Multimeter VIMP

7/21/2019 Multimeter VIMP

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12/8/151

Prof.S.Lakshminarayana,M.Tech.,Ph.D.

Galvanometer/Alications



12/8/15

SYLLABUS

UNIT I

• Defnitions, Errors in Measurement.• DC Ammeters.• DC voltmeters.• Ohmmeter – Series type, Shunt type,

Caliration. Multimeter as• DC voltmeter,• DC Ammeter• AC voltmeter, AC voltmeter usin! re"tifer• Ohmmeter,• Di!ital voltmeter –• #amp,• Inte!ratin!• Continuous $alan"e Type. %$asi" Con"epts&



!nstr"mentation an$ Meas"rement

D% Meter



TYPES OF INSTRUMENTS

12/8/15

1. Permanent Ma&net Movin& %oil. 'nly forD.%.Meas"rements.

2. Movin& !ron. A.%/D.%

(. )lectro*Dynamometer. A.%/D.%

+. ot-ire. A.%/D.%5. Thermoco"le. A.%/D.%

. !n$"ction. A.%.Meas"rements.

. )lectrostatic. A.%/D.%8. 0ectier. A.%/D.%



hat is a meter3

5

A meter is any 'evi"e uilt to

a""urately 'ete"t an' 'isplayan ele"tri"al (uantity in a )ormrea'ale y a human ein!.

Usually this *rea'ale )orm* isvisual+motion o) a pointer on a s"ale,a series o) li!hts arran!e' to )orm

a *ar !raph,* orsome sort o) 'isplay "ompose' o)numeri"al f!ures.

In the analysis an' testin! o)

"ir"uits, there are meters



Most modern meters are "digital" in design, meaning

that their readable display is in the form of

numerical digits. (Less Error)

Older designs of meters are mechanical in nature,

using some kind of pointer device to sho !uantity of

measurement.

n either case, the principles applied in adapting a

display unit to the measurement of (relatively) large

!uantities of voltage, current, or resistance are the

same.



Display Me"hanism

The display mechanism of a mete is often efeed to

as a movement ! "oo#in$ fom its mechanical nat%eto move a pointe alon$ a scale so that a meas%ed

&al%e may "e ead'

Tho%$h moden di$ital metes ha&e no mo&in$ pats!the tem (mo&ement( may "e applied to the same

"asic de&ice pefomin$ the display f%nction'

Most mechanical mo&ements ae "ased on the

pinciple of electoma$netism) that electric currentthrough a conductor produces a magnetic field

perpendicular to the axis of electron flow.



8

The $eate the electic c%ent! the ston$e the

ma$netic field pod%ced'

If the magnetic field formed by the conductor is

allowed to interact with another magnetic field, a

physical force will be generated between the two

sources of fields' If one of these sources is free to move with

respect to the other, it will do so as current is

conducted through the wire, the motion (usuallyagainst the resistance of a spring) being

proportional to strength of current.



Galvanometer

4

#alvanometer is a $ermanent Magnet

Moving %oil electric current detector

($MM%).

&hen a current is passed through a coil in

a magnetic field, the coil e'periences a

tor!ue proportional to the current.

f the coils movement is opposed by a coilspring, then the amount of deflection of a

needle attached to the coil is proportional

to the current passing through the coil.

uch "meter movements" ere at the heart

of the moving coil meters (such asvoltmeters and ammeters) until they ere

largely replaced ith solid state meters.

*he accuracy of moving coil meters is

dependent upon having a uniform and

constant magnetic field.



1

The foce on the coil F* nBiSin

+ nBi if +--

n=No. of turns of the coil.

B=Magnetic field of inductioni=urrent through the coil.

=!eflection.

The ma$netic moment +*foce , #idth of coil

+* nBil," * nBiA -A* l ,".+ θ*nBiA

i*/+0-nBA.1 + k



a vanomeer

11

An incease in meas%ed c%ent

#ill di&e the needle to pointf%the to the i$ht and a decease

#ill ca%se the needle to dop "ac2

do#n to#ad its estin$ point on

the left'

The ac on the mete display isla"eled #ith n%m"es to indicate

the &al%e of the 3%antity "ein$

meas%ed! #hate&e that 3%antity

is'

In othe #ods! if it ta2es 45 6A of c%ent to di&e theneedle f%lly to the i$ht -ma2in$ this a (45 6A f%ll7scale

mo&ement(.! the scale #o%ld ha&e 5 6A #itten at the &ey

left end and 45 6A at the &ey i$ht! 84 6A "ein$ ma2ed in

the middle of the scale'



alvanometer use' as Ammeter

Shunt #esistan"e.

