Voltage and Current. Objectives 2.1 INTRODUCTION 2.2 ATOMS AND THEIR STRUCTURE. I e V

Transcription

1 oltag and Currnt 2 Objctivs Bcom awar of th basic atomic structur of conductors such as coppr and aluminum and undrstand why thy ar usd so xtnsivly in th fild. Undrstand how th trminal voltag of a battry or any dc supply is stablishd and how it crats a flow of charg in th systm. Undrstand how currnt is stablishd in a circuit and how its magnitud is affctd by th charg flowing in th systm and th tim involvd. Bcom familiar with th factors that affct th trminal voltag of a battry and how long a battry will rmain ffctiv. B abl to apply a voltmtr and ammtr corrctly to masur th voltag and currnt of a ntwork. 2.1 NTRODUCTON Now that th foundation for th study of lctricity/lctronics has bn stablishd, th concpts of voltag and currnt can b invstigatd. Th trm voltag is ncountrd practically vry day. W hav all rplacd battris in our flashlights, answring machins, calculators, automobils, and so on, that had spcific voltag ratings. W ar awar that most outlts in our homs ar 120 volts. Although currnt may b a lss familiar trm, w know what happns whn w plac too many appliancs on th sam outlt th circuit brakr opns du to th xcssiv currnt that rsults. t is fairly common knowldg that currnt is somthing that movs through th wirs and causs sparks and possibly fir if thr is a short circuit. Currnt hats up th coils of an lctric hatr or th rang of an lctric stov; it gnrats light whn passing through th filamnt of a bulb; it causs twists and kinks in th wir of an lctric iron ovr tim, and so on. All in all, th trms voltag and currnt ar part of th vocabulary of most individuals. n this chaptr, th basic impact of currnt and voltag and th proprtis of ach ar introducd and discussd in som dtail. Hopfully, any mystris surrounding th gnral charactristics of ach will b liminatd, and you will gain a clar undrstanding of th impact of ach on an lctric/lctronics circuit. 2.2 ATOMS AND THER STRUCTURE A basic undrstanding of th fundamntal concpts of currnt and voltag rquirs a dgr of familiarity with th atom and its structur. Th simplst of all atoms is th hydrogn atom, mad up of two basic particls, th proton and th lctron, in th rlativ positions shown in Fig. 2.1(a). Th nuclus of th hydrogn atom is th proton, a positivly chargd particl. Th orbiting lctron carris a ngativ charg qual in magnitud to th positiv charg of th proton.

2 34 OLTAGE AND CURRENT Elctron Elctron Nuclus Protons Nutrons Proton Elctron Nuclus (a) Hydrogn atom (b) Hlium atom FG. 2.1 Hydrogn and hlium atoms. n all othr lmnts, th nuclus also contains nutrons, which ar slightly havir than protons and hav no lctrical charg. Th hlium atom, for xampl, has two nutrons in addition to two lctrons and two protons, as shown in Fig. 2.1(b). n gnral, th atomic structur of any stabl atom has an qual numbr of lctrons and protons. Diffrnt atoms hav various numbrs of lctrons in concntric orbits calld shlls around th nuclus. Th first shll, which is closst to th nuclus, can contain only two lctrons. f an atom has thr lctrons, th xtra lctron must b placd in th nxt shll. Th numbr of lctrons in ach succding shll is dtrmind by 2n 2 whr n is th shll numbr. Each shll is thn brokn down into subshlls whr th numbr of lctrons is limitd to 2, 6, 10, and 14 in that ordr as you mov away from th nuclus. Coppr is th most commonly usd mtal in th lctrical/lctronics industry. An xamination of its atomic structur will rval why it has such widsprad application. As shown in Fig. 2.2, it has 29 lctrons in orbits around th nuclus, with th 29th lctron apparing all by itslf in th 4th shll. Not that th numbr of lctrons in ach shll and subshll 3rd shll 18 lctrons Nuclus 29 protons 29 nutrons + 1st shll 2 lctrons 4th shll (Maximum = 32 lctrons) 29th lctron 2nd shll 8 lctrons FG. 2.2 Th atomic structur of coppr.

3 OLTAGE 35 is as dfind abov. Thr ar two important things to not in Fig First, th 4th shll, which can hav a total of 2n 2 2(4) 2 32 lctrons, has only on lctron. Th outrmost shll is incomplt and, in fact, is far from complt bcaus it has only on lctron. Atoms with complt shlls (that is, a numbr of lctrons qual to 2n 2 ) ar usually quit stabl. Thos atoms with a small prcntag of th dfind numbr for th outrmost shll ar normally considrd somwhat unstabl and volatil. Scond, th 29th lctron is th farthst lctron from th nuclus. Opposit chargs ar attractd to ach othr, but th farthr apart thy ar, th lss th attraction. n fact, th forc of attraction btwn th nuclus and th 29th lctron of coppr can b dtrmind by Coulomb s law dvlopd by Charls Augustin Coulomb (Fig. 2.3) in th lat 18th cntury: (nwtons, N) (2.1) whr F is in nwtons (N), k a constant N m 2 /C 2, Q 1 and Q 2 ar th chargs in coulombs (a unit of masur discussd in th nxt sction), and r is th distanc btwn th two chargs in mtrs. At this point, th most important thing to not is that th distanc btwn th chargs appars as a squard trm in th dnominator. First, th fact that this trm is in th dnominator clarly rvals that as it incrass, th forc will dcras. Howvr, sinc it is a squard trm, th forc will drop dramatically with distanc. For instanc, if th distanc is doubld, th forc will drop to 1/4 bcaus (2) 2 4. f th distanc is incrasd by a factor of 4, it will drop by 1/16, and so on. Th rsult, thrfor, is that th forc of attraction btwn th 29th lctron and th nuclus is significantly lss than that btwn an lctron in th first shll and th nuclus. Th rsult is that th 29th lctron is loosly bound to th atomic structur and with a littl bit of prssur from outsid sourcs could b ncouragd to lav th parnt atom. f this 29th lctron gains sufficint nrgy from th surrounding mdium to lav th parnt atom, it is calld a fr lctron. n 1 cubic in. of coppr at room tmpratur, thr ar approximatly fr lctrons. Expandd, that is 1,400,000,000,000,000,000,000,000 fr lctrons in a 1 in. squar cub. Th point is that w ar daling with normous numbrs of lctrons whn w talk about th numbr of fr lctrons in a coppr wir not just a fw that you could lisurly count. Furthr, th numbrs involvd ar clar vidnc of th nd to bcom proficint in th us of powrs of tn to rprsnt numbrs and us thm in mathmatical calculations. Othr mtals that xhibit th sam proprtis as coppr, but to a diffrnt dgr, ar silvr, gold, and aluminum, and som rarr mtals such as tungstn. Additional commnts on th charactristics of conductors ar in th following sctions. 2.3 OLTAGE F k Q 1Q 2 r 2 f w sparat th 29th lctron in Fig. 2.2 from th rst of th atomic structur of coppr by a dashd lin as shown in Fig. 2.4(a), w crat rgions that hav a nt positiv and ngativ charg as shown in Fig. 2.4(b) and (c). For th rgion insid th dashd boundary, th numbr of protons in th nuclus xcds th numbr of orbiting lctrons by 1, so th nt charg is positiv as shown in both figurs. This positiv rgion cratd by sparating th fr lctron from th basic atomic structur is calld a FG. 2.3 Charls Augustin Coulomb. Courtsy of th Smithsonian nstitution, Photo No. 52,597 Frnch (Angoulèm, Paris) ( ) Scintist and nvntor Military Enginr, Wst ndis Attndd th nginring school at Mzirs, th first such school of its kind. Formulatd Coulomb s law, which dfins th forc btwn two lctrical chargs and is, in fact, on of th principal forcs in atomic ractions. Prformd xtnsiv rsarch on th friction ncountrd in machinry and windmills and th lasticity of mtal and silk fibrs. Positiv ion + (a) + (b) + (c) + (d) Positiv rgion qual in charg to th isolatd lctron Fr lctron FG. 2.4 Dfining th positiv ion.

