教學(xué)課件-信息工程專業(yè)英語(黃小莉)

Unit1CircuitComponents1.1Text1.2ReadingMaterials

1.1Text

Resistors：Aresistorisanelectricalcomponentthatresiststheflowofelectricalcurrent.Theamountofcurrent(I)flowinginacircuitisdirectlyproportionaltothevoltageacrossitandinverselyproportionaltotheresistanceofthecircuit.ThisisOhmlawandcanbeexpressedasaformula: .Theresistorisgenerallyalineardeviceanditscharacteristicsformastraightlinewhenplottedonagraph.

Resistorsareusedtolimitcurrentflowingtoadevice,therebypreventingitfromburningout,asvoltagedividerstoreducevoltageforothercircuits,astransistorbiasingcircuits,andtoserveascircuitloads.

Generally,resistors(Fig1.1)consistofcarboncomposition,wire-wound,andmetalfilm.Thesizeofresistorsdependsonpowerratings.Largersizesarereferredtoaspowerresistors.Variableresistorsareadjustable:rheostats,potentiometers,andtrimmerpots.Precisionresistorshaveatoleranceof1%orless.

Fig1.1VariousResistors

IfyouareabitseriousabouttheelectronicshobbyIrecommendlearningthe“ColorCode”.Itmakesaloteasier.Thesamecolorcodeisusedforeverythingelse,likecoils,capacitorsetc.Again,justthecolorcodeassociatedwithanumber,like:black=0brown=1red=2,etc.Fig1.2isanexample;itisa4-bandresistor.

Canyou“create”yourownresistors?Ofcauseandnotdifficult.Hereishowtodoit.Drawalineonapieceofpaperwithasoftpencil,HBor2HBwilldofine.Makethelinethickandabout2inches(5cm)long.Withyourmultimeter,measuretheohm’svalueofthislinebyputtingaprobeoneachsideoftheline;makesuretheprobesaretouchingthecarbonfromthepencil.Thevaluewouldprobablybearoundthe800kto1.5Mdependingline.Theresistancewilldropconsiderably,ifyouerasesomeofit(length-wiseobviously!).Youcanalsousecarbonwithsiliconglueandwhenitdriesmeasuretheresistance,etc.

Fig1.2Anexampleofresistorcolorcode

Capacitors：Acapacitorisanelectricaldevicethatcantemporarilystoreelectricalenergy.Basically,acapacitorconsistsoftwoconductors(metalplates)separatedbyadielectricinsulatingmaterial(Fig1.3(a)),whichincreasestheabilitytostoreacharge.

Fig1.3Capacitor

Thedielectriccanbepaper,plasticfilm,mica,ceramic,airoravacuum.Theplatescanbealuminumdiscs,aluminumfoilorathinfilmofmetalappliedtooppositesidesofasoliddielectric.Theconductor-dielectric-conductorsandwichcanberolledintoacylinderorleftflat，thesymbolsofcapacitorareshowninFig1.3(b).

AcapacitorwillblockDCcurrent,butappearstopassACcurrentbycharginganddischarging.ItdevelopsanACresistance,knownascapacitivereactance,whichisaffectedbythecapacitanceandACfrequency.TheformulaforcapacitivereactanceisXC?=?1/(2

fC),withunitsofohms.

Inductors:Aninductorisanelectricaldevice,whichcantemporarilystoreelectromagneticenergyinthefieldaboutitaslongascurrentisflowingthroughit.Theinductorisacoilofwirethatmayhaveanaircoreoranironcoretoincreaseitsinductance.Apoweredironcoreintheshapeofacylindermaybeadjustedinandoutofthecore.

Aninductortendstoopposeachangeinelectricalcurrent,ithasnoresistancetoDCcurrentbuthasanACresistancetoACcurrent,knownasinductivereactance,thisinductivereactanceisaffectedbyinductanceandtheACfrequencyandisgivenbytheformulaXL?=?2

fL,withunitsofohms.InductorsareusedforfilteringACcurrent,increasingtheoutputoftheRF(RadioFrequency)amplifier.