12/8/1512

Tyical &alvanometer have an internalresistance of the or$er of 0m6 7* that co"l$

si&nicantly $ist"r 9re$"ce: a c"rrentmeas"rement.

;"ilt to have f"ll scale for small c"rrent !m 6 1

mA or less.M"st therefore e mo"nte$ in arallel -ith a

small resistor or sh"nt resistor.Galvanometer Ω

0



12/8/151(

Galvanometer

Ω

0

• Let9s con&et a :5 7! ; mA f%ll scale $al&anomete to an

ammete that can meas%e %p to 8 A c%ent'• R p m%st "e selected s%ch that #hen 8 A passes tho%$h the

ammete! only 5'55; A $oes tho%$h the $al&anomete'

• R p is athe small< * ImR m0-I 7 Im.

• The e3%i&alent esistance of the cic%it is also small<

<M 6 9.1A:97: 6 91.444A: 0

0 6 .(2 7

<

M

!

!M



12/8/151+

=al&anomete %sed as >oltmete • Finite intenal esistance of a $al&anomete m%st

also addessed if one #ishes to %se it as &oltmete'• M%st mo%nted a la$e esisto in seies to limit the

c%ent $oin$ tho%$h the &oltmete to ; mA'

• M%st also ha&e a la$e esistance to a&oid

dist%"in$ cic%it #hen meas%ed in paallel'

Galvanometer Ω0s



12/8/15

Galvanometer Ω0s

Ma?im%m &olta$e acoss $al&anomete)

( ) ( )ma? 5'55; :5 5'5:V A V ∆ = Ω =

S%ppose one #ish to ha&e a &oltmete that can meas%e

&olta$e diffeence %p to ;55 >)

Lar&e resistance1< 6 9.1A:90s=Ω:

0s 6 444+



<oltmeter Desi&n

1

Some D-Arsonval movements have )ulls"ale 'e/e"tion "urrentratin!s as little as 01 2A, 3ith an %internal& 3ire resistan"e o) lessthan 4111 5. This ma6es )or a voltmeter 3ith a )ulls"ale ratin! o)only 01 mill volts %01 2A 7 4111 5&8

In or'er to uil' voltmeters 3ith pra"ti"al %hi!her volta!e& s"ales

)rom su"h sensitive movements, 3e nee' to fn' some 3ay tore'u"e the measure' (uantity o) volta!e 'o3n to a level themovement "an han'le.

Usin! Ohm-s 9a3 %E:I R&, 3e "an 'etermine ho3 mu"h volta!e 3ill'rive this meter movement 'ire"tly to )ull s"ale+

E : I R

E : %4 mA&%011 5& E : 1.0 volts



1

To $et an effecti&e &oltmete mete an$e in

e?cess of ;08 &olt! #e@ll need to desi$n a cic%it

allo#in$ only a pecise popotion of meas%ed

&olta$e to dop acoss the mete mo&ement'

This #ill e?tend the mete mo&ement@s an$e to

"ein$ a"le to meas%e hi$he &olta$es than "efoe'

+oespondin$ly! #e #ill need to e7la"el the

scale on the mete face to indicate its ne#meas%ement an$e #ith this popotionin$

cic%it connected'



Multimeter or <OM%Simpson Mo'el =>1& >%'?T)?TS@

http://var/www/apps/conversion/tmp/scratch_3/LECTURE%203.ppt#2.%20CONTENTS




Schematic $ia&ram of theSimson Mo$el 2m"ltimeter

$c voltmeter section of theSimson Mo$el 2m"ltimeter



$c ammeter section of theSimson Mo$el 2

m"ltimeter

'hmmeter section of theSimson Mo$el 2m"ltimeter



Caliration o) DC Instruments >%'?T)?TS@

Potentiometer metho$ ofcaliratin& a $c ammeter ***

Potentiometer metho$ ofcaliratin& a $c voltmeter ***





ntroduction to lternating/current

ndicating nstruments.