4 36 OLTAGE AND CURRENT positiv ion. f th fr lctron thn lavs th vicinity of th parnt atom as shown in Fig. 2.4(d), rgions of positiv and ngativ charg hav bn stablishd. This sparation of charg to stablish rgions of positiv and ngativ charg is th action that occurs in vry battry. Through chmical action, a havy concntration of positiv charg (positiv ions) is stablishd at th positiv trminal, with an qually havy concntration of ngativ charg (lctrons) at th ngativ trminal. n gnral, vry sourc of voltag is stablishd by simply crating a sparation of positiv and ngativ chargs. t is that simpl: f you want to crat a voltag lvl of any magnitud, simply stablish a rgion of positiv and ngativ charg. Th mor th rquird voltag, th gratr th quantity of positiv and ngativ charg. n Fig. 2.5(a), for xampl, a rgion of positiv charg has bn stablishd by a packagd numbr of positiv ions, and a rgion of ngativ charg by a similar numbr of lctrons, both sparatd by a distanc r. Sinc it would b inconsquntial to talk about th voltag stablishd by th sparation of a singl lctron, a packag of lctrons calld a coulomb (C) of charg was dfind as follows: On coulomb of charg is th total charg associatd with lctrons. A coulomb of positiv charg would hav th sam magnitud but opposit polarity. n Fig. 2.5(b), if w tak a coulomb of ngativ charg nar th surfac of th positiv charg and mov it toward th ngativ charg, nrgy must b xpndd to ovrcom th rpulsiv forcs of th largr ngativ charg and th attractiv forcs of th positiv charg. n th procss of moving th charg from point a to point b in Fig. 2.5(b): if a total of 1 joul (J) of nrgy is usd to mov th ngativ charg of 1 coulomb (C), thr is a diffrnc of 1 volt () btwn th two points. Th dfining quation is W Q volts () W jouls (J) (2.2) Q coulombs (C) 1 coulomb of charg 1 coulomb of charg 2 jouls of nrgy 1 coulomb of charg 4.8 jouls of nrgy 4.8 r + b 1 a 1 joul of nrgy (a) (b) (c) (d) FG. 2.5 Dfining th voltag btwn two points.

5 OLTAGE 37 Tak particular not that th charg is masurd in coulombs, th nrgy in jouls, and th voltag in volts. Th unit of masurmnt, volt, was chosn to honor th fforts of Alssandro olta, who first dmonstratd that a voltag could b stablishd through chmical action (Fig. 2.6). f th charg is now movd all th way to th surfac of th largr ngativ charg as shown in Fig. 2.5(c), using 2 jouls of nrgy for th whol trip, thr ar 2 volts btwn th two chargd bodis. f th packag of positiv and ngativ charg is largr, as shown in Fig. 2.5(d), mor nrgy will hav to b xpndd to ovrcom th largr rpulsiv forcs of th larg ngativ charg and attractiv forcs of th larg positiv charg. As shown in Fig. 2.5(d), 4.8 jouls of nrgy wr xpndd, rsulting in a voltag of 4.8 btwn th two points. W can thrfor conclud that it would tak 12 jouls of nrgy to mov 1 coulomb of ngativ charg from th positiv trminal to th ngativ trminal of a 12 car battry. Through algbraic manipulations, w can dfin an quation to dtrmin th nrgy rquird to mov charg through a diffrnc in voltag: W Q (jouls, J) (2.3) Finally, if w want to know how much charg was involvd: Q W (coulombs, C) (2.4) EXAMPLE 2.1 Find th voltag btwn two points if 60 J of nrgy ar rquird to mov a charg of 20 C btwn th two points. FG. 2.6 Count Alssandro olta. Courtsy of th Smithsonian nstitution, Photo No. 55,393 talian (Como, Pavia) ( ) Physicist Profssor of Physics, Pavia, taly Bgan lctrical xprimnts at th ag of 18 working with othr Europan invstigators. Major contribution was th dvlopmnt of an lctrical nrgy sourc from chmical action in For th first tim, lctrical nrgy was availabl on a continuous basis and could b usd for practical purposs. Dvlopd th first condnsr known today as th capacitor. Was invitd to Paris to dmonstrat th voltaic cll to Napolon. Th ntrnational Elctrical Congrss mting in Paris in 1881 honord his fforts by choosing th volt as th unit of masur for lctromotiv forc. Solution: Eq. (2.2): W Q 60 J 20 C 3 EXAMPLE 2.2 Dtrmin th nrgy xpndd moving a charg of 50 mc btwn two points if th voltag btwn th points is 6. Solution: Eq. (2.3): W Q ( C)(6 ) J 300 MJ Thr ar a varity of ways to sparat charg to stablish th dsird voltag. Th most common is th chmical action usd in car battris, flashlight battris, and, in fact, all portabl battris. Othr sourcs us mchanical mthods such as car gnrators and stam powr plants or altrnativ sourcs such as solar clls and windmills. n total, howvr, th sol purpos of th systm is to crat a sparation of charg. n th futur, thrfor, whn you s a positiv and a ngativ trminal on any typ of battry, you can think of it as a point whr a larg concntration of charg has gathrd to crat a voltag btwn th two points. Mor important is to rcogniz that a voltag xists btwn two points for a battry btwn th positiv and ngativ trminals. Hooking up just th positiv or th ngativ trminal of a battry and not th othr would b maninglss. Both trminals must b connctd to dfin th applid voltag.

6 38 OLTAGE AND CURRENT As w movd th 1 coulomb of charg in Fig. 2.5(b), th nrgy xpndd would dpnd on whr w wr in th crossing. Th position of th charg is thrfor a factor in dtrmining th voltag lvl at ach point in th crossing. Sinc th potntial nrgy associatd with a body is dfind by its position, th trm potntial is oftn applid to dfin voltag lvls. For xampl, th diffrnc in potntial is 4 btwn th two points, or th potntial diffrnc btwn a point and ground is 12, and so on. 2.4 CURRENT Th qustion, Which cam first th chickn or th gg? can b applid hr also bcaus th layprson has a tndncy to us th trms currnt and voltag intrchangably as if both wr sourcs of nrgy. t is tim to st things straight: Th applid voltag is th starting mchanism th currnt is a raction to th applid voltag. n Fig. 2.7(a), a coppr wir sits isolatd on a laboratory bnch. f w cut th wir with an imaginary prpndicular plan, producing th circular cross sction shown in Fig. 2.7(b), w would b amazd to find that thr ar fr lctrons crossing th surfac in both dirctions. Thos fr lctrons gnratd at room tmpratur ar in constant motion in random dirctions. Howvr, at any instant of tim, th numbr of lctrons crossing th imaginary plan in on dirction is xactly qual to that crossing in th opposit dirction, so th nt flow in any on dirction is zro. Evn though th wir sms dad to th world sitting by itslf on th bnch, intrnally, it is quit activ. Th sam would b tru for any othr good conductor. Now, to mak this lctron flow do work for us, w nd to giv it a dirction and b abl to control its magnitud. This is accomplishd by simply applying a voltag across th wir to forc th lctrons to mov toward th positiv trminal of th battry, as shown in Fig Th instant th wir is placd across th trminals, th fr lctrons in th wir drift toward th positiv trminal. Th positiv ions in th coppr wir simply oscillat in a man fixd position. As th lctrons pass through th wir, th ngativ trminal of th battry acts as a supply of additional solatd coppr wir maginary plan Prpndicular plan for Fig. 2.7(b) Prpndicular surfac cut by plan (a) (b) FG. 2.7 Thr is motion of fr carrirs in an isolatd pic of coppr wir, but th flow of charg fails to hav a particular dirction.

7 CURRENT 39 Coppr wir Battry trminals Chmical action FG. 2.8 Motion of ngativly chargd lctrons in a coppr wir whn placd across battry trminals with a diffrnc in potntial of volts (). lctrons to kp th procss moving. Th lctrons arriving at th positiv trminal ar absorbd, and through th chmical action of th battry, additional lctrons ar dpositd at th ngativ trminal to mak up for thos that lft. To tak th procss a stp furthr, considr th configuration in Fig. 2.9, whr a coppr wir has bn usd to connct a light bulb to a battry to crat th simplst of lctric circuits. Th instant th final connction is mad, th fr lctrons of ngativ charg drift toward th positiv trminal, whil th positiv ions lft bhind in th coppr wir simply oscillat in a man fixd position. Th flow of charg (th lctrons) through th bulb hats up th filamnt of th bulb through friction to th point that it glows rd-hot and mits th dsird light. n total, thrfor, th applid voltag has stablishd a flow of lctrons in a particular dirction. n fact, by dfinition, if lctrons (1 coulomb) pass through th imaginary plan in Fig. 2.9 in 1 scond, th flow of charg, or currnt, is said to b 1 ampr (A). Coppr wir convntional lctron Battry Chmical activity maginary plan FG. 2.9 Basic lctric circuit.