Inductorsareavailableinvarietyofshapes(Fig1.4):aircore,ironcore(whichmaylooklikeatransformer,buthasonlytwoleads),toroidal(doughnutshaped),smalltubularwithepoxy,RFchokewithseparatecoilsonacylinder,andtunableRFcoilwithascrewdriveradjustment.

Fig1.4Variousinductors

TechnicalWordsandPhrases

reactance n.電抗

resistance n.【電】電阻值，中文有時(shí)簡稱電阻

resistor

n.【電】電阻器，中文有時(shí)簡稱電阻

voltage

n.【電工】電壓，伏特?cái)?shù)；電壓(電位差)

capacitance n.電容量

capacitor n.電容器

charge n.負(fù)荷，電荷，充電v.裝滿，充電

dielectric n.電介質(zhì)，絕緣體adj.非傳導(dǎo)性的

electrical adj.電的，有關(guān)電的

formula n.公式，規(guī)則，客套話

inductance n.電感量，電感值

inductor n.誘導(dǎo)物，感應(yīng)器，電感器

insulating adj.絕緣的

multimeter n.萬用表vt.多點(diǎn)測(cè)量

probe n.探針，外太空探測(cè)器；探測(cè)飛船，探索

beinverselyproportionalto 與……成反比

beaffectedby… 受……影響

beavailable 是可利用的，可用的

bedirectlyproportionalto

與……成正比

beexpressedasaformula 用公式表示成……

beproportionalto 與……成比例

beusedto…

被用于……

invariousshapesandsizes 各種形狀和尺寸

toserveas… 用作……

1.1.1Exercises

1.PutthePhrasesintoEnglish

(1)運(yùn)算放大器； (2)傳遞函數(shù)；

(3)電子器件； (4)集成電路；

(5)反饋電路； (6)性能參數(shù)。

2.PutthePhrasesintoChinese

(1)electricalcomponents; (2)Ohmlaw;

(3)limitcurrent; (4)voltagedivider;

(5)transistorbiasingcircuits.

3.Translation

(1)Resistorsareusedtolimitcurrentflowingtoadevice,therebypreventingitfromburningout,asvoltagedividerstoreducevoltageforothercircuits,astransistorbiasingcircuits,andtoserveascircuitloads.

(2)IfyouareabitseriousabouttheelectronicshobbyIrecommendlearningthe“ColorCode”.

(3)AcapacitorwillblockDCcurrent,butappearstopassACcurrentbycharginganddischarging.

(4)Capacitorsareavailableinvariousshapesandsizes.

(5)Capacitorsareusedforfiltering,bypassingsignals,fortimingcircuits,andforradio-frequency(RF)tuningcircuits.

(6)Aninductorisanelectricaldevice,whichcantemporarilystoreelectromagneticenergyinthefieldaboutitaslongascurrentisflowingthroughit.

(7)Theinductivereactanceisaffectedbyinductanceandtheacfrequency.

(8)Theinputvoltageistotheprimarywindingandtheinducedvoltageistakenoffthesecondarywinding.

1.1.2參考譯文

電阻器：電阻器是一種能阻礙電流流動(dòng)的電子器件。電阻器中流過的電流與加在電阻兩端的電壓成正比，與電阻的阻值成反比。這就是歐姆定律，可以用公式表示成I=UR/R。

電阻器通常是線性器件，它的(伏安)特性曲線是一條直線。

電阻器常用作限流器，以限制流過器件的電流，以防止器件因流過的電流過大而燒壞。電阻器也可以用作分壓器，以減小其他電路的電壓，如晶體管偏置電路。電阻器還可用作電路的負(fù)載。

一般來說，電阻有碳(膜)電阻、線繞電阻和金屬膜電阻(如圖1.1所示)，電阻器尺寸的大小與電阻的(額定)功率有關(guān)，尺寸比較大的電阻器通常是高功率電阻器。可變電阻器是電阻值可調(diào)節(jié)的電阻器，如變阻器、電位器和微調(diào)電位器。精密電阻器是指其誤差率在1%或更小的電阻器。