The d@Ason&al mo&ement esponds to the a&ea$e o dc&al%e of the c%ent tho%$h the mo&in$ coil

If the mo&ement caies an altenatin$ c%ent #ith positi&e

and ne$ati&e half cycles! the di&in$ to3%e #o%ld "e in

one diection fo the positi&e altenation and othe diectionfo the ne$ati&e altenation

If the fe3%ency of the ac is &ey lo#! the pointe #o%ld

s#in$ "ac2 and foth ao%nd eo point on the mete scale

At hi$he fe3%encies! the inetia of the coil is so $eat thatthe pointe cannot follo# the apid e&esals of the di&in$

to3%e and ho&es ao%nd the eo ma2! &i"atin$ sli$htly'



Measurin!?i!her<olta!es

2(

hat -e nee$ is a voltage divider circ"it to

roortion the total meas"re$ volta&e into alesser fraction across the meter movementsconnection oints.

Bno-in& that volta&e $ivi$er circ"its are "ilt

from series resistances, -ell connect a resistor inseries -ith the meter movement.

'hms La- co"l$ e "se$ to $etermine resistance

9R6E/I: for the m"ltilierC 6 E/I − Rm

6 −

R m



M%lti7

Ran$e

>oltmete

2+

o- $o yo"calc"late all Ri 9i 6

1,2,(,+:3



<oltmeter

Loa$in&)Eect

25

Since voltmeters are al-aysconnecte$ in arallel -iththe comonent or

comonents "n$er test, anyc"rrent thro"&h thevoltmeter -ill contri"te tothe overall c"rrent in theteste$ circ"it, otentiallyaEectin& the volta&e ein&meas"re$.

A perfect voltmeter hasinnite resistance, so that itdraws no current from thecircuit under test.

The eFamle reresente$ inthe $ia&ram on the ri&ht is

very eFtreme case



<oltmeter #atin!

2

>oltmetes #ith electomechanical mo&ements ae typically$i&en atin$s in "ohms per volt0(ensitivity of meter) of

an$e to desi$nate the amo%nt of cic%it impact ceated "y thec%ent da# of the mo&ement'

Beca%se s%ch metes ely on diffeent &al%es of m%ltiplieesistos to $i&e diffeent meas%ement an$es! their lead/to/lead resistances ill change depending on hat rangetheyre set to.

On any + &olta$e an$e)

nalogue Meter 1esistance + ensitivity 2 Ma'. reading

of range

e'$' a mete #ith 8520> sensiti&ity on its ;5> an$e has aesistance of 8520> C ;5> * 8552'

By contast! digital multimeters ha&e a constant esistance of

at least ;M -often ;5M. on all thei + &olta$e an$es' This is

moe than eno%$h fo almost all cic%its'



2

Di!ital voltmeters, on the other han', o)tene@hiit a "onstant resistan"e across their testlea$s re&ar$less of ran&e settin& 9"t not al-ays:,

an$ as s"ch are "s"ally rate$ simly in ohms o)input resistan"e, rather than *ohms per volt*sensitivity.hat Hohms er voltH means is ho- many ohms oflea$*to*lea$ resistance for every volt of range

setting on the selector s-itch.)FamleC 'n the 1 volt scale, the totalresistance is 1 M 9444.5 k = 5:, &ivin&1,, er 1 volts of ran&e, or 1 ohmser volt 91 k/<:. This ohmspervolt

*sensitivity* ratin! remains "onstant )or anyran!e o) this meter.

The ast"te oserver -ill notice that the ohms*er*volt ratin& of any meter is $etermine$ y a sin&lefactor+ the full-scale current o) the movement,

in this "ase 4 mA.Sensitivity o) a meter : 4Bull s"ale De/e"tion Current. %*ohms per



Minimiin!9oa'in! Ee"t

28

To minimiIe the loa$in& of avoltmeter on any circ"it, the$esi&ner m"st seek tominimiIe the c"rrent $ra- ofits movement.

This can e accomlishe$ y

re*$esi&nin& the movementitself for maFim"msensitivity 9less c"rrentreJ"ire$ for f"ll*scale$eKection:, "t the tra$eoEhere is tyically r"&&e$nessC

a more sensitive movementten$s to e more fra&ile.

Another aroach is toelectronically oost thec"rrent sent to themovement, so that very littlec"rrent nee$s to e $ra-nfrom the circ"it "n$er test.

This secial electronic circ"itis kno-n as an amplier ,

an$ the voltmeter th"sconstr"cte$ is an ampliedvoltmeter .