8 40 OLTAGE AND CURRENT Th unit of currnt masurmnt, ampr, was chosn to honor th fforts of André Ampèr in th study of lctricity in motion (Fig. 2.10). Using th coulomb as th unit of charg, th currnt in amprs can b dtrmind using th following quation: Q t amprs (A) Q coulombs (C) (2.5) t tim (s) FG André Mari Ampèr. Courtsy of th Smithsonian nstitution, Photo No. 76,524 Frnch (Lyon, Paris) ( ) Mathmatician and Physicist Profssor of Mathmatics, Écol, Polytchniqu in Paris On Sptmbr 18, 1820, introducd a nw fild of study, lctrodynamics, dvotd to th ffct of lctricity in motion, including th intraction btwn currnts in adjoining conductors and th intrplay of th surrounding magntic filds. Constructd th first solnoid and dmonstratd how it could bhav lik a magnt (th first lctromagnt). Suggstd th nam galvanomtr for an instrumnt dsignd to masur currnt lvls. Th capital lttr was chosn from th Frnch word for currnt, intnsité. Th S abbrviation for ach quantity in Eq. (2.5) is providd to th right of th quation. Th quation clarly rvals that for qual tim intrvals, th mor charg that flows through th wir, th largr th rsulting currnt. Through algbraic manipulations, th othr two quantitis can b dtrmind as follows: and Q t t Q (coulombs, C) (2.6) (sconds, s) (2.7) EXAMPLE 2.3 Th charg flowing through th imaginary surfac in Fig. 2.9 is 0.16 C vry 64 ms. Dtrmin th currnt in amprs. Solution: Eq. (2.5): Q t 0.16 C s C 2.50 A s EXAMPLE 2.4 Dtrmin how long it will tak lctrons to pass through th imaginary surfac in Fig. 2.9 if th currnt is 5 ma. Solution: Dtrmin th charg in coulombs: 1 C lctrons a lctrons b C Eq. (2.7): t Q C A 1.28 s 6.41 mc n summary, thrfor, th applid voltag (or potntial diffrnc) in an lctrical/lctronics systm is th prssur to st th systm in motion, and th currnt is th raction to that prssur. A mchanical analogy oftn usd to xplain th abov is th simpl gardn hos. n th absnc of any prssur, th watr sits quitly in th hos with no gnral dirction, just as lctrons do not hav a nt dirction in th absnc of an applid voltag. Howvr, rlas th spigot, and th

9 OLTAGE SOURCES 41 applid prssur forcs th watr to flow through th hos. Similarly, apply a voltag to th circuit, and a flow of charg or currnt rsults. A scond glanc at Fig. 2.9 rvals that two dirctions of charg flow hav bn indicatd. On is calld convntional flow, and th othr is calld lctron flow. This txt discusss only convntional flow for a varity of rasons; namly, it is th most widly usd at ducational institutions and in industry, it is mployd in th dsign of all lctronic dvic symbols, and it is th popular choic for all major computr softwar packags. Th flow controvrsy is a rsult of an assumption mad at th tim lctricity was discovrd that th positiv charg was th moving particl in mtallic conductors. B assurd that th choic of convntional flow will not crat grat difficulty and confusion in th chaptrs to follow. Onc th dirction of is stablishd, th issu is droppd and th analysis can continu without confusion. Safty Considrations t is important to raliz that vn small lvls of currnt through th human body can caus srious, dangrous sid ffcts. Exprimntal rsults rval that th human body bgins to ract to currnts of only a fw milliamprs. Although most individuals can withstand currnts up to prhaps 10 ma for vry short priods of tim without srious sid ffcts, any currnt ovr 10 ma should b considrd dangrous. n fact, currnts of 50 ma can caus svr shock, and currnts of ovr 100 ma can b fatal. n most cass, th skin rsistanc of th body whn dry is sufficintly high to limit th currnt through th body to rlativly saf lvls for voltag lvls typically found in th hom. Howvr, if th skin is wt du to prspiration, bathing, and so on, or if th skin barrir is brokn du to an injury, th skin rsistanc drops dramatically, and currnt lvls could ris to dangrous lvls for th sam voltag shock. n gnral, thrfor, simply rmmbr that watr and lctricity don t mix. Grantd, thr ar safty dvics in th hom today [such as th ground fault circuit intrrupt (GFC) brakr, discussd in Chaptr 4] that ar dsignd spcifically for us in wt aras such as th bathroom and kitchn, but accidnts happn. Trat lctricity with rspct not far. 2.5 OLTAGE SOURCES Th trm dc, usd throughout this txt, is an abbrviation for dirct currnt, which ncompasss all systms whr thr is a unidirctional (on dirction) flow of charg. This sction rviws dc voltag supplis that apply a fixd voltag to lctrical/lctronics systms. Th graphic symbol for all dc voltag sourcs is shown in Fig Not that th rlativ lngth of th bars at ach nd dfin th polarity of th supply. Th long bar rprsnts th positiv sid; th short bar, th ngativ. Not also th us of th lttr E to dnot voltag sourc. t coms from th fact that an lctromotiv forc (mf) is a forc that stablishs th flow of charg (or currnt) in a systm du to th application of a diffrnc in potntial. E 12 n gnral, dc voltag sourcs can b dividd into thr basic typs: (1) battris (chmical action or solar nrgy), (2) gnrators (lctromchanical), and (3) powr supplis (rctification a convrsion procss to b dscribd in your lctronics courss). FG Standard symbol for a dc voltag sourc.

10 42 OLTAGE AND CURRENT Battris Gnral nformation For th layprson, th battry is th most common of th dc sourcs. By dfinition, a battry (drivd from th xprssion battry of clls ) consists of a combination of two or mor similar clls, a cll bing th fundamntal sourc of lctrical nrgy dvlopd through th convrsion of chmical or solar nrgy. All clls can b dividd into th primary or scondary typs. Th scondary is rchargabl, whras th primary is not. That is, th chmical raction of th scondary cll can b rvrsd to rstor its capacity. Th two most common rchargabl battris ar th lad-acid unit (usd primarily in automobils) and th nickl-mtal hydrid (NiMH) battry (usd in calculators, tools, photoflash units, shavrs, and so on). Th obvious advantags of rchargabl units ar th savings in tim and mony of not continually rplacing dischargd primary clls. All th clls discussd in this chaptr (xcpt th solar cll, which absorbs nrgy from incidnt light in th form of photons) stablish a potntial diffrnc at th xpns of chmical nrgy. n addition, ach has a positiv and a ngativ lctrod and an lctrolyt to complt th circuit btwn lctrods within th battry. Th lctrolyt is th contact lmnt and th sourc of ions for conduction btwn th trminals. Primary Clls Th popular alkalin primary battry uss a powdrd zinc anod ( ); a potassium (alkali mtal) hydroxid lctrolyt; and a mangans dioxid, carbon cathod ( ) as shown in Fig. 2.12(a). n Fig. 2.12(b), not that for th cylindrical typs (AAA, AA, C, and D), th voltag is th sam for ach, but th ampr-hour (Ah) rating incrass significantly with siz. Th ampr-hour rating is an indication of th lvl of currnt that th battry can provid for a spcifid priod of tim (to b discussd in dtail in Sction 2.6). n particular, not that for th larg, lantrn-typ battry, th voltag is only 4 tims that of th AAA Positiv covr: platd stl Elctrolyt: potassium hydroxid/watr Cathod: mangans dioxid and carbon Sparator: nonwovn fabric Mtal washr Can: stl Mtalizd plastic film labl Anod: powdrd zinc Currnt collctor: brass pin Sal: nylon nnr cll covr: stl Ngativ covr: platd stl 6 26 Ah 6 52 Ah 6 26 Ah Mtal spur (a) D cll Ah C cll mah (b) AA cll mah mah AAA cll mah FG Alkalin primary cll: (a) Cutaway of cylindrical Enrgizr cll; (b) various typs of Evrady Enrgizr primary clls. ( Evrady Battry Company, nc., St. Louis Missouri)

11 OLTAGE SOURCES mah Standard drain: 30 µa mah Standard drain: 200 µa mah Standard drain: 2.5 ma mah Standard drain: 150 ma FG Lithium primary battris. battry, but th ampr-hour rating of 52 Ah is almost 42 tims that of th AAA battry. Anothr typ of popular primary cll is th lithium battry, shown in Fig Again, not that th voltag is th sam for ach, but th siz incrass substantially with th ampr-hour rating and th ratd drain currnt. n gnral, thrfor, for battris of th sam typ, th siz is dictatd primarily by th standard drain currnt or ampr-hour rating, not by th trminal voltag rating. Lad-Acid Scondary Cll For th scondary lad-acid unit shown in Fig. 2.14, th lctrolyt is sulfuric acid, and th lctrods ar spongy lad (Pb) and lad proxid (PbO 2 ). Whn a load is applid to th battry trminals, thr is a transfr of lctrons from th spongy lad lctrod Hat-sald covr Ngativ trminal Positiv trminal Elctrolyt rsrvoir Flam arrstor vnt Sparator nvlop Extrusion-fusion intrcll connction Wrought lad-calcium grid Clls (ach 2.1 ) FG Maintnanc-fr 12 (actually 12.6 ) lad-acid battry. (Courtsy of Rmy ntrnational, nc.)