如果你對(duì)電子技術(shù)頗有興趣，建議學(xué)會(huì)電阻的“彩色條形碼”識(shí)別方法，這樣會(huì)帶來很多方便。而且這種彩色條形碼標(biāo)注方法在其他器件上也適用，如線圈(電感)、電容等。這種方法是用彩色表示數(shù)字，如黑色=0，棕色=1，紅色=2等，圖1.2是個(gè)彩色條形碼電阻的例子。

自己可以制作一個(gè)電阻(器)嗎？當(dāng)然可以，而且也不難。這里教你如何做一個(gè)電阻(器)，用一支軟鉛筆(HB鉛筆或用2HB鉛筆更好)，在紙上畫一段大約2英寸(5cm)長的粗線。用萬用表測(cè)量這段線的歐姆值，(方法是)把萬用表的兩個(gè)探筆分別與鉛筆線的兩端相接觸，一定要讓探筆與線端的碳接觸。根據(jù)線的粗細(xì)，電阻值大約為800kΩ～1.5MΩ。如果你擦掉一些線，使線明顯變短，電阻值就會(huì)變小。你也可以用摻有硅膠的碳粉來制作電阻器，當(dāng)硅膠干了以后測(cè)量其電阻值等。

電容器：電容器是可以暫時(shí)存儲(chǔ)電能的電子器件。電容器一般由兩塊導(dǎo)體(金屬極板)組成(見圖1.3(a))，中間用一層不導(dǎo)電的絕緣材料隔開，這層絕緣材料可以增加電容存儲(chǔ)電荷的本領(lǐng)(即增大電容量)。

絕緣材料可以是紙、塑料片、云母、陶瓷材料、空氣或真空。極板可以是薄鋁板、鋁箔或在一片兩面各貼上一層金屬薄膜的絕緣板?？梢灾苯影岩粋€(gè)這種導(dǎo)體-絕緣體-導(dǎo)體(三明治式)制成平板電容器，也可以把它卷起來成為圓柱形電容器。電容器的符號(hào)如圖1.3(b)所示。

電容器隔直流電，但能以充電和放電的方式通過交流電。它構(gòu)成的交流電阻抗稱為容抗。容抗與電容量和交流電的頻率有關(guān)，容抗的公式為XC=1/(2πfC)，其單位為歐姆。

電感器：當(dāng)電流流過電感器時(shí)，電感器周圍就有電磁場，電感器是以電磁場的形式暫時(shí)存儲(chǔ)電磁能量的電子器件。電感器是一組線圈，有的電感器線圈中有可增加其電感量的鐵芯，可調(diào)電感有一個(gè)強(qiáng)磁的圓柱狀鐵芯，通過調(diào)節(jié)鐵芯可以增加或減少電感量。

電感量有阻礙電流變化的趨勢(shì)，對(duì)直流電而言，電感器是沒有阻礙作用的，但對(duì)交流電來說，電感器有一個(gè)交流阻抗，稱為感抗。這個(gè)感抗與電感量和交流電的頻率有關(guān)，可以用公式表示為XL=2πfL，其單位為歐姆。電感器可以用來濾波、增加RF(無線電頻率)放大器的輸出。電感器有各式各樣的形狀(如圖1.4所示)，空芯的、鐵芯的(鐵芯的有時(shí)看起來像個(gè)變壓器，但只有兩個(gè)端口)、環(huán)狀的(圓環(huán)形的)、管狀的(以環(huán)氧樹脂為材料)，RF扼流圈是由圓柱上分開的線圈構(gòu)成，而且可調(diào)的線圈帶有一把調(diào)整螺絲刀。

1.2ReadingMaterials

1.2.1HowSemiconductorsWork

Semiconductorshavehadamonumentalimpactonoursociety.Youfindsemiconductorsattheheartofmicroprocessorchipsaswellastransistors.Anythingthat’scomputerizedorusesradiowavesdependsonsemiconductors.