Soli$*state transistoramlier circ"its accomlishthe same task in $i&ital

meter $esi&ns. hile thisaroach 9of "sin& anamlier to oost themeas"re$ si&nal c"rrent:-orks -ell, it vastlycomlicates the $esi&n ofthe meter.



MinimiIin&Loa$in& )Eect

24

An in&enio"s sol"tion to therolem of voltmeter loa$in&is that of the potentiometric or null-balance instr"ment.

!n a otentiometric

instr"ment, a recisiona$"stale volta&e so"rce iscomare$ a&ainst themeas"re$ volta&e, an$ asensitive $evice calle$ a nulldetector is "se$ to in$icate

-hen the t-o volta&es areeJ"al.

!n some circ"it $esi&ns, arecision potentiometer is"se$ to rovi$e thea$"stale volta&e, hence the

lael potentiometric.

hen the volta&es are eJ"al,there -ill e Iero c"rrent $ra-n

from the circ"it "n$er test, an$th"s the meas"re$ volta&esho"l$ e "naEecte$.

The Hn"ll $etectorH is asensitive $evice caale ofin$icatin& the resence of very

small volta&es. ?"ll $etectors are tyically

$esi&ne$ to e as sensitive asossile in or$er to morerecisely in$icate a Hn"llH orHalanceH 9Iero volta&e:con$ition.



AmmeterDesi!n

(

!n ammeter $esi&ns, eFternalresistors a$$e$ to eFten$ the"sale ran&e of the movementare connecte$ in parallel -iththe movement rather than inseries as is the case for

voltmeters. This is eca"se -e -ant to

$ivi$e the meas"re$ c"rrent9not the meas"re$ volta&e:&oin& to the movement, an$eca"se c"rrent $ivi$er

circ"its are al-ays forme$ yarallel resistances.

Takin& the same metermovement as the voltmetereFamle, -e can see that it-o"l$ make a very limite$instr"ment y itself, f"ll*scale$eKection occ"rrin& at only 1

sin& 5 ams as an eFten$e$ran&e for o"r samlemovement, lets $etermine theamo"nt of arallel resistancenecessary to Hsh"nt,H oryass, the maority of c"rrentso that only 1 mA -ill &othro"&h the movement -ith atotal c"rrent of 5 A.

Rsh 6 E/9Ish N IG:

6 .5</95A N 1mA:

6 .5</+.5444A

6 1.2 mΩ

l i



M"lti*0an&eAmmeter

(1

As is the case -ithm"ltile*ran&evoltmeters, ammeterscan e &iven more thanone "sale ran&e yincororatin& severalsh"nt resistors s-itche$-ith a m"lti*ole s-itch.

?otice that the ran&eresistors are connecte$thro"&h the s-itch so as

to e in parallel 3iththe meter movement,rather than in series as it-as in the voltmeter$esi&n.

The ve*osition s-itch makes

contact -ith only one resistor at atime, of co"rse. )ach resistor issiIe$ accor$in&ly for a $iEerent f"ll*scale ran&e, ase$ on the artic"larratin& of the meter movement 91mA, 5 :.

Oor an ammeter -ith ran&es of 1mA, 1 A, 1 A, an$ 1 A, the sh"ntresistances -o"l$ e as s"chC

Sh F



Shunts Fo3erDissipation

(2

'ne thin& to e a-are of -hensiIin& ammeter sh"nt resistorsis the factor of o-er$issiation.

nlike the voltmeter, an

ammeters ran&e resistors haveto carry lar&e amo"nts ofc"rrent.

!f those sh"nt resistors are notsiIe$ accor$in&ly, they mayoverheat an$ s"Eer $ama&e, or

at the very least lose acc"racy$"e to overheatin&.

Oor the eFamle meter aove,the o-er $issiations at f"ll*scale in$ication are resente$.

U i < lt t



Usin! <oltmeter asAmmeter

((

Sometimes, sh"nt resistors are"se$ in con"nction -ithvoltmeters of hi&h in"tresistance to meas"re c"rrent.

!n these cases, the c"rrentthro"&h the voltmeter movement

is small eno"&h to e consi$ere$ne&li&ile, an$ the sh"ntresistance can e siIe$ accor$in&to ho- many volts or millivolts of$ro -ill e ro$"ce$ er am ofc"rrent.

!f, for eFamle, the sh"nt resistorin the aove circ"it -ere siIe$ atrecisely 1 , there -o"l$ e 1volt $roe$ across it for everyam of c"rrent thro"&h it.

