multimeter & cro

8/8/2019 Multimeter & CRO

1/8

1

MultimetersandCROs

1

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.

3

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

4

permanent magnet

4. The swinging coil

5. Returning hairspring.


2/8

2

Characteristics

Inputimpedance(internalresistance)inthecoildeterminesthecurrent.

Currentincoilproducesatorqueinthepresenceofamagneticfield.

Theironcoreconvergesthemagneticlinesintheradialdirection.

Shapeofthetwopolepiecesallowsfieldlinestobealwaysparalleltotheplaneofthecoil. Hence

torquepro uce

epen s

on y

on

t e

strengt

othecurrent.

Deflectionofpointer torqueproduced currentflowingincoil appliedvoltage.

5

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

6

Effectsofinternalresistance

Thecombinedresistanceofr&RisalwayssmallerthenrorR.HencetheinsertionofthevoltmeteralwayslowersthepotentialbetweenA&B.

RrrR

7

rrRRr

>>=+

Example

Thevoltageacrossthe1M resistorismeasuredbyavoltmeterhavin aninternalresistanceof20k V1.Whatarethereadingswhen10Vf.s.d.and2.5Vf.s.d.areused?

8


3/8

3

10Vf.s.d.

Internalresistance=10 x 20 k = 200 k.

V0.86V6167.01

167.0ABacrossvoltage

M167.0k200M1

ABacrossResistance

=+

=

=+

=

9

2.5Vf.s.d.

Internalresistance=2.5 x 20 k = 50 k.

V0.27V6047.01

047.0ABacrossvoltage

M047.0ABacrossResistance

=+

=

=

10

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

.

12


4/8 4

D.C.andA.C.measurement

D.C.voltageisapplieddirectlytotheswingingcoil.

. . . .followingtwostepsbeforeconnectedtothecoil.

Rectification

Filtering

13

Rectification

14

(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.

15

Ammeter

erysma n erna res s ance

CanmeasureD.C.andA.C.currents

Theaddition

of

the

ammeter

will

inevitably

increase

thetotalresistancebetweenABandthereforelowersthecurrentflow.

16

rRifR

V

Rr

Vi >>

+=


5/8 5

D.C.andA.C.current

Theammetercanbethoughtasavoltmetermeasuringthep.d.betweenpointsa&b.

HencedescriptiononD.C.andA.C.voltage. . . .

currentmeasurement

Scaleislinear,i.e.

deflection magnetictorque current

17

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.

19

Changingmagneticflux

BdAd

20densityfluxmagneticB

coiltheofareasectional-crossA

by Aenclosedfluxmagneticwhere

dtdt

=

=

=


6/8 6

Notesonclipmeter

Thewireneedsnotbeplacedatthecentreoftheclip.

themagneticfieldlines. ForuseinHongKong,ithastobecalibratedfor50Hz.

21

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

23

Usinganohmmeter

comparableareR&rwhereRr

rV

+

rR

rcurrentdeflectionHence,

+

, . .islarge,risalsochosentobelargeandviceversa.

Setzerobyshortingthetwoterminals.

Scaleisusuallynonlinear.

24


7/8 7

Scaleofanohmmeter

Theresistanceisthemeterreadingmultipliedbytherangefactor.

In(a),

if

x10

k isused,resistanceis7x10kwhich

is70k. In(b),thesameresistanceismeasuredinthex1

k range.

25

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.

26

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

28

Sawtoothwave

plates


8/8 8

ControlsofaCRO Intensity Focus

Voltagerangeselector(Vcm1 ormVcm1)

Timebase(scm1,mscm1 orscm1)

XshiftandYshift

Yinput

29

npu

Lissajous Figures

Compare

voltages;

phases

and

frequencies

30

OtherControls

Sweepmode:auto,normal,single

, , ,

Coupling AC,DC,TV,HFrej

Slope

+ve,ve

Triggeringlevel

Doublebeamdisplay

31

Choppingmode:goodforlowfrequencywaves

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