Today,mostsemiconductorchipsandtransistorsarecreatedwithsilicon.Youmayhaveheardexpressionslike“SiliconValley”andthe“siliconeconomy,”andthat’swhy—siliconistheheartofanyelectronicdevice.

Adiodeisthesimplestpossiblesemiconductordevice,andisthereforeanexcellentbeginningpointifyouwanttounderstandhowsemiconductorswork.Inthisarticle,you’lllearnwhatasemiconductoris,howdopingworksandhowadiodecanbecreatedusingsemiconductors.Butfirst,let’stakeacloselookatsilicon.

Siliconisaverycommonelement—forexample,itisthemainelementinsandandquartz.Ifyoulook“silicon”upintheperiodictable(Fig1.5),youwillfindthatitsitsnexttoaluminum,belowcarbonandabovegermanium.

Fig1.5Siliconsitsnexttoaluminumandbelowcarbonintheperiodictable

Carbon,siliconandgermanium(germanium,likesilicon,isalsoasemiconductor)haveauniquepropertyintheirelectronstructure—eachhasfourelectronsinitsouterorbital.Thisallowsthemtoformnicecrystals.Thefourelectronsformperfectcovalentbondswithfourneighboringatoms,creatingalattice.Incarbon,weknowthecrystallineformasdiamond.Insilicon,thecrystallineformisasilvery,metallic-lookingsubstance.

Inasiliconlattice(Fig1.6),allsiliconatomsbondperfectlytofourneighbors,leavingnofreeelectronstoconductelectriccurrent.Thismakesasiliconcrystalaninsulatorratherthanaconductor.

Fig1.6Siliconlattice

Metalstendtobegoodconductorsofelectricitybecausetheyusuallyhave“freeelectrons”thatcanmoveeasilybetweenatoms,andelectricityinvolvestheflowofelectrons.Whilesiliconcrystalslookmetallic,theyarenot,infact,metals.Alloftheouterelectronsinasiliconcrystalareinvolvedinperfectcovalentbonds,sotheycan’tmovearound.Apuresiliconcrystalisnearlyaninsulator—verylittleelectricitywillflowthroughit.

Butyoucanchangeallthisthroughaprocesscalleddoping.Youcanchangethebehaviorofsiliconandturnitintoaconductorbydopingit.Indoping,youmixasmallamountofanimpurityintothesiliconcrystal.

Therearetwotypesofimpurities:

N-type-InN-typedoping,phosphorusorarsenicisaddedtothesiliconinsmallquantities.Phosphorusandarseniceachhavefiveouterelectrons,sothey’reoutofplacewhentheygetintothesiliconlattice.Thefifthelectronhasnothingtobondto,soit’sfreetomovearound.Ittakesonlyaverysmallquantityoftheimpuritytocreateenoughfreeelectronstoallowanelectriccurrenttoflowthroughthesilicon.N-typesiliconisagoodconductor.Electronshaveanegativecharge,hencethenameN-type.

P-type-InP-typedoping,boronorgalliumisthedopant.Boronandgalliumeachhaveonlythreeouterelectrons.Whenmixedintothesiliconlattice,theyform“holes”inthelatticewhereasiliconelectronhasnothingtobondto.Theabsenceofanelectroncreatestheeffectofapositivecharge,hencethenameP-type.Holescanconductcurrent.Aholehappilyacceptsanelectronfromaneighbor,movingtheholeoveraspace.P-typesiliconisagoodconductor.

AminuteamountofeitherN-typeorP-typedopingturnsasiliconcrystalfromagoodinsulatorintoaviable(butnotgreat)conductor—hencethename“semiconductor”.

N-typeandP-typesiliconarenotthatamazingbythemselves;butwhenyouputthemtogether,yougetsomeveryinterestingbehavioratthejunction.That’swhathappensinadiode.