The voltmeter in$ication co"l$

then e taken as a $irectin$ication of c"rrent thro"&h the

A t ! t



Ammeter !mact onMeas"re$ %irc"it

(+

"st like voltmeters, ammeters ten$ to inK"ence the amo"nt ofc"rrent in the circ"its theyre connecte$ to.

o-ever, "nlike the i$eal voltmeter, the i$eal ammeter has Ierointernal resistance, so as to $ro as little volta&e as ossile aselectrons Ko- thro"&h it.

?ote that this i$eal resistance val"e is eFactly oosite as that ofa voltmeter. ith voltmeters, -e -ant as little c"rrent to e $ra-nas ossile from the circ"it "n$er test.

ith ammeters, we want as littlevoltage to be dropped as possible -hile con$"ctin& c"rrent.

ith the ammeter $isconnecte$

from this circ"it, the c"rrentthro"&h the ( resistor -o"l$ e. mA, an$ the c"rrentthro"&h the 1.5 resistor -o"l$e 1.(( ams.

A t ! t



Ammeter !mact onMeas"re$ %irc"it

(5

!f the ammeter ha$ an internalresistance of 1/2 , an$ it -ereinserte$ into one of the ranchesof this circ"it, tho"&h, itsresistance -o"l$ serio"sly aEectthe meas"re$ ranch c"rrentC

avin& eEectively increase$ theleft ranch resistance from ( to(.5 , the ammeter -ill rea$51.+( mA instea$ of . mA.Placin& the same ammeter in theri&ht ranch -o"l$ aEect thec"rrent to an even &reater eFtentC



ClampOn Ammeter

(

'ne in&enio"s -ay to re$"ce the imact that a c"rrent*meas"rin& $evice has on a circ"it is to "se the circ"it -ire asart of the ammeter movement itself.

All c"rrent*carryin& -ires ro$"ce a ma&netic el$, thestren&th of -hich is in $irect roortion to the stren&th of thec"rrent.

;y "il$in& an instr"ment that meas"res the stren&th of thatma&netic el$, a no*contact ammeter can e ro$"ce$.

S"ch a meter is ale to meas"re the c"rrent thro"&h acon$"ctor -itho"t even havin& to make hysical contact -iththe circ"it, m"ch less reak contin"ity or insert a$$itionalresistance.

Ammeters of this $esi&n are ma$e, an$ are calle$ Hclamp-onHmeters eca"se they have Ha-sH -hich can e oene$ an$then sec"re$ aro"n$ a circ"it -ire.

%lam*on ammeters make for J"ick an$ safe c"rrentmeas"rements, esecially on hi&h*o-er in$"strial circ"its.

;eca"se the circ"it "n$er test has ha$ no a$$itional resistanceinserte$ into it y a clam*on meter, there is no error in$"ce$



Ammeter

(

The act"al movement mechanism of a clam*onammeter is m"ch the same as for an iron*vane

instr"ment, eFcet that there is no internal -irecoil to &enerate the ma&netic el$.

More mo$ern $esi&ns of clam*on ammeters"tiliIe a small ma&netic el$ $etector $evicecalle$ a Hall-eect sensor to acc"rately

$etermine el$ stren&th. Some clam*on meters contain electronic

amlier circ"itry to &enerate a small volta&eroortional to the c"rrent in the -ire et-een the a-s, that small volta&e connecte$ to a voltmeterfor convenient rea$o"t y a technician.

Th"s, a clam*on "nit can e an accessory $eviceto a voltmeter, for c"rrent meas"rement.

D i



Desi!n

(8

The "rose of an ohmmeter isto meas"re the resistancelace$ et-een its lea$s. Thisresistance rea$in& is in$icate$thro"&h a mechanical metermovement -hich oerates onelectric c"rrent.

The ohmmeter m"st then havean internal so"rce of volta&e tocreate the necessary c"rrent tooerate the movement, an$also have aroriate ran&in&resistors to allo- "st the ri&ht

amo"nt of c"rrent thro"&h themovement at any &ivenresistance.

hen there is inniteresistance 9no contin"ityet-een test lea$s:, there is

Iero c"rrent thro"&h the metermovement, an$ the nee$le

!f the test lea$s of thisohmmeter are $irectly shorte$

to&ether 9meas"rin& Iero :, themeter movement -ill have amaFim"m amo"nt of c"rrentthro"&h it, limite$ only y theattery volta&e an$ themovements internal resistance.