12 44 OLTAGE AND CURRENT to th lad proxid lctrod through th load. This transfr of lctrons will continu until th battry is compltly dischargd. Th discharg tim is dtrmind by how dilutd th acid has bcom and how havy th coating of lad sulfat is on ach plat. Th stat of discharg of a lad storag cll can b dtrmind by masuring th spcific gravity of th lctrolyt with a hydromtr. Th spcific gravity of a substanc is dfind to b th ratio of th wight of a givn volum of th substanc to th wight of an qual volum of watr at 4 C. For fully chargd battris, th spcific gravity should b somwhr btwn 1.28 and Whn th spcific gravity drops to about 1.1, th battry should b rchargd. Sinc th lad storag cll is a scondary cll, it can b rchargd at any point during th discharg phas simply by applying an xtrnal dc currnt sourc across th cll that passs currnt through th cll in a dirction opposit to that in which th cll supplid currnt to th load. This rmovs th lad sulfat from th plats and rstors th concntration of sulfuric acid. Th output of a lad storag cll ovr most of th discharg phas is about 2.1. n th commrcial lad storag battris usd in automobils, 12.6 can b producd by six clls in sris, as shown in Fig n gnral, ladacid storag battris ar usd in situations whr a high currnt is rquird for rlativly short priods of tim. At on tim, all lad-acid battris wr vntd. Gass cratd during th discharg cycl could scap, and th vnt plugs providd accss to rplac th watr or lctrolyt and to chck th acid lvl with a hydromtr. Th us of a grid mad from a wrought ladcalcium alloy strip, rathr than th lad-antimony cast grid commonly usd, has rsultd in maintnanc-fr battris, shown in Fig Th ladantimony structur was suscptibl to corrosion, ovrcharg, gasing, watr usag, and slf-discharg. mprovd dsign with th lad-calcium grid has ithr liminatd or substantially rducd most of ths problms. t would sm with all th yars of tchnology surrounding battris that smallr, mor powrful units would now b availabl. Howvr, whn it coms to th lctric car, which is slowly gaining intrst and popularity throughout th world, th lad-acid battry is still th primary sourc of powr. A station car, manufacturd in Norway and usd on a tst basis in San Francisco for typical commutr runs, has a total wight of 1650 pounds, with 550 pounds (a third of its wight) for th lad-acid rchargabl battris. Although th station car will travl at spds of 55 mph, its rang is limitd to 65 mils on a charg. t would appar that long-distanc travl with significantly rducd wight factors for th battris will dpnd on a nw, innovativ approach to battry dsign. NicklMtal Hydrid Scondary Clls Th rchargabl battry has bn rciving normous intrst and dvlopmnt in rcnt yars. For applications such as flashlights, shavrs, portabl tlvisions, powr drills, and so on, rchargabl battris such as th nicklmtal hydrid (NiMH) battris shown in Fig ar th scondary battris of choic. Ths battris ar so wll mad that thy can surviv ovr 1000 charg/discharg cycls ovr a priod of tim and can last for yars. t is important to rcogniz that if an applianc calls for a rchargabl battry such as a NiMH battry, a primary cll should not b usd. Th applianc may hav an intrnal charging ntwork that would b dysfunctional with a primary cll. n addition, not that NiMH battris ar about 1.2 pr cll, whras th common primary clls ar typically 1.5 pr cll.

13 OLTAGE SOURCES 45 D cll ma C cll ma AA cll ma AAA cll ma 9 (7.2 nominal) ma FG Nicklmtal hydrid (NiMH) rchargabl battris. ( Evrady Battry Company, nc., St. Louis, Missouri) Thr is som ambiguity about how oftn a scondary cll should b rchargd. Gnrally, th battry can b usd until thr is som indication that th nrgy lvl is low, such as a dimming light from a flashlight, lss powr from a drill, or a low-battry indicator. Kp in mind that scondary clls do hav som mmory. f thy ar rchargd continuously aftr bing usd for a short priod of tim, thy may bgin to bliv thy ar short-trm units and actually fail to hold th charg for th ratd priod of tim. n any vnt, always try to avoid a hard discharg, which rsults whn vry bit of nrgy is draind from a cll. Too many harddischarg cycls will rduc th cycl lif of th battry. Finally, b awar that th charging mchanism for nickl-cadmium clls is quit diffrnt from that for lad-acid battris. Th nickl-cadmium battry is chargd by a constant currnt sourc, with th trminal voltag staying fairly stady through th ntir charging cycl. Th lad-acid battry is chargd by a constant voltag sourc, prmitting th currnt to vary as dtrmind by th stat of th battry. Th capacity of th NiMH battry incrass almost linarly throughout most of th charging cycl. Ni-Cad battris bcom rlativly warm whn charging. Th lowr th capacity lvl of th battry whn charging, th highr th tmpratur of th cll. As th battry approachs ratd capacity, th tmpratur of th cll approachs room tmpratur. Othr typs of rchargabl battris includ th nickl-cadmium (Ni- Cad) and nickl hydrogn (Ni-H) battris. n rality, howvr, th NiMH battry is a hybrid of th nickl-cadmium and nickl-hydrogn clls, combining th positiv charactristics of ach to crat a product with a high powr lvl in a small packag that has a long lif. Anothr typ of rchargabl battry is th lithium-ion varity shown in Fig. 2.16, usd in th BM laptop computr. Solar Cll Th SX 20 and SX 30 solar moduls (a combination of connctd clls) shown in Fig provid 20 W and 30 W of lctrical powr, rspctivly. Th siz and orintation of such units ar important bcaus th maximum availabl wattag on an avrag bright, sunlit day is 100 mw/cm 2. Sinc convrsion fficincis ar currntly only at 10% to 14%, th maximum availabl powr pr squar cntimtr from most 10.8, 10.8 Ah Charg tim: Systm oprational: 6 h max. Powr off: 2.5 h max. FG BM ThinkPad T-20 lithium-ion rchargabl battry. (Courtsy of BM.) 16.4" 19.8" SX W 1.19 A 19.8" SX W 1.78 A FG Photovoltaic solar modul. (Photograph courtsy of BP Solar.) 23.1"