Adiode(Fig1.7)isthesimplestpossiblesemiconductordevice.Adiodeallowscurrenttoflowinonedirectionbutnottheother.Youmayhaveseenturnstilesatastadiumorasubwaystationthatletpeoplegothroughinonlyonedirection.Adiodeisaone-wayturnstileforelectrons.

Fig1.7Diode

WhenyouputN-TYPEandP-TYPEsilicontogetherasshowninthisdiagram,yougetaveryinterestingphenomenonthatgivesadiodeitsuniqueproperties.

EventhoughN-TYPEsiliconbyitselfisaconductor,andP-TYPEsiliconbyitselfisalsoaconductor,thecombinationshowninthediagramdoesnotconductanyelectricity.ThenegativeelectronsintheN-typesilicongetattractedtothepositiveterminalofthebattery.

ThepositiveholesintheP-TYPEsilicongetattractedtothenegativeterminalofthebattery.Nocurrentflowsacrossthejunctionbecausetheholesandtheelectronsareeachmovinginthewrongdirection.Ifyouflipthebatteryaround,thediodeconductselectricityjustfine.ThefreeelectronsintheN-TYPEsiliconarerepelledbythenegativeterminalofthebattery.TheholesintheP-TYPEsiliconarerepelledbythepositiveterminal.AtthejunctionbetweentheN-TYPEandP-TYPEsilicon,holesandfreeelectronsmeet.Theelectronsfilltheholes.Thoseholesandfreeelectronsceasetoexist,andnewholesandelectronsspringuptotaketheirplace.Theeffectisthatcurrentflowsthroughthejunction.

1.2.2ThePowerSupply

Apowersupplyisadevicethattakesanincomingelectricalcurrentandamplifiesittolevelsrequiredbyvariousdevices.Inmanyinstances,apowersupplyisalsoimplementedtotaketheincomingelectricityanddeliveritacrossmanyotherelectronicdevices,oftenatdifferentpresetlevels.Thisdeviceallowsmanufacturerstocreateelectronicsandmachinerythatcanhandlemanydifferenttasksfromasinglesourceofpower,withouttheneedforvariousadaptersandadditionalhardware.Withinotherdevices,apowersupplyisusedtotransformvarioustypesofpowerintoacompatibleformattobestored,likesolarenergytoelectricalenergy.

Perhapsthemostcommonuseofapowersupplyiswithincomputersystems.Aselectricityentersthepowersupply,itismomentarilystoredandthendistributedtonumerousfunctionsthroughoutthesystem,allowingthemotherboard,harddrive,andothervariousdevicestoreceiveelectricityinordertofunction.Eachoneoftheseitemsrequiresaseparatevoltage(amperes),anditisdeliveredthroughspecializedconnectorsthatattachinacertainmanner.Forexample,motherboardsrequireeithera20-pinpowersupplyora24-pinpowersupply,andtheyarenotinterchangeablewithoutthepurchaseofanadditionaladapter.

Modernvehiclesalsorequireatypeofpowersupplyinordertofunction,anditisreferredtoasanalternator(Fig1.8).Althoughthewiringanddesignmaybedifferent,itessentiallyworksintheexactsamemannerbytakingincomingpoweranddeliveringitthroughoutthevehicleatthenecessarylevels.Alternatorscanbefoundoneverythingfromlawnmowerstoseacraftandindustrialequipment,andwithoutthem,thedeviceswouldberendereduseless.

Fig1.8Alternatorformodernvehicles

Anothercommontypeofpowersupplycanbefoundonwindmillsandsolarpanels(Fig1.9),anditsprimaryfunctionistoconvertvarioustypesofenergyintoelectricitysothatitcanbestoredanddistributedacrossagrid.Thistypeofpowersupplyisreferredtoasagenerator,anditisoftenafree-standingobjectthatisinstalledbetweenthepowersourceandthestorageunit.Homeandcommercialgenerators,usedduringpoweroutages,alsoworkoffofthissamepremisebytransformingpetroleumproductsintoelectricalenergybymeansofanengine.Manytypesofindustrialtoolsalsoimplementatypeofgenerator.Othercommontypesofpowersuppliesareusedwithincircuitbreakers,battery-powereditemsandtransformers.