Dith &olts of "attey potential andonly 455 of mo&ement esistance!o% cic%it c%ent #ill "e ;G mA!#hich is fa "eyond the f%ll7scaleatin$ of the mo&ement' S%ch ane?cess of c%ent #ill li2ely dama$e

the mete'



Desi!n

(4

De need a #ay to ma2e it so that

the mo&ement H%st e$istes f%ll7scale #hen the test leads ae

shoted to$ethe' This is

accomplished "y adding a series

resistance to the mete@s cic%it

The total cic%it esistance neededto limit c%ent to ; mA -f%ll7scale

deflection on the mo&ement. #ith

&olts of potential fom the

"attey can "e calc%lated fom)

On the left side of the scale #e

ha&e (infinity( and on the i$ht

side #e ha&e eo'

;esi$es ein& Hack-ar$sHfrom the scales of voltmetersan$ ammeters, this scale &oesfrom nothin& to innity.

Infinity cannot "e appoached in a

linea -e&en.' Dith a lo$aithmic

scale! the amo%nt of esistance

spanned fo any $i&en distance on

the scale inceases as the scale

po$esses to#ad infinity! ma2in$

infinity an attaina"le $oal'



%ertain disadvantage 3 &hen the battery is old, the full/scale current drops

and the meter does not read "-" hen " and B are shorted. 14 is ad5usted to

get the full scale reading.

f 16 is to be ad5usted to get the full scale deflection calibration ill change. o

it ill be better to ad5ust 14 only.

*he design can be approach by recogni7ing that, if introducing # $ reduces the

meter current to (8 .fsd ). 1 h is half of full scale deflection resistance.

E

R R I R R

hm fsd

h −=;

Series*Tye 'hmmeter



<al"es of 01 an$ 02 can e $etermine$ from theval"e of 0F -hich &ives half the f"ll scale $eKection.

-here 0h 6 half of f"ll scale $eKection resistance.

The total resistance resente$ to the attery theneJ"als 20

h

The attery c"rrent nee$e$ to s"ly half scale $eKection is

To ro$"ce f"ll scale c"rrent, the attery c"rrent m"st

e $o"le$. Therefore, the total c"rrent of the ckt, !t6 </0h

The sh"nt c"rrent thro"&h 02 is &iven y 9!2 6 lt N lfs$:

!h6< /2 0h .



The volta&e across sh"nt, <sh , is eJ"al to the

volta&e across the meter. Therefore <sh6 <m

!202 6 !fs$0m

;"t !26!t*!fs$

S"stit"tin& the vale of02

S"stit"te

6

;"t



Patic%laly s%ited to the

meas%ement of low%value resistors

Dhen R?* the f%ll7scale mete

c%ent #ill "e

The mete c%ent fo any &al%e of

R x ! e?pessed as a faction of the

f%ll7scale c%ent! is

At half7scale eadin$ R h of the mete

m

fsd R R

E I

+

=

;

p x

x

R R

R s

+=

m

mh

R R

R R R

+=

;

;

ShuntTypeOhmmeter

• By shotin$ AB! Im *5' So

left side is J59'

•If R ? is open! ma?im%mc%ent flo#s tho%$h the

mete and f%ll scale

deflection is infinity is on the

i$ht most! adH%sted "y R ;'



Another tye of ohmmeter is the S?UNTO?MMETE#. !n the sh"nt ohmmeter, the resistanceto e meas"re$ sh"nts 9is in arallel -ith: the meter

movement of the ohmmeter. The most ovio"s -ayto tell the $iEerence et-een the series an$ sh"ntohmmeters is y the scale of the meter.



Potentiometer

metho$ ofcaliratin& a

'" ammeter***

Potentiometermetho$ ofcaliratin& a

'" voltmeter%"rrent thro"&h 0is a$"ste$ to&ive a stan$ar$

volta&e across0.

Caliration o) DCInstruments

M



+

M"ltimeter is an instr"ment-ith the caaility ofmeas"rin& volta&e, c"rrent,an$ resistance.

ith all three f"n$amentalf"nctions availale, this

m"ltimeter may also ekno-n as a volt-ohm-milliammeter .

'n an analo& m"ltimeter, themeter movement is marke$

-ith several scales, each one"sef"l for at least one ran&esettin&.

M"ltimeter %onstr"ction

A% <olta&e Q %"rrentC



A% <olta&e Q %"rrentCPeak to Peak <al"e

+

e enco"nter a meas"rement rolem if -e try to eFress ho-lar&e or small an A% J"antity is.