14 46 OLTAGE AND CURRENT Applid torqu nput FG dc gnrator. Output 120 voltag commrcial units is btwn 10 mw and 14 mw. For a squar mtr, howvr, th rturn would b 100 W to 140 W. Th units shown in Fig ar typically usd for rmot tlmtry, isolatd instrumntation systms, scurity snsors, signal sourcs, and land-basd navigation aids. A mor dtaild dscription of th solar cll will appar in your lctronics courss, but for now it is important to raliz that a fairly stady sourc of lctrical dc powr can b obtaind from th sun. Gnrators Th dc gnrator is quit diffrnt from th battry, both in construction (Fig. 2.18) and in mod of opration. Whn th shaft of th gnrator is rotating at th namplat spd du to th applid torqu of som xtrnal sourc of mchanical powr, a voltag of ratd valu appars across th xtrnal trminals. Th trminal voltag and powr-handling capabilitis of th dc gnrator ar typically highr than thos of most battris, and its liftim is dtrmind only by its construction. Commrcially usd dc gnrators ar typically 120 or 240. For th purposs of this txt, th sam symbols ar usd for a battry and a gnrator. FG dc laboratory supply (30, 3 A). (mag complimnts of Ladr nstrumnts Corporation.) Powr Supplis Th dc supply ncountrd most frquntly in th laboratory uss th rctification and filtring procsss as its mans toward obtaining a stady dc voltag. Both procsss will b covrd in dtail in your basic lctronics courss. n total, a tim-varying voltag (such as ac voltag availabl from a hom outlt) is convrtd to on of a fixd magnitud. A dc laboratory supply of this typ is shown in Fig Most dc laboratory supplis hav a rgulatd, adjustabl voltag output with thr availabl trminals, as indicatd horizontally at th bottom of Fig 2.19 and vrtically in Fig 2.20(a). Th symbol for ground or zro potntial (th rfrnc) is also shown in Fig. 2.20(a). f 10 abov ground potntial ar rquird, th connctions ar mad as shown in Fig. 2.20(b). f 15 blow ground potntial ar rquird, th connctions ar mad as shown in Fig. 2.20(c). f connctions ar as shown in Fig. 2.20(d), w say w hav a floating voltag of 5 sinc th rfrnc lvl is not includd. Sldom is th configuration in Fig. 2.20(d) usd sinc it fails to 10 (+10 ) Gnd (0 ) Jumpr 10 (a) (b) Jumpr (15 ) ( Floating ) (c) (d) FG dc laboratory supply: (a) availabl trminals; (b) positiv voltag with rspct to (w.r.t.) ground; (c) ngativ voltag w.r.t. ground; (d) floating supply.

15 OLTAGE SOURCES 47 protct th oprator by providing a dirct low-rsistanc path to ground and to stablish a common ground for th systm. n any cas, th positiv and ngativ trminals must b part of any circuit configuration. Ful Clls On of th most xciting dvlopmnts in rcnt yars has bn th stadily rising intrst in ful clls as an altrnativ nrgy sourc. Ful clls ar now bing usd in th small stationary powr plants, transportation (buss), and a wid varity of applications whr portability is a major factor, such as th spac shuttl. Millions ar now bing spnt by major automobil manufacturrs to build affordabl ful-cll vhicls. Ful clls hav th distinct advantag of oprating at fficincis of 70% to 80% rathr than th typical 20% to 25% fficincy of currnt intrnal combustion ngin of today s automobils. Thy also hav no moving parts, produc littl or no pollution, gnrat vry littl nois, and us fuls such as hydrogn and oxygn that ar radily availabl. Ful clls ar considrd primary clls (of th continuous-fd varity) bcaus thy cannot b rchargd. Thy hold thir charactristics as long as th ful (hydrogn) and oxygn ar supplid to th cll. Th only byproducts of th convrsion procss ar small amounts of hat (which is oftn usd lswhr in th systm dsign), watr (which may also b rusd), and ngligibl lvls of som oxids, dpnding on th componnts of th procss. Ovrall, ful clls ar nvironmntally frindly. Th opration of th ful cll is ssntially opposit to that of th chmical procss of lctrolysis. Elctrolysis is th procss whrby lctric currnt is passd through an lctrolyt to brak it down into its fundamntal componnts. An lctrolyt is any solution that will prmit conduction through th movmnt of ions btwn adjoining lctrods. For instanc, passing currnt through watr rsults in a hydrogn gas by th cathod (ngativ trminal) and oxygn gas at th anod (positiv trminal). n 1839, Sir William Grov blivd this procss could b rvrsd and dmonstratd that th propr application of th hydrogn gas and oxygn rsults in a currnt through an applid load connctd to th lctrods of th systm. Th first commrcial unit was usd in a tractor in 1959, followd by an nrgy pack in th 1965 Gmini program. n 1996, th first small powr plant was dsignd, and today it is an important componnt of th shuttl program. Th basic componnts of a ful cll ar dpictd in Fig. 2.21(a) with dtails of th construction in Fig. 2.21(b). Hydrogn gas (th ful) is supplid to th systm at a rat proportional to th currnt rquird by th load. At th opposit nd of th cll, oxygn is supplid as ndd. Th nt rsult is a flow of lctrons through th load and a discharg of watr with a rlas of som hat dvlopd in th procss. Th amount of hat is minimal although it can also b usd as a componnt in th dsign to improv th fficincy of th cll. Th watr (vry clan) can simply b dischargd or usd for othr applications such as cooling in th ovrall application. f th sourc of hydrogn or oxygn is rmovd, th systm braks down. Th flow diagram of th systm is rlativly simpl, as shown in Fig. 2.21(a). n an actual cll, shown in Fig. 2.21(b), th hydrogn gas is applid to a porous lctrod calld th anod that is coatd with a platinum catalyst. Th catalyst on th anod srvs to spd up th procss of braking down th hydrogn atom into positiv hydrogn ions and fr lctrons. Th lctrolyt btwn th lctrods is a solution or mmbran that prmits th passag of positiv hydrogn ions, but not lctrons. Facing this wall, th

16 48 OLTAGE AND CURRENT Hydrogn DC Powr FUEL CELL Oxygn H 2 (Hydrogn) H + H + H+ H + + O 2 O 2 O 2 (Oxygn) Watr and Hat Anod H + H + Cathod O 2 O 2 Watr (H 2 O) Hat (a) Elctrolyt (b) FG Ful cll (a) componnts; (b) basic construction. lctrons choos to pass through th load and light up th bulb, whil th positiv hydrogn ions migrat toward th cathod. At th porous cathod (also coatd with th catalyst), th incoming oxygn atoms combin with th arriving hydrogn ions and th lctrons from th circuit to crat watr (H 2 O) and hat. Th circuit is, thrfor, complt. Th lctrons ar gnratd and thn absorbd. f th hydrogn supply is cut off, th sourc of lctrons is shut down, and th systm is no longr an oprating ful cll. n som ful clls, ithr a liquid or moltn lctrolyt mmbran is usd. Dpnding on which th systm uss, th chmical ractions will chang slightly, but not dramatically from that dscribd abov. Th phosphoric acid ful cll is a popular cll using a liquid lctrolyt, whil th PEM uss a polymr lctrolyt mmbran. Th liquid or moltn typ is typically usd in stationary powr plants, whil th mmbran typ is favord for vhicular us. Th output from a singl ful cll is a low voltag, high currnt dc output. Stacking th clls in sris or paralll incrass th output voltag or currnt lvl. Ful clls ar rciving an normous amount of attntion and dvlopmnt ffort today. t is crtainly possibl that ful clls may som day rplac battris in th vast majority of applications rquiring a portabl nrgy sourc. Fig shows th componnts of a hydrogn ful-cll automobil. 2.6 AMPERE-HOUR RATNG Th most important pic of data for any battry (othr than its voltag rating) is its ampr-hour (Ah) rating. You hav probably notd in th photographs of battris in this chaptr that both th voltag and th ampr-hour rating hav bn providd for ach battry. Th ampr-hour (Ah) rating provids an indication of how long a battry of fixd voltag will b abl to supply a particular currnt. A battry with an ampr-hour rating of 100 will thortically provid a currnt of 1 A for 100 hours, 10 A for 10 hours, or 100 A for 1 hour. Quit

17 BATTERY LFE FACTORS 49 Hydrogn tanks Ful clls us hydrogn and oxygn to crat a raction that producs lctricity to run th ngin. Watr vapor is th primary mission. Air comprssor pumps air into th ful cll. FG Hydrogn ful-cll automobil. obviously, th gratr th currnt, th shortr th tim. An quation for dtrmining th lngth of tim a battry will supply a particular currnt is th following: ampr-hour 1Ah2 rating Lif 1hours2 amprs drawn 1A2 (2.8) EXAMPLE 2.5 How long will a 9 transistor battry with an amprhour rating of 520 mah provid a currnt of 20 ma? Solution: Eq. (2.8): Lif 520 mah 20 ma h 26 h EXAMPLE 2.6 How long can a 1.5 flashlight battry provid a currnt of 250 ma to light th bulb if th ampr-hour rating is 16 Ah? Solution: Eq. (2.8): Lif 16 Ah 250 ma 16 3 h 64 h BATTERY LFE FACTORS Th prvious sction mad it clar that th lif of a battry is dirctly rlatd to th magnitud of th currnt drawn from th supply. Howvr, thr ar factors that affct th givn ampr-hour rating of a battry, so w may find that a battry with an ampr-hour rating of 100 can supply a currnt of 10 A for 10 hours but can supply a currnt of 100 A for only 20 minuts rathr than th full 1 hour calculatd using Eq. (2.8). n othr words, th capacity of a battry (in ampr-hours) will chang with chang in currnt dmand. This is not to say that Eq. (2.8) is totally invalid. t can always b usd to gain som insight into how long a battry can supply a particular currnt. Howvr, b awar that thr ar factors that affct th amprhour rating. Just as with most systms, including th human body, th