Fig1.9Solarpanels

1.2.3MOSFETSwitches

TheN-channel,Enhancement-modeMOSFEToperatesusingapositiveinputvoltageandhasanextremelyhighinputresistance(almostinfinite)makingitpossibletointerfacewithnearlyanylogicgateordrivercapableofproducingapositiveoutput.Also,duetothisveryhighinput(Gate)resistancewecanparalleltogethermanydifferentMOSFET’suntilweachievethecurrenthandlinglimitrequired.WhileconnectingtogethervariousMOSFET’smayenableustoswitchhighcurrentorhighvoltageloads,doingsobecomesexpensiveandimpracticalinbothcomponentsandcircuitboardspace.ToovercomethisproblemPowerFieldEffectTransistorsorPowerFET’swheredeveloped.

WenowknowthattherearetwomaindifferencesbetweenFET’s,Depletion-modeforJFET’sandEnhancement-modeforMOSFET’sandonthispagewewilllookatusingtheEnhancement-modeMOSFETasaSwitch.

ByapplyingasuitabledrivevoltagetotheGateofanFETtheresistanceoftheDrain-Sourcechannelcanbevariedfroman“OFF-resistance”ofmanyhundredsofk?’s,effectivelyanopencircuit,toan“ON-resistance”oflessthan1?,effectivelyashortcircuit.WecanalsodrivetheMOSFETtoturn“ON”fastorslow,ortopasshighcurrentsorlowcurrents.ThisabilitytoturnthepowerMOSFET“ON”and“OFF”allowsthedevicetobeusedasaveryefficientswitchwithswitchingspeedsmuchfasterthanstandardbipolarjunctiontransistors.

AnexampleofusingtheMOSFETasaswitch:

Inthiscircuit(Fig1.10)arrangementanEnhancement-modeN-channelMOSFETisbeingusedtoswitchasimplelamp“ON”and“OFF”(couldalsobeanLED).ThegateinputvoltageVGSistakentoanappropriatepositivevoltageleveltoturnthedeviceandthelampeitherfully“ON”,(VGS=+ve)orazerovoltageleveltoturnthedevicefully“OFF”(VGS=0).Iftheresistiveloadofthelampwastobereplacedbyaninductiveloadsuchasacoilorsolenoid,a“Flywheel”diodewouldberequiredinparallelwiththeloadtoprotecttheMOSFETfromanyback-emf.

Fig1.10CircuitwithanEnhancement-modeN-channelMOSFET

AboveshowsaverysimplecircuitforswitchingaresistiveloadsuchasalamporLED.ButwhenusingpowerMOSFET’stoswitcheitherinductiveorcapacitiveloadssomeformofprotectionisrequiredtopreventtheMOSFETdevicefrombecomingdamaged.Drivinganinductiveloadhastheoppositeeffectfromdrivingacapacitiveload.Forexample,acapacitorwithoutanelectricalchargeisashortcircuit,resultinginahigh“inrush”ofcurrentandwhenweremovethevoltagefromaninductiveloadwehavealargereversevoltagebuildupasthemagneticfieldcollapses,resultinginaninducedback-emfinthewindingsoftheinductor.

ForthepowerMOSFETtooperateasananalogueswitchingdevice,itneedstobeswitchedbetweenits“Cut-offRegion”whereVGS=0andits“SaturationRegion”whereVGS(on).

ThepowerdissipatedintheMOSFET(PD)dependsuponthecurrentflowingthroughthechannelIDatsaturationandalsothe“ON-resistance”ofthechannelgivenasRDS(on).

Forexample:

Letsassumethatthelampisratedat6V,24Wandisfully“ON”andthestandardMOSFEThasachannel“ON-resistance”(RDS(on))valueof0.1ohms.CalculatethepowerdissipatedintheMOSFETswitch.

