminesthecurrent.
Currentincoilproducesatorqueinthepresenceofamagneticfield.
Theironcoreconvergesthemagneticlinesintheradialdirection.
Shapeofthetwopolepiecesallowsfieldlinestobealwaysparalleltotheplaneofthecoil. Hence
torquepro uce
epen s
on y
on
t e
strengt
othecurrent.
Deflectionofpointer torqueproduced currentflowingincoil appliedvoltage.
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Voltmeter Large
internal
resistance
(or
input
impedance
for
A.C.measurement).
.k V1.
CanmeasureD.C.aswellasA.C.voltages.
Internalresistanceofvarioustypesofvoltmeter Vacuumtubevoltmeter(VTVM) : 11M Cathoderayoscilloscope(CRO) : 10M
Digitalvoltmeter
(DVM)
:
10M
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Effectsofinternalresistance
Thecombinedresistanceofr&RisalwayssmallerthenrorR.HencetheinsertionofthevoltmeteralwayslowersthepotentialbetweenA&B.
RrrR
7
rrRRr
>>=+
Example
Thevoltageacrossthe1M resistorismeasuredbyavoltmeterhavin aninternalresistanceof20k V1.Whatarethereadingswhen10Vf.s.d.and2.5Vf.s.d.areused?
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10Vf.s.d.
Internalresistance=10 x 20 k = 200 k.
V0.86V6167.01
167.0ABacrossvoltage
M167.0k200M1
ABacrossResistance
=+
=
=+
=
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2.5Vf.s.d.
Internalresistance=2.5 x 20 k = 50 k.
V0.27V6047.01
047.0ABacrossvoltage
M047.0ABacrossResistance
=+
=
=
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ChangingVoltageRange
Im Rm Rs
Vm
AseriesresistorRsisconnected.
V~Im(Rm+Rs)
e.g.iffullscaledeflectionrequires1mA,andRm =
V
1000,thenfullscalevoltageisImRm =1V. IfRs =9000,thenfullscaledeflectionoccursat
V=
11
10 V.
ToMeasureHighTension
I Rm Rs
Vm
Rs becomesahighvoltageprobewithresistanceas
V
.
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D.C.andA.C.measurement
D.C.voltageisapplieddirectlytotheswingingcoil.
. . . .followingtwostepsbeforeconnectedtothecoil.
Rectification
Filtering
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Rectification
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(a) Half-wave rectifier (b) Full-wave rectifier
Filtering
Aspecialcircuitusedtosmoothouttheoscillatingattern orri les sothatastableandconstant
D.C.voltageisfedtothecoil.
Thefinal
reading
is
the
root
mean
squared
(R.M.S.)valueoftheA.C.
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Ammeter
erysma n erna res s ance
CanmeasureD.C.andA.C.currents
Theaddition
of
the
ammeter
will
inevitably
increase
thetotalresistancebetweenABandthereforelowersthecurrentflow.
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rRifR
V
Rr
Vi >>
+=
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D.C.andA.C.current
Theammetercanbethoughtasavoltmetermeasuringthep.d.betweenpointsa&b.
HencedescriptiononD.C.andA.C.voltage. . . .
currentmeasurement
Scaleislinear,i.e.
deflection magnetictorque current
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ChangingCurrentRange
RmIm
Rs
Is
A shunt resistor Rs is connected.
Since IsRs = ImRm
e.g.iffullscaledeflectioncorrespondsto1mA,thenwhenRm =999 andRs =1,fullscaledeflectioncorrespondsto?
181 A
I = Im+Is = Im+ImRm/Rs = Im(1+Rm/Rs)
A.C.clipmeter
ThewirecarryingtheA.C.currentisclipped
yt emeter.
Thechangingcurrentwill
produce
achanging
magneticfluxintheclip,henceinducinganem n ec p.
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Changingmagneticflux
BdAd
20densityfluxmagneticB
coiltheofareasectional-crossA
by Aenclosedfluxmagneticwhere
dtdt
=
=
=
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Notesonclipmeter
Thewireneedsnotbeplacedatthecentreoftheclip.
themagneticfieldlines. ForuseinHongKong,ithastobecalibratedfor50Hz.
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A.C.Parameters
Peakvalue:theamplitudeormaximumvalue
Avera evalue:
Effectivevalue:IfaDCcurrentandanACcurrentareappliedtothesameresistorR.Withinoneperiod,theyproduceequalthermalheating:
==T
0
T
0dt)t(V
T
1Vanddt)t(i
T
1i
22
=
==
T
0
2
T
0
2T
0
22
dt)t(vT
1V,Similarly
dt)t(iT
1IorRdt)t(iRTI
Ohmmeter
1. Ohm-meter
2. Voltmeter
3. Range selector
4. Internal battery
. resistance R
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Usinganohmmeter
comparableareR&rwhereRr
rV
+
rR
rcurrentdeflectionHence,
+
, . .islarge,risalsochosentobelargeandviceversa.
Setzerobyshortingthetwoterminals.
Scaleisusuallynonlinear.
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Scaleofanohmmeter
Theresistanceisthemeterreadingmultipliedbytherangefactor.
In(a),
if
x10
k isused,resistanceis7x10kwhich
is70k. In(b),thesameresistanceismeasuredinthex1
k range.
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Precautions
Choose the suitable mode A.C. or D.C., voltage orcurrent or resistance.
Note that the pointer is at the zero position of the scalewhen the two input terminals are shorted together. Adjustthe position if necessary.
To measure D.C., connect the terminals such that currententers the meter from the + terminal and leaves from the- terminal.
Always start with the range that gives the smallestdeflection of the meter. Then decrease the rangestepwise until suitable deflection is obtained. Rememberthat larger deflection gives smaller percentage error.
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Precautionsforohmmeters
Adjust the zero-ohm position by shorting the two terminalstogether.
Note that the internal battery has its positive poleconnected to the - terminal of the meter such that current
flows out from that terminal to the external resistor. Thispoint is especially important when the meter is used tomeasure diodes or transistors.
Do not measure the resistance of a component that isconnected to a circuit.
Always switch off the power supply to that component.
After measurement, turn the range of the meter to theother modes such as voltmeter or ammeter to prevent
accidentally draining the internal battery. 27
SketchofaCathodeRayTube
Fluorescentscreen
Electron gun
Luminous
traceElectron
beam
Deflectin
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Sawtoothwave
plates
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ControlsofaCRO Intensity Focus
Voltagerangeselector(Vcm1 ormVcm1)
Timebase(scm1,mscm1 orscm1)
XshiftandYshift
Yinput
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npu
Lissajous Figures
Compare
voltages;
phases
and
frequencies
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OtherControls
Sweepmode:auto,normal,single
, , ,
Coupling AC,DC,TV,HFrej
Slope
+ve,ve
Triggeringlevel
Doublebeamdisplay
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Choppingmode:goodforlowfrequencywaves