18 50 OLTAGE AND CURRENT mor w dmand, th shortr th tim that th output lvl can b maintaind. This is clarly vrifid by th curvs in Fig for th Evrady Enrgizr D cll. As th constant currnt drain incrasd, th ampr-hour rating dcrasd from about 18 Ah at 25 ma to around 12 Ah at 300 ma. Ah 20 18@25 ma 17@100 ma 15@200 ma Ampr-hour rating 10 12@300 ma 9.5@400 ma ma (constant currnt drain) FG Ampr-hour rating (capacity) vrsus drain currnt for an Enrgizr D cll. Anothr factor that affcts th ampr-hour rating is th tmpratur of th unit and th surrounding mdium. n Fig. 2.24, th capacity of th sam battry plottd in Fig shows a pak valu nar th common room tmpratur of 68 F. At vry cold tmpraturs and vry warm tmpraturs, th capacity drops. Clarly, th ampr-hour rating will b providd at or nar room tmpratur to giv it a maximum valu, but b awar that it will drop off with an incras or dcras in tmpratur. Most of us hav notd that th battry in a car, radio, two-way radio, flashlight, or whatvr sms to hav lss powr in rally cold wathr. t would sm, thn, that th battry capacity would incras with highr tmpraturs apparntly not th cas. n gnral, thrfor, Ah @32 F 18@68 F 10 Room tmpratur (68 F) Frzing F FG Ampr-hour rating (capacity) vrsus tmpratur for an Enrgizr D cll.

19 CONDUCTORS AND NSULATORS 51 th ampr-hour rating of a battry will dcras from th roomtmpratur lvl with vry cold and vry warm tmpraturs. Anothr intrsting factor that affcts th prformanc of a battry is how long it is askd to supply a particular voltag at a continuous drain currnt. Not th curvs in Fig. 2.25, whr th trminal voltag droppd at ach lvl of drain currnt as th tim priod incrasd. Th lowr th currnt drain, th longr it could supply th dsird currnt. At 100 ma, it was limitd to about 100 hours nar th ratd voltag, but at 25 ma, it did not drop blow 1.2 until about 500 hours had passd. That is an incras in tim of 5 : 1, which is significant. Th rsult is that th trminal voltag of a battry will vntually drop (at any lvl of currnt drain) if th tim priod of continuous discharg is too long oltag () Trminal voltag ma 50 ma 25 ma Hours Discharg tim FG Trminal voltag vrsus discharg tim for spcific drain currnts for an Enrgizr D cll. 2.8 CONDUCTORS AND NSULATORS Diffrnt wirs placd across th sam two battry trminals allow diffrnt amounts of charg to flow btwn th trminals. Many factors, such as th dnsity, mobility, and stability charactristics of a matrial, account for ths variations in charg flow. n gnral, howvr, conductors ar thos matrials that prmit a gnrous flow of lctrons with vry littl xtrnal forc (voltag) applid. n addition, good conductors typically hav only on lctron in th valnc (most distant from th nuclus) ring. Sinc coppr is usd most frquntly, it srvs as th standard of comparison for th rlativ conductivity in Tabl 2.1. Not that aluminum, which has sn som commrcial us, has only 61% of th conductivity lvl of coppr. Th choic of matrial must b wighd against th cost and wight factors, howvr. nsulators ar thos matrials that hav vry fw fr lctrons and rquir a larg applid potntial (voltag) to stablish a masurabl currnt lvl. TABLE 2.1 Rlativ conductivity of various matrials. Mtal Rlativ Conductivity (%) Silvr 105 Coppr 100 Gold 70.5 Aluminum 61 Tungstn 31.2 Nickl 22.1 ron 14 Constantan 3.52 Nichrom 1.73 Calorit 1.44

20 52 OLTAGE AND CURRENT (a) (d) (b) (c) () FG arious typs of insulators and thir applications: (a) Corning Glass Works Pyrx powr lin insulator; (b) Fi-Shock xtndr insulator; (c) Lapp powr lin insulator; (d) Fi-Shock cornr insulator; () Fi-Shock scrw-in post insulator. TABLE 2.2 Brakdown strngth of som common insulators. Matrial Avrag Brakdown Strngth (k/cm) Air 30 Porclain 70 Oils 140 Baklit 150 Rubbr 270 Papr (paraffin-coatd) 500 Tflon 600 Glass 900 Mica 2000 A common us of insulating matrial is for covring currnt-carrying wir, which, if uninsulatd, could caus dangrous sid ffcts. Powr lin workrs war rubbr glovs and stand on rubbr mats as safty masurs whn working on high voltag transmission lins. A numbr of diffrnt typs of insulators and thir applications appar in Fig B awar, howvr, that vn th bst insulator will brak down (prmit charg to flow through it) if a sufficintly larg potntial is applid across it. Th brakdown strngths of som common insulators ar listd in Tabl 2.2. According to this tabl, for insulators with th sam gomtric shap, it would rquir 270/30 9 tims as much potntial to pass currnt through rubbr compard to air and approximatly 67 tims as much voltag to pass currnt through mica as through air. 2.9 SEMCONDUCTORS Smiconductors ar a spcific group of lmnts that xhibit charactristics btwn thos of insulators and thos of conductors. Th prfix smi, includd in th trminology, has th dictionary dfinition of half, partial, or btwn, as dfind by its us. Th ntir lctronics industry is dpndnt on this class of matrials sinc th lctronic dvics and intgratd circuits (Cs) ar constructd of smiconductor matrials. Although silicon (Si) is th most xtnsivly mployd matrial, grmanium (G) and gallium arsnid (GaAs) ar also usd in many important dvics. Smiconductor matrials typically hav four lctrons in th outrmost valnc ring.

21 AMMETERS AND OLTMETERS 53 Smiconductors ar furthr charactrizd as bing photoconductiv and having a ngativ tmpratur cofficint. Photoconductivity is a phnomnon whr th photons (small packags of nrgy) from incidnt light can incras th carrir dnsity in th matrial and thrby th charg flow lvl. A ngativ tmpratur cofficint rvals that th rsistanc (a charactristic to b dscribd in dtail in th nxt chaptr) dcrass with an incras in tmpratur (opposit to that of most conductors). A grat dal mor will b said about smiconductors in th chaptrs to follow and in your basic lctronics courss AMMETERS AND OLTMETERS t is important to b abl to masur th currnt and voltag lvls of an oprating lctrical systm to chck its opration, isolat malfunctions, and invstigat ffcts impossibl to prdict on papr. As th nams imply, ammtrs ar usd to masur currnt lvls; voltmtrs, th potntial diffrnc btwn two points. f th currnt lvls ar usually of th ordr of milliamprs, th instrumnt will typically b rfrrd to as a milliammtr, and if th currnt lvls ar in th microampr rang, as a microammtr. Similar statmnts can b mad for voltag lvls. Throughout th industry, voltag lvls ar masurd mor frquntly than currnt lvls, primarily bcaus masurmnt of th formr dos not rquir that th ntwork connctions b disturbd. Th potntial diffrnc btwn two points can b masurd by simply conncting th lads of th mtr across th two points, as indicatd in Fig An up-scal rading is obtaind by placing th positiv lad of th mtr to th point of highr potntial of th ntwork and th common or ngativ lad to th point of lowr potntial. Th rvrs connction rsults in a ngativ rading or a blow-zro indication. Ammtrs ar connctd as shown in Fig Sinc ammtrs masur th rat of flow of charg, th mtr must b placd in th ntwork such that th charg flows through th mtr. Th only way this can b accomplishd is to opn th path in which th currnt is to b masurd and plac th mtr btwn th two rsulting trminals. For th configuration in Fig. 2.28, th voltag sourc lad ( ) must b disconnctd COM 20mA A + COM E 12 E 40 Systm FG oltmtr connction for an up-scal ( ) rading. FG Ammtr connction for an up-scal ( ) rading.