'ne -ay to eFress the intensity, or ma&nit"$e 9also calle$ theamplitude:, of an A% J"antity is to meas"re its eak hei&ht on a-aveform &rah. This is kno-n as the pea or crest val"e of anA% -aveform.

Another -ay is to meas"re the total hei&ht et-een oositeeaks. This is kno-n as the pea-to-pea 9P*P: val"e of an A%-aveform.

nfort"nately, either one of these eFressions of -aveformamlit"$e can e mislea$in& -hen comarin& t-o $iEerent tyes

of -aves.

A% <olta&e Q %"rrentC



A% <olta&e Q %"rrentC0MS an$ Avera&e

+8

A sJ"are -ave eakin& at 1volts is ovio"sly a &reateramo"nt of volta&e for a &reateramo"nt of time than a trian&le-ave eakin& at 1 volts.

The eEects of these t-o A%

volta&es o-erin& a loa$ -o"l$e J"ite $iEerent.

'ne -ay of eFressin& theamlit"$e of $iEerent-aveshaes in a more eJ"ivalentfashion is to mathematicallyavera&e the val"es of all theoints on a -aveforms &rah toa sin&le, a&&re&ate n"mer.

!f -e avera&e all the oints onthe -aveform al&eraically 9thatis, to consi$er their sign, eitherositive or ne&ative:, the

avera&e val"e for most

% 9oltage : %urrent3 1M and verage



% 9oltage : %urrent3 1M and verage

!iodes are non linear at low values of forward current.

&ence the readings are crowed at the lower range of voltmeter.

In this part of the meter has low sensitivity because of high forwardresistance.

'he capacitance of the reverse biased diode bypass higher freuencies

and the error is .*+ decreases for every -& rise in freuency.

!m



RMS and +

5

!n the t-o circ"its aove, -e have the same amo"nt of loa$

resistance 92 : $issiatin& the same amo"nt of o-er in theform of heat 95 -atts:, one o-ere$ y A% an$ the other y D%.

;eca"se the A% volta&e so"rce ict"re$ aove is eJ"ivalent 9interms of o-er $elivere$ to a loa$: to a 1 volt D% attery, -e-o"l$ call this a H1 voltH A% so"rce.

More specifically, we would denote its voltage value as being volts #M/. 'heualifier 0#M/0 stands for #oot Mean /uare.

Sometimes the alternative terms e!uivalent or "# e!uivalent are"se$ instea$ of H0MS,H "t the J"antity an$ rincile are oth thesame.

Dith iode! thescenaio is diffeent



51

%onversion %oefficient Fo (p%e( #a&efoms! simple

con&esion coefficients e?ist foe3%atin$ Pea2! Pea27to7Pea2!A&ea$e -pactical! not al$e"aic.!and RMS meas%ements to oneanothe'

The crest factor of an A+ #a&efom

is the atio of its pea2 -cest. &al%edi&ided "y its RMS &al%e'

The form factor of an A+ #a&efomis the atio of its pea2 &al%e di&ided

"y its a&ea$e &al%e' S3%ae7shaped

#a&efoms al#ays ha&e cest andfom factos e3%al to ;! since the pea2 is the same as the RMS anda&ea$e &al%es' Sin%soidal#a&efoms ha&e cest factos of;'K;K -the s3%ae oot of 8. and

fom factos of ;'4; -08.'



52

% 9OL*ME*E1 ;<# =L> &9E 1E%*>E1

If a diode D1 is added to the dc &oltmete! meas%in$ ;5> f%ll scale

deflection' The sensiti&ity of the dc &oltmete is $i&en "y

dc + l?fsd+l?lm+lk

6- 9 dc inp%t #o%ld ca%se e?actly f%ll scale deflection #hen

connected #ith pope polaity' Ass%me ! to "e an ideal diode

#ith ne$li$i"le fo#ad "ias esistance' If this dc input is replaced

by a 6- 9 rms sine ave input. The &olta$es appeain$ at the

o%tp%t is d%e to the &e half cycle d%e to ectifyin$ action'

The pea2 &al%e of ;5 > ms sine #a&e is

Ep + 6- 9 rms ' 6.@6@+ [email protected]@ 9 peak

The dc #ill espond to the a&ea$e &al%e of the ac inp%t! theefoe 1 av = 1 p ' -.ABA + [email protected]@ ' -.ABA + C.