Thecurrentflowingthroughthelampiscalculatedas:

ThenthepowerdissipatedintheMOSFETwillbegivenas:

Youmaythink,wellsowhat!butwhenusingtheMOSFETasaswitchtocontrolDCmotorsorhighinrushcurrentdevicesthe“ON”channelresistance(RDS(on))isveryimportant.Forexample,MOSFET’sthatcontrolDCmotors,aresubjectedtoahighin-rushcurrentasthemotorfirstbeginstorotate.ThenahighRDS(on)channelresistancevaluewouldsimplyresultinlargeamountsofpowerbeingdissipatedwithintheMOSFETitselfresultinginanexcessivetemperaturerise,andwhichinturncouldresultintheMOSFETbecomingveryhotanddamagedduetoathermaloverload.

ButalowRDS(on)valueontheotherhandisalsodesirabletohelpreducetheeffectivesaturationvoltage(VDS(sat)=ID×RDS(on))acrosstheMOSFET.WhenusingMOSFET’soranytypeofFieldEffectTransistorforthatmatterasaswitchingdevice,itisalwaysadvisabletoselectonesthathaveaverylowRDS(on)valueoratleastmountthemontoasuitableheatsinktohelpreduceanythermalrunawayanddamage.

PowerMOSFETMotorControl

BecauseoftheextremelyhighinputorGateresistancethattheMOSFEThas,itsveryfastswitchingspeedsandtheeaseatwhichtheycanbedrivenmakesthemidealtointerfacewithop-ampsorstandardlogicgates.However,caremustbetakentoensurethatthegate-sourceinputvoltageiscorrectlychosenbecausewhenusingtheMOSFETasaswitchthedevicemustobtainalowRDS(on)channelresistanceinproportiontothisinputgatevoltage.

Forexample,donotapplya12Vsignalifa5Vsignalvoltageisrequired.PowerMOSFET’scanbeusedtocontrolthemovementofDCmotorsorbrushlesssteppermotorsdirectlyfromcomputerlogicorPulse-widthModulation(PWM)typecontrollers.AsaDCmotoroffershighstartingtorqueandwhichisalsoproportionaltothearmaturecurrent,MOSFETswitchesalongwithaPWMcanbeusedasaverygoodspeedcontrollerthatwouldprovidesmoothandquietmotoroperation.

SimplePowerMOSFETMotorController

AsimplepowerMOSFETmotorcontrollerisshowninFig1.11.Asthemotorloadisinductive,asimple“Freewheeling”diodeisconnectedacrosstheloadtodissipateanybackemfgeneratedbythemotorwhentheMOSFETturnsit“OFF”.

TheZenerdiodeisusedtopreventexcessivegate-sourceinputvoltages.

Fig1.11SimplePowerMOSFETMotorController

1.2.4FrequencyDividers

Ifweapplyafixed-frequencypulsetraintoacounter,ratherthanindividualpulsescomingatrandomintervals,webegintonoticesomeinterestingcharacteristics,andsomeusefulrelationshipsbetweentheinputclocksignalandtheoutputsignals.

Considerasingleflip-flopwithacontinuoussuccessionofclockpulsesatafixedfrequency,suchastheoneshowninFig1.12.WenotethreeusefulfactsabouttheoutputsignalsseenatQand:

1.Theyareexactlyinvertedfromeachother.

2.Theyareperfectsquarewaves(50%dutycycle).

3.Theyhaveafrequencyjusthalfthatoftheclockpulsetrain.

Fig1.12Clockpulsesatafixedfrequencyofasingleflip-flop

Thedutycycleofanyrectangularwaveformreferstothepercentageofthefullcyclethatthesignalremainsatlogic1.Ifthesignalspendshalfitstimeatlogic1andtheotherhalfatlogic0,wehaveawaveformwitha50%dutycycle.Thisdescribesaperfect,symmetricalsquarewave.