22 54 OLTAGE AND CURRENT FG olt-ohm-milliammtr (OM) analog mtr. (Courtsy of Simpson Elctric Co.) from th systm, and th ammtr insrtd as shown. An up-scal rading will b obtaind if th polaritis on th trminals of th ammtr ar such that th currnt of th systm ntrs th positiv trminal. Th introduction of any mtr into an lctrical/lctronic systm raiss a concrn about whthr th mtr will affct th bhavior of th systm. This qustion and othrs will b xamind in Chaptrs 5 and 6 aftr additional trms and concpts hav bn introducd. For th momnt, lt it b said that sinc voltmtrs and ammtrs do not hav intrnal componnts, thy will affct th ntwork whn introducd for masurmnt purposs. Th dsign of ach, howvr, is such that th impact is minimizd. Thr ar instrumnts dsignd to masur just currnt or just voltag lvls. Howvr, th most common laboratory mtrs includ th volt-ohmmilliammtr (OM) and th digital multimtr (DMM) in Figs and 2.30, rspctivly. Both instrumnts masur voltag and currnt and a third quantity, rsistanc (introducd in th nxt chaptr). Th OM uss an analog scal, which rquirs intrprting th position of a pointr on a continuous scal, whil th DMM provids a display of numbrs with dcimal-point accuracy dtrmind by th chosn scal. Commnts on th charactristics and us of various mtrs will b mad throughout th txt. Howvr, th major study of mtrs will b lft for th laboratory sssions. FG Digital multimtr (DMM). (Courtsy of Fluk Corporation. Rproducd with prmission.) 2.11 APPLCATONS Throughout th txt, Applications sctions such as this on hav bn includd to prmit a furthr invstigation of trms, quantitis, or systms introducd in th chaptr. Th primary purpos of ths Applications is to stablish a link btwn th thortical concpts of th txt and th ral, practical world. Although th majority of componnts that appar in a systm may not hav bn introducd (and, in fact, som componnts will not b xamind until mor advancd studis), th topics wr chosn vry carfully and should b quit intrsting to a nw studnt of th subjct mattr. Sufficint commnt is includd to provid a surfac undrstanding of th rol of ach part of th systm, with th undrstanding that th dtails will com at a latr dat. Sinc xrciss on th subjct mattr of th Applications do not appar at th nd of th chaptr, th contnt is dsignd not to challng th studnt but rathr to stimulat his or hr intrst and answr som basic qustions such as how th systm looks insid, what rol spcific lmnts play in th systm, and, of cours, how th systm works. n ssnc, thrfor, ach Applications sction provids an opportunity to bgin to stablish a practical background byond simply th contnt of th chaptr. Do not b concrnd if you do not undrstand vry dtail of ach application. Undrstanding will com with tim and xprinc. For now, tak what you can from th xampls and thn procd with th matrial. Flashlight Although th flashlight uss on of th simplst of lctrical circuits, a fw fundamntals about its opration do carry ovr to mor sophisticatd systms. First, and quit obviously, it is a dc systm with a liftim totally dpndnt on th stat of th battris and bulb. Unlss it is th rchargabl typ, ach tim you us it, you tak som of th lif out of it. For many hours, th brightnss will not diminish noticably. Thn, howvr, as it rachs th nd of its ampr-hour capacity, th light bcoms dimmr at an incrasingly rapid rat (almost xponntially). Th standard two-battry flashlight is shown in Fig. 2.31(a) with its lctrical

23 APPLCATONS 55 Contact Bulb Rflctor Mtal support Bulb (a) Sliding switch 1.5 D battry 1.5 D battry Switch bulb Spring (b) (c) FG (a) Evrady D cll flashlight; (b) lctrical schmatic of flashlight of part (a); (c) Duracll Powrchck D cll battry. schmatic in Fig. 2.31(b). Each 1.5 battry has an ampr-hour rating of about 18 as indicatd in Fig Th singl-contact miniatur flangbas bulb is ratd at 2.5 and 300 ma with good brightnss and a liftim of about 30 hours. Thirty hours may not sm lik a long liftim, but you hav to considr how long you usually us a flashlight on ach occasion. f w assum a 300 ma drain from th battry for th bulb whn in us, th liftim of th battry, by Eq. (2.8), is about 60 hours. Comparing th 60 hour liftim of th battry to th 30 hour lif xpctancy of th bulb suggsts that w normally hav to rplac bulbs mor frquntly than battris. Howvr, most of us hav xprincd th opposit ffct. W can chang battris two or thr tims bfor w nd to rplac th bulb. This is simply on xampl of th fact that on cannot b guidd solly by th spcifications of ach componnt of an lctrical dsign. Th oprating conditions, trminal charactristics, and dtails about th actual rspons of th systm for short and long priods of tim must b considrd. As mntiond arlir, th battry loss som of its powr ach tim it is usd. Although th trminal voltag may not chang much at first, its ability to provid th sam lvl of currnt drops with ach usag. Furthr, battris slowly discharg du to lakag currnts vn if th switch is not on. Th air surrounding th battry is not clan in th sns that moistur and othr lmnts in th air can provid a conduction path for lakag currnts through th air through th surfac of th battry itslf, or through othr narby surfacs, and th battry vntually dischargs. How oftn hav w lft a flashlight with nw battris in a car for a long priod of tim only to find th light vry dim or th battris dad whn w nd th flashlight th most? An additional problm is acid laks that appar as brown stains or corrosion on th casing of th battry. Ths laks also affct th lif of th battry. Furthr, whn th flashlight is turnd on, thr is an initial surg in currnt that drains th battry mor than continuous us for a priod of tim. n othr words, continually turning th flashlight on and off has a vry dtrimntal ffct on its lif. W must also raliz that th 30 hour rating of th bulb is for continuous us, that is, 300 ma flowing through th bulb for a continuous 30 hours. Crtainly, th filamnt in th bulb and th bulb itslf will gt hottr with tim,

24 56 OLTAGE AND CURRENT and this hat has a dtrimntal ffct on th filamnt wir. Whn th flashlight is turnd on and off, it givs th bulb a chanc to cool down and rgain its normal charactristics, thrby avoiding any ral damag. Thrfor, with normal us w can xpct th bulb to last longr than th 30 hours spcifid for continuous us. Evn though th bulb is ratd for 2.5 opration, it would appar that th two battris would rsult in an applid voltag of 3 which suggsts poor oprating conditions. Howvr, a bulb ratd at 2.5 can asily handl 2.5 to 3. n addition, as was pointd out in this chaptr, th trminal voltag drops with th currnt dmand and usag. Undr normal oprating conditions, a 1.5 battry is considrd to b in good condition if th loadd trminal voltag is 1.3 to 1.5. Whn it drops to th rang from 1 to 1.1, it is wak, and whn it drops to th rang from 0.8 to 0.9, it has lost its ffctivnss. Th lvls can b rlatd dirctly to th tst band now apparing on Duracll battris, such as on th on shown in Fig. 2.31(c). n th tst band on this battry, th uppr voltag ara (grn) is nar 1.5 (labld 100%); th lightr ara to th right, from about 1.3 down to 1 ; and th rplac ara (rd) on th far right, blow 1. B awar that th total supplid voltag of 3 will b obtaind only if th battris ar connctd as shown in Fig. 2.31(b). Accidntally placing th two positiv trminals togthr will rsult in a total voltag of 0, and th bulb will not light at all. For th vast majority of systms with mor than on battry, th positiv trminal of on battry will always b connctd to th ngativ trminal of anothr. For all low-voltag battris, th nd with th nippl is th positiv trminal, and th nd with th flat nd is th ngativ trminal. n addition, th flat or ngativ nd of a battry is always connctd to th battry casing with th hlical coil to kp th battris in plac. Th positiv nd of th battry is always connctd to a flat spring connction or th lmnt to b opratd. f you look carfully at th bulb, you will find that th nippl connctd to th positiv nd of th battry is insulatd from th jackt around th bas of th bulb. Th jackt is th scond trminal of th battry usd to complt th circuit through th on/off switch. f a flashlight fails to oprat proprly, th first thing to chck is th stat of th battris. t is bst to rplac both battris at onc. A systm with on good battry and on naring th nd of its lif will rsult in prssur on th good battry to supply th currnt dmand, and, in fact, th bad battry will actually b a drain on th good battry. Nxt, chck th condition of th bulb by chcking th filamnt to s whthr it has opnd at som point bcaus a long-trm, continuous currnt lvl occurrd or bcaus th flashlight was droppd. f th battry and bulb sm to b in good shap, th nxt ara of concrn is th contacts btwn th positiv trminal and th bulb and th switch. Claning both with mry cloth oftn liminats this problm. 12 Car Battry Chargr Battry chargrs ar a common houshold pic of quipmnt usd to charg vrything from small flashlight battris to havy-duty, marin, lad-acid battris. Sinc all ar pluggd into a 120 ac outlt such as found in th hom, th basic construction of ach is quit similar. n vry charging systm, a transformr (Chaptr 22) must b includd to cut th ac voltag to a lvl appropriat for th dc lvl to b stablishd. A diod (also calld rctifir) arrangmnt must b includd to convrt th ac voltag which varis with tim to a fixd dc lvl such as dscribd in this chaptr. Diods and/or rctifirs will b discussd in dtail in your first lctronics cours. Som dc chargrs also includ a rgulator to provid