9



Since the diode cond%cts only d%in$ the positi&e half cycle!

the a&ea$e &al%e o&e the entie cycle * a&ea$e &al%e of G' >!

i'e' a"o%t K'4>'-alf #a&e.

• The mete eadin$ fo a f%ll scale is ;5> dc'

• K'4 > #hen a ;5 > ms sin%soidal si$nal is applied'

• This means that an ac &oltmete is not as sensiti&e as a dc

&oltmete'

)Famle +.4 %alc"late the val"e of them"ltilier resistor for a 1< rms ran&e on thevoltmeter sho-n inSolution

Fo D ectifie! The ac sensiti&ity is almost half of + sensiti&ity'

% 9OL*ME*E1 ;<# >;LL



% 9OL*ME*E1 ;<# >;LL

&9E 1E%*>E1

The pea2 &al%e of a ;5 > ms

si$nal E p* ;'K;K?Ems* ;'K;K?;5 *

;K';K> pea2'

A&ea$e >al%e Ea&* 5':Q: ?

E pea2 * 5':Q: ?;K';K*G'>

Theefoe! #e can see that a ;5 > ms &olta$e is e3%al to a > dc fo

f%ll scale deflection! i'e' the pointe #ill deflect to 5 of f%ll scale!o

Sensiti&ity -ac. * 5' ? Sensiti&ity -dc.

Fi$' ac >oltmete Usin$ F%ll

Da&e Rectifie



E@ample G.41 %alc"late the val"e of the m"ltilier resistor for a 1 < rms acran&e on the voltmeter in Oi&. )F. +.5. $olution

The $c sensitivity is &iven y$dc6 1/!fs$ 6 1/1 mA 6 1 kΩ/< Therefore A% sensitivity 6 .4 F $c sensitivity

9O.:$ac 6 .4 F 1 k Ω /< 6 .4 k Ω /< Therefore A% sensitivity 6 .+5 F $c

sensitivity 9.: The m"ltilier resistor is &iven y Rs 6 $ac F ran&e N Rm 6 .4 k Ω /< F 1 < * 25

6 4 F 1 * 25

6 4 * 25 6 85

Fo f%ll #a&e ectifie! the sensiti&ity is

almost same li2e +'



5

% 9oltmeter

1E*O1 9L;E

% 9oltage3 *ice the

value of D% ranges. &hy

=i&es RMS

>al%e of >olta$e



5

D% 9oltmeter

1E*O1 9L;E



Dia!ram o) aMultimeter

1E*O1 9L;E

% 9oltage3 =alf the value of D%

ranges. &hy



54

% 9oltmeter

Fi$%e a"o&e sho#s the ac &oltmete section of a

m%ltimete' To meas%e ac &olta$e! the o%tp%t ac&olta$e is ectified "y a half #a&e ectifie -=een

+icle. "efoe the c%ent passes tho%$h the

mete' Acoss the mete! the othe diode -Red+icle.se&es as potection' The diode cond%cts

#hen a e&ese &olta$e appeas acoss the diodes!

so that c%ent "ypasses the mete in the e&esediection'

Thee ae se&eal points to note fom this typical analo$%e



Thee ae se&eal points to note fom this typical analo$%e

m%ltimete specification)

The lo# &olta$e A+ &olta$e! and in this e?ample the ;5> A+

an$e may ha&e a diffeent scale to the othes' The eason fo this

is that at lo# &olta$es a ectifie is non7linea and this needs to

"e ta2en into consideation' It is also fo this eason that no 8'4>

A+ an$e #as incl%ded'

The ;555> o ;2> an$es #ill often %se a diffeent inp%tconnection to ena"le the eadin$ to "e ta2en tho%$h a diffeent

sh%nt and 2ept a#ay fom the otay s#itch that may not "e a"le

to handle a &olta$e this hi$h'

A+ c%ent is often not incl%ded in the lo#e end metes "eca%se

of the diffic%lties of %ndeta2in$ the meas%ement #itho%t a

tansfome to step %p any &olta$e acoss a seies sensin$ esisto

fo ectification'



Nomally the sensiti&ity of an analo$%e mete is m%ch less on

A+ than +' A mete #ith a + sensiti&ity of K2 ohms pe &olt

on + mi$ht only ha&e a sensiti&ity of 82 ohm pe &olt on A+'

Documents

Multimeter VIMP