Frequencydivisionbyanoddnumberisalsopossible.ThecircuitshowedinFig1.13isademonstrationofadivide-by-3counter.NogatesarerequiredtocontrolthesequenceifJKflip-flopsareused;feedingtheoutputsignalsbacktotheappropriateinputsissufficient.

Ofcourse,itisnotpossibletogetasymmetrical(50%dutycycle)squarewavewiththiscircuit.TheAoutputisatlogic1fortwoclockpulsesoutofthree;theBoutputisatlogic1foroneclockpulseoutofthree.Thus,dutycyclesof1/3(33.333%)and2/3(66.667%)areavailable.

Fig1.13Adivide-by-3counter

Thisrenditionofadivide-by-5counter(Fig1.14)actuallyfollowsthenormaldecimal(orbinary)countfromzerothroughfour.TheprimarycontrolfeatureisthefeedbackfromtheC’outputtoflip-flopA’sJinput.Thisfeedbackpreventsflip-flopAfromswitchingfromlogic0tologic1inanefforttogofromacountoffourtoacountoffive.Atthesametime,theCoutputisappliedtoflip-flopC’sKinputtoforceflip-flopCtoresetonthenextclockpulse.

Thisparticulararrangementisoftencombinedwithasingleflip-flopinanICpackage.Thecombinationcanthenbeusedeitherasanormaldecimalcounterorasadivide-by-10counterwithatruesquare-waveoutput.

Fig1.14Adivide-by-5counter

Ifitisnotnecessarytomaintainastandardbinarycountingsequence,wecanofteninterconnecttheflip-flopssoastoeliminatetheneedforanyextragates,asshowntotheleft.NotethattheKinputstobothflip-flopsAandBareconnectedtologic1.Asaresult,outputsAandBwillremainatlogic1foronlyoneclockpulseatatime,andwillthenresettologic0.OutputCwilltoggleafterBgoestologic1.

OutputChasa40%dutycycle.OutputsAandBproducetwooutputpulsesforeachpulsefromC,butnotatequalintervals.Thecountingsequenceis0,1,2,5,6,0,etc.

Thiscountercircuitactuallyhasaflawasshown:ifitpowersupinstate4(A=0,B=0,

C=1),itwillremaininthatstateandbeunabletochangeatall.Tocorrectthis,wecandisconnectC’sKinputfromoutputB,andconnectittooutputA’instead.Nowthefirstclockpulsewillforcethecircuittostate0(000),fromwhichthecountwillproceednormally.Thischangewillnotaffectthenormalcountingsequence,becausealogic1attheKinputcannotpreventtheflip-flopfromchangingtoalogic1,andwouldforceCbacktoalogic0atthesametimeitwouldchangeanyway.

Othercountingsequencesarealsopossible,ofcourse.Ifaneedexiststohavetwoormoresignalsinaparticularfrequencyrelationshipwitheachother,someextensionorvariationonthecircuitsshownherecanbedesignedtosupplytheneed.

Unit2TheBasicsofComputerSystem2.1Text2.2ReadingMaterials

2.1Text

TheVonNeumannArchitectureofComputerSystemAllcomputerssharethesamebasicarchitecture,whetheritisamainframeoraPalmPilot.Allhavememory,anI/Osystem,andarithmetic/logicunit,andacontrolunit.ThistypeofarchitectureisnamedVonNeumann’sArchitectureafterthemathematician,JohnVonNeumannwhoconceivedofthedesign.

JohnVonNeumannbeginshis“PreliminaryDiscussion”withabroaddescriptionofthegeneral-purposecomputingmachinecontainingfourbasiccomponents.Theseareknownasrelatingtoarithmetic,memory,control,andconnectionwiththehumanoperator.Inotherwords,thearithmetic/logicalunit(ALU),thememory,thecontrolunit(CU),andtheinput-output(I/O)devicesthatweseeintheclassicalmodelofwhatacomputer“l(fā)ookslike”.VonNeumannArchitectureofComputerSystemisshowninFig2.1.Fig2.1VonNeumannArchitectureofComputerSystem

Memory

ComputerMemoryisthesubsystem