25 APPLCATONS 57 Circuit brakr Mtr lads Hat sink Rctifir (diod) configuration Transformr Control switch Groundd to chassis (a) (b) FG Battry chargr: (a) xtrnal apparanc; (b) intrnal construction. an improvd dc lvl (on that varis lss with tim or load). Th car battry chargr, on of th most common, is dscribd hr. Th outsid apparanc and th intrnal construction of a Sars 6/2 AMP Manual Battry Chargr ar providd in Fig Not in Fig. 2.32(b) that th transformr (as in most chargrs) taks up most of th intrnal spac. Th additional air spac and th hols in th casing ar thr to nsur an outlt for th hat that will dvlop du to th rsulting currnt lvls. Th schmatic in Fig includs all th basic componnts of th chargr. Not first that th 120 from th outlt ar applid dirctly across th primary of th transformr. Th charging rat of 6 A or 2 A is dtrmind by th switch, which simply controls how many windings of th primary will b in th circuit for th chosn charging rat. f th battry is charging at th 2 A lvl, th full primary will b in th circuit, and th ratio of th turns in th primary to th turns in th scondary will b a maximum. f it is charging at th 6 A lvl, fwr turns of th primary ar in th circuit, and th ratio drops. Whn you study transformrs, you will find that th voltag at th primary and scondary is dirctly rlatd to th turns ratio. f th ratio from primary to scondary drops, th voltag drops also. Th rvrs ffct occurs if th turns on th scondary xcd thos on th primary. Th gnral apparanc of th wavforms appars in Fig for th 6 A charging lvl. Not that so far, th ac voltag has th sam wav shap across th primary and scondary. Th only diffrnc is in th pak valu of th wavforms. Now th diods tak ovr and convrt th ac wavform which has zro avrag valu (th wavform abov quals th wavform blow) to on that has an avrag valu (all abov th axis) as shown in th sam figur. For th momnt simply rcogniz that diods ar smiconductor lctronic dvics that prmit only convntional currnt to flow through thm in th dirction indicatd by th arrow in th symbol. Evn though th wavform rsulting from th diod

26 58 OLTAGE AND CURRENT Pak = ac 2 A 6 A 13 Diods (rctifirs) Positiv clamp of chargr Transformr (stp-down) 12 FG Elctrical schmatic for th battry chargr of Fig Circuit brakr Currnt mtr Ngativ clamp of chargr action has a pulsing apparanc with a pak valu of about 18, it chargs th 12 battry whnvr its voltag is gratr than that of th battry, as shown by th shadd ara. Blow th 12 lvl, th battry cannot discharg back into th charging ntwork bcaus th diods prmit currnt flow in only on dirction. n particular, not in Fig. 2.32(b) th larg plat that carris th currnt from th rctifir (diod) configuration to th positiv trminal of th battry. ts primary purpos is to provid a hat sink (a plac for th hat to b distributd to th surrounding air) for th diod configuration. Othrwis, th diods would vntually mlt down and slf-dstruct du to th rsulting currnt lvls. Each componnt of Fig has bn carfully labld in Fig. 2.32(b) for rfrnc. Whn currnt is first applid to a battry at th 6 A charg rat, th currnt dmand as indicatd by th mtr on th fac of th instrumnt may ris to 7 A or almost 8 A. Howvr, th lvl of currnt dcrass as th battry chargs until it drops to a lvl of 2 A or 3 A. For units such as this that do not hav an automatic shutoff, it is important to disconnct th chargr whn th currnt drops to th fully chargd lvl; othrwis, th battry bcoms ovrchargd and may b damagd. A battry that is at its 50% lvl can tak as long as 10 hours to charg, so don t xpct it to b a 10-minut opration. n addition, if a battry is in vry bad shap with a lowr than normal voltag, th initial charging currnt may b too high for th dsign. To protct against such situations, th circuit brakr opns and stops th charging procss. Bcaus of th high currnt lvls, it is important that th dirctions providd with th chargr b carfully rad and applid. FG Answring machin/phon 9 dc supply. Answring Machins/Phons dc Supply A wid varity of systms in th hom and offic rciv thir dc oprating voltag from an ac/dc convrsion systm pluggd right into a 120 ac outlt. Laptop computrs, answring machins/phons, radios, clocks, cllular phons, CD playrs, and so on, all rciv thir dc powr from a packagd systm such as shown in Fig Th convrsion from ac to

27 PROBLEMS 59 dc occurs within th unit which is pluggd dirctly into th outlt. Th dc voltag is availabl at th nd of th long wir which is dsignd to b pluggd into th oprating unit. As small as th unit may b, it contains basically th sam componnts as in th battry chargr in Fig n Fig. 2.35, you can s th transformr usd to cut th voltag down to appropriat lvls (again th largst componnt of th systm). Not that two diods stablish a dc lvl, and a capacitiv filtr (Chaptr 10) is addd to smooth out th dc as shown. Th systm can b rlativly small bcaus th oprating currnt lvls ar quit small, prmitting th us of thin wirs to construct th transformr and limit its siz. Th lowr currnts also rduc th concrns about hating ffcts, prmitting a small housing structur. Th unit in Fig. 2.35, ratd at 9 at 200 ma, is commonly usd to provid powr to answring machins/phons. Furthr smoothing of th dc voltag is accomplishd by a rgulator built into th rciving unit. Th rgulator is normally a small C chip placd in th rciving unit to sparat th hat that it gnrats from th hat gnratd by th transformr, thrby rducing th nt hat at th outlt clos to th wall. n addition, its placmnt in th rciving unit rducs th possibility of picking up nois and oscillations along th long wir from th convrsion unit to th oprating unit, and it nsurs that th full ratd voltag is availabl at th unit itslf, not a lssr valu du to losss along th lin. Capacitor Diods (2) 120 ac plug Transformr 9 dc output FG ntrnal construction of th 9 dc supply in Fig COMPUTER ANALYSS n som txts, th procdur for choosing a dc voltag sourc and placing it on th schmatic using computr mthods is introducd at this point. This approach, howvr, rquirs studnts to turn back to this chaptr for th procdur whn th first complt ntwork is installd and xamind. Thrfor, th procdur is introducd in Chaptr 4 whn th first complt ntwork is xamind, thrby localizing th matrial and rmoving th nd to rrad this chaptr and Chaptr 3. PROBLEMS SECTON 2.2 Atoms and Thir Structur 1. Th numbrs of orbiting lctrons in aluminum and silvr ar 13 and 47, rspctivly. Draw th lctronic configuration for ach, and discuss brifly why ach is a good conductor. 2. Find th forc of attraction in nwtons btwn th chargs Q 1 and Q 2 in Fig whn a. r 1 m b. r 3 m c. r 10 m d. Did th forc drop off quickly with an incras in distanc? 1 C 2 C Q 1 r Q 2 FG Problm 2. *3. Find th forc of rpulsion in nwtons btwn Q 1 and Q 2 in Fig whn a. r 1 mi b. r 10 ft c. r 1/16 in. 8 C 40 C Q 1 r Q 2 FG Problm 3. *4. Plot th forc of attraction (in nwtons) vrsus sparation (in mtrs) for two chargs of 2 C and 8 C. St r to 0.5 m and 1 m, followd by 1 m intrvals to 10 m. Commnt on th shap of th curv. s it linar or nonlinar? What dos it tll you about th forc of attraction btwn chargs as thy ar sparatd? What dos it tll you about any function plottd against a squard trm in th dnominator?