Shopwork Basic Instruments

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An Introduction to

Electrical Circuits

Basic Electrical Measurement Instruments,Safety Measures, Resistance, and Ohms Law

(2nd week)


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Objectives

At the end of this topic you should be able to:Appreciate what an ammeter, a voltmeter, an

ohmmeter, a multimeter, an oscilloscope, awattmeter, a bridge megger;

istinguish between linear and non!lineardevices;

escri!e a c"nduct"r and an insulat"r, #i$in#e%am&les "f each'

Learn t" measure resistance and $"lta#e usin#an anal"# multimeter (OM)'

Enumerate safety de$ices and &recauti"ns'

"tate the basic "I units;

#ecogni$e derived "I units; and


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Objectives

At the end of this topic you should be able to:

&&reciate that electrical &"wer * is #i$en !y *+ I + I2R

+ 2

R watts'-alculate electrical &"wer'

E%&lain the im&"rtance "f fuses in electricalcircuits' and

&&reciate the dan#ers "f c"nstant hi#hcurrent ."w with insulati"n materials/


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)asic Electrical *easuringInstruments

An ammeter is aninstrument used to measurecurrent and must beconnected in series with thecircuit(

+igure ( shows anammeter connected inseries with the lamp tomeasure the current -owingthrough it(

"ince all the current in the

circuit passes through the


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A voltmeter is aninstrument used to measurep(d( and must be connectedin parallel with the part ofthe circuit whose p(d( isre.uired(

In +igure (, a voltmeter isconnected in parallel withthe lamp to measure thep(d( across it(

/o avoid a signi&cant

current -owing through it a


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An ohmmeter is an instrument for measuringresistance(


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A multi!meter, or universal instrument, may beused to measure voltage, current and resistance(


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/he oscilloscope may be used to observewaveforms and to measure voltages and currents(

/he display of an oscilloscope involves a spot of

light moving across a screen(


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/he amount by which the spot is de-ected fromits initial position depends on the p(d( applied tothe terminals of the oscilloscope and the rangeselected( /he displacement is calibrated in 0voltsper cm1(

+or e'ample, if the spot is de-ected 2 cm and thevolts3cm switch is on 45 63cm then the magnitudeof the p(d( is 2cm74563cm, i(e( 256(


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A wattmeter is an instrument for themeasurement of power in an electrical circuit(


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A )*85 or a 95 *I/ megger or a bridge meggermay be used to measure both continuity andinsulation resistance(

Continuity testing is the measurement of theresistance of a cable to discover if the cable iscontinuous, i(e( that it has no breas or highresistance joints(


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Insulation resistance testing is the measurementof resistance of the insulation between cables,individual cables to earth or metal plugs andsocets, and so on(

An insulation resistance in e'cess of 4 * isnormally acceptable(


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*%


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DIGITAL MULTITESTER

A piece of test e.uipment used for measuri

ng voltage, current, resistance, and possiblyotherelectrical .uantities and displaying the value in number form(


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ANALOG MULTITESTER

Although older than thecommonly preferreddigital multimeter, docome with severaluni.ue advantages(

Although both digitaland analog multimetersmeasure varying units,

such as voltage, current,and resistance, theydisplay their readings indistinctly di>erent ways(


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PARTS OF MULTITESTER

POINTER

/he needle!shape rodthat moves over the

scale of a meter( It ismechanicallyconnected to themoving coil(

It indicates themeasured values onthe multimeter(


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RANGE SELECTORKNOB

?"elector "witch@

maes it possible toselect di>erentfunction and range ofthe meter(


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ADJUSTMENT SCREW

maes it possible toadjust the pointer to

the $ero position ofthe scale(


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SCALE

is a series of maringused for reading the

value of .uantity(

"cale can havedi>erent types of

scale, for voltage andcurrent readings thescales have mostlylinear which mean

e.ual division forresistance(


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TEST PROBES

ositive ?red@,negative ?blac@, is

use to connect thecircuit to theelectricalcomponents being

tested(


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ZERO-OHMADJUSTMENT KNOB

is used to $ero!in the

pointer beforemeasuringresistance(

#OE# CA#E AB


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#OE# CA#E AB*AIB/EBABCE O+ /E

*%


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RESISTANCE READING


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VOLTAGE READING


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CURRENT READING


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Conductors and Insulators

A conductor is a material having a low resistancewhich allows electric current to -ow in it(

All metals are conductors and some e'amples

include copper, aluminum, brass, platinum, silver,gold and carbon(

An insulator is a material having a high resistance

which does not allow electric current to -ow in it(

"ome e'amples of insulators include plastic,rubber, glass, porcelain, air, paper, cor, mica,ceramics and certain oils(


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+use

If there is a fault in a piece of e.uipment thene'cessive current may -ow(

/his will cause overheating and possibly a &re;

fuses protect against this happening(

Current from the supply to the e.uipment -owsthrough the fuse(

/he fuse is a piece of wire which can carry astated current; if the current rises above thisvalue it will melt(


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+use


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+use


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+use

If the fuse melts ?blows@ then there is an opencircuit and no current can then -ow G thusprotecting the e.uipment by isolating it from thepower supply(

/he fuse must be able to carry slightly more thanthe normal operating current of the e.uipment toallow for tolerances and small current surges(

Dith some e.uipment there is a very large surgeof current for a short time at switch on(


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+use

If a fuse is &tted to withstand thislarge current there would be noprotection against faults whichcause the current to rise slightlyabove the normal value(

/herefore special anti!surge fusesare &tted(

/hese can stand 45 times therated current for 45 milliseconds(

If the surge lasts longer than thisthe fuse will blow(

SchematicSym!"


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+use

E'ample

If A, 45A and 42A fuses are available, statewhich is most appropriate for the following

appliances which are both connected to a 95 6supply

?a(@ Electric toaster having a power rating of 4D

?b(@ Electric &re having a power rating of 2 D(

P!#e$ P % &I' ($!m #hich' c)$$e*t I % P + &

?a(@ the toaster, current I % P + & % , +% -


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+use

E'ample

If A, 45A and 42A fuses are available, statewhich is most appropriate for the following

appliances which are both connected to a 95 6supply

?a(@ Electric toaster having a power rating of 4D

?b(@ Electric &re having a power rating of 2 D(

P!#e$ P % &I' ($!m #hich' c)$$e*t I % P + &

?b(@ +or the &re, current I + * + 011121 +-

Insulation and the angers of


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Insulation and the angers ofConstant igh Current +low

/he use of insulation materials on electricale.uipment, whilst being necessary, also has thee>ect of preventing heat loss, i(e( the heat is notable to dissipate, thus creating possible danger of

&re(

In addition, the insulating material has ama'imum temperature rating H this is heat it can

withstand without being damaged(

Insulation and the angers of


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Insulation and the angers ofConstant igh Current +low

/he current rating for all e.uipment and electricalcomponents is therefore limited to eep the heatgenerated within safe limits(

In addition, the ma'imum voltage present needsto be considered when choosing insulation(

International "ystem of %nits


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International "ystem of %nits?"I units@

/he system of units used in engineering andscience is the "ystme Internationale d1%nitJs?International system of units@, usuallyabbreviated to "I units, and is based on themetric system(

/his was introduced in 4KF5 and is now adoptedby the majority of countries as the oLcial systemof measurement(

erived "I units use combinations of basic unitsand there are many of them(



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/wo e'amples are:

6elocity G meters per second ?m3s@

Acceleration G meters per second s.uared

?m3s

@

"I units may be made larger or smaller by usingpre&'es which denote multiplication or division by

a particular amount(



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