UJT Triggering Circuit

UJT Triggering Circuit

It includes UJT triggering, SCR (Silicon Controlled Rectifer), Load resistor.

A unijunction transistor (UJT) is an electronic electronic semiconductor device that has only one junction. The UJT has three terminals: terminals: an emitter (E) and two bases (B1 and B). The base is !ormed by li"htly do#ed n$ty#e bar o! silicon. Two ohmic contacts B1 and B are attached at its ends. The emitter is o! #$ty#e and it is heavily do#ed. The resistance between B1 and B% when the emitter is o#en$circuit is called interbase resistance.

Working o Capacitor

&nitially the ca#acitor char"es throu"h ' whose volta"e is a##lied to the emitter o! UJT. hen the ca#acitor volta"e reaches #ea #oint volta"e o! UJT. the UJT will switch to on condition. *ow the ca#acitor dischar"es throu"h throu"h the out#ut resistance. Thus the #ulse is "enerated in the circuit.

La !"peri#ent Aim: The UJT circuit is a relaxation oscillator which creates a train of pulses depending on the RC timing

circuit. These pulses are generated across R0 and are connected to the gate of the SCR. this in turn switch on the SCR appropriately. This This can be viewed by see in the voltage drop across the load.

 To  To +tudy and veri!y UJT Tri""erin" Tri""erin" circuit o! +,'. +,'.

$pparatus re%uired

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UJT Tri""erin" circuit it -atch cards

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-ower card

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/0 Bulb

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TheSi l i c onCont r ol l edRec t i fi er( SCR)

Shoc kl eydi odesar ec ur i ousdev i c es ,butr at herl i mi t edi nappl i c at i on.Thei rus ef ul nes smay beex panded,ho we v er ,b yequi ppi ngt hem wi t hano t hermeansofl at c hi ng.I ndo i ngso,eac h bec omest r ueampl i f y i ngde vi c es( i fonl yi nanon/ offmode) ,andwer ef ert ot hes e ass . i l i c onc ont r ol l edr ec t i fi er s,orSCRs Thepr ogr es s i onf r om Shoc k l e ydi odet oSCRi sac hi e v edwi t hones mal l add i t i on,ac t ua l l y not hi ngmor et hanat hi r dwi r ec onnec t i ont ot hee xi s t i ngPNPNs t r uc t ur e:( Fi gur ebel ow)

TheSi l i c onCont r ol l edRec t i fi er( SCR)

I fanSCR' sgat ei sl ef tfl ( di s connec t ed) ,i tbehav esex ac t l yasaShoc kl eydi ode.I t oat i ng  ma ybel at c hedbybr eak ov erv ol t ageorbyex c eedi ngt hec r i t i c al r at eofv ol t ager i s e b et we ena no dea ndc a t h od e,j u s ta swi t ht h eSho c k l e ydi o de .Dr o po uti sac c omp l i s he db y r educ i ngc ur r entunt i l oneorbot hi nt er nal t r ans i s t or sf al l i nt oc ut offmode,al s ol i k et he Shoc k l e ydi ode.Ho wev er ,bec aus et hegat et er mi nal c onnec t sdi r ec t l yt ot hebas eoft he l owert r ans i s t or ,i tma ybeus edasanal t er nat i v emeanst ol at c ht heSCR.Byappl y i nga s mal l v ol t agebet weengat eandc at hode,t hel owert r ans i s t orwi l l bef or c edon  b yt he r es ul t i ngbas ec ur r ent ,whi c hwi l l c aus et heuppert r ans i s t ort oc onduc t ,whi c ht hens uppl i es t hel owert r ans i s t or ' sbas ewi t hc ur r ents ot hati tnol ongerneedst obeac t i v at edbyagat e v ol t age.Thenec es sar ygat ec ur r entt oi ni t i at el at c hup,ofc our s e,wi l l bemuc hl owert han t h ec u r r e ntt h r o ug ht h eSCRf r o mc a t h od et oan od e,s ot h eSCRd oe sa c hi e v eame as u r eof ampl i fi cat i on.

Thi sme t h odo fs e cu r i n gSCRc on du ct i o ni sc al l edt ti sbyf art hemos t r i gger i ng,andi c ommo nwa yt h atSCRsar el a t c h edi na ct u al p r a ct i c e.I nf a ct ,SCRsar eu s ua l l yc ho s ens o t hatt hei rbr eak ov erv ol t agei sf arbey ondt hegr eat es tv ol t ageex pec t edt obee xper i enc ed f r om t hepo wers our c e,s ot hati tc anbet ur nedononl byani nt ent i onal v ol t agepul s e y  appl i edt ot hegat e. I tshoul dbement i onedt hatSCRsmaysomet bet ur nedoffbydi r ec t l ys hor t i ngt hei r i mes  gat eandc at hodet er mi nal st oget her ,orby" r ev er s et r i gger i ng"t hegat ewi t hanegat i v e v ol t age( i nr ef er enc et ot hec at hode) ,s ot hatt hel owert r ans i s t ori sf or c edi nt oc ut off.Is ay t hi si s" s omet i mes "pos si bl ebec aus ei ti nv ol v esshunt i ngal l oft heuppert r ans i s t or ' s c ol l ec t orc ur r entpas tt hel owert r ans i s t or ' sbas e.Thi scur r entmaybes ubs t ant i al ,mak i ng t r i gger eds hut offofanSCRdi ffic ul tatbes t .Av ar i at i onoft heSCR,c al l edaGat eT ur nt hy r i s t or ,o rGTO,mak est hi st as keas i er .Butev enwi t haGTO,t hegat ec ur r entr equi r ed Off  t ot u r ni to ffma ybeasmu c ha s2 0% o ft h ea no de( l o ad )c u r r e nt !Th es c he ma t i cs y mb ol f o r aGTO i sshowni nt hef ol l owi ngi l l us t r at i on:( Fi gur ebel o w)

TheGa t eT ur n Offt h y r i s t o r( GTO)

SCRsa ndGTOss h ar et hes ameeq ui v a l e nts c he ma t i c s( t wot r a ns i s t o r sc o nn ec t e di na pos i t i v ef eedbac kf as hi on) ,t heonl ydi ffer enc esbei ngdet ai l sofc ons t r uc t i ondes i gnedt o gr antt heNPNt r ans i s t oragr eat erβt hant hePNP .Thi sal l owsas mal l ergat ec ur r ent ( f or war dorr ev er s e)t oex er tagr eat erdegr eeofc ont r ol ov erc onduc t i onf r om c at hodet o anode,wi t ht hePNPt r ans i s t or ' sl at c heds t at ebe i ngmor edepend entupont heNPN' st han v i c ev er s a.TheGat eT ur nOfft h yr i s t ori sal s ok no wnb yt henameofGat eCont r ol l ed Swi t c h,orGCS.

Ar udi ment ar yt es tofSCRf unc t i on,oratl eas tt er mi nal i dent i fi c at i on,ma ybeper f or med wi t hanohmme t er .Bec au set hei nt er na lc onne ct i onbe t weengat eandc at hodei sas i ngl e PNj unc t i on,amet ers houl di ndi c at ec ont i nui t ybet weent hes et er mi nal swi t ht her edt es t l eadont hegat eandt hebl ac kt es tl eadont hec at hodel i k et hi s :( Fi gur ebel ow)

Rud i men t a r yt e s tofSCR

Al l o t h erc o nt i n ui t yme as u r e me nt spe r f o r me do na nSCRwi l l s h o w" o pe n"( " OL "o ns o me di gi t al mul t i met erdi s pl ay s) .I tmus tbeunder s t oodt hatt hi st es ti sver ycr udeand doesnot   c ons t i t ut eac ompr ehens i v eas s es s mentoft heSCR.I ti spos s i b l ef oranSCRt o gi v egoodohmmet eri ndi c at i onsands t i l l bedef ec t i v e.Ul t i mat el y ,t heonl ywayt ot es tan SCRi st os ubj ec ti tt oal oadc ur r ent . I fy ouar eus i ngamul t i met erwi t ha" di odec hec k"f unc t i on,t hegat et oc at hodej unc t i on v ol t agei ndi c at i ony ouge tma yorma yno tc or r es pondt owhat ' sex pec t edofas i l i c onPN  j unc t i on( appr ox i mat el y0. 7v ol t s ) .I ns omec as es ,y ouwi l l r eadamuc hl owerj unc t i on v ol t age:mer ehundr edt hsofav ol t .Thi si sduet oani nt er nal r es i s t orc onnec t edbet weent he g at ean dc at h od ei n co r p or a t edwi t h i ns omeSCRs .Th i sr e si s t o ri sa dd edt oma k et h eSCR l es ss us cept i bl et of al s et r i gger i ngbys pur i ousv ol t ages pi k es ,f r om c i r c ui t" noi s e"orf r om s t at i cel ec t r i cdi s char ge.I not herwor ds ,hav i ngar es i s t orc onnec t edac r os st hegat ec at hodej unc t i onr equi r est hatas t r i gger i ngs i gnal ( s ubs t ant i al c ur r ent )beappl i edt o t r ong  l at c ht heSCR.Thi sf eat ur ei sof t enf oundi nl ar gerSCRs ,no tons mal l SCRs .Beari nmi nd t hatanSCRwi t hani nt er nal r es i s t orc onnec t edbe t weengat eandc at hodewi l l i ndi c at e c ont i nui t yi bet weent hos et wot er mi nal s :( Fi gur ebel o w) nbot hdi r ec t i ons 

Lar g erSCRsha v eg at et oc at hoder es i s t or .

" Nor mal "SCRs ,l ac ki ngt hi si nt er nal r es i s t or ,ar es omet i mesr ef er r edt oass ens i t i v e SCRsduet ot hei rabi l i t yt obet r i gger edbyt hes l i ght es tpos i t i v egat es i gnal .  gat e  Thet es tc i r c ui tf oranSCRi sbot hpr ac t i c al asadi agnos t i ct ool f orc hec ki ngs us pec t ed SCRsa ndal s oanex c e l l e nta i dt ou nd er s t a ndi n gba s i cSCRo pe r a t i o n.ADCv o l t a ge s our c ei sus edf orpower i ngt hec i r c ui t ,andt wopus hbut t ons wi t c hesar eus edt ol at c hand unl at c ht heSCR,r es pec t i v el y :( Fi gur ebel ow)

SCRt es t i ngc i r c ui t

Ac t uat i ngt henor mal l y open" o n"pus hbut t ons wi t c hc onnec t st hegat et ot heanode, al l owi ngc ur r entf r om t henegat i v et er mi nal oft hebat t er y ,t hr ought hec at hodegat ePN  j unc t i on,t hr ought hes wi t c h,t hr ought hel oadr es i s t or ,andbac kt ot hebat t er y .Thi sgat e c ur r ents houl df or c et heSCRt ol at c hon,al l owi ngc ur r entt ogodi r ec t l yf r om c at hodet o anodewi t houtf ur t hert r i gger i ngt hr ought hegat e.Whent he" on"pus hbut t oni sr el eas ed,t he l o ads h ou l dr e ma i nen er g i z ed . Pus hi ngt henor mal l y c l os ed" off"pus hbut t ons wi t c hbr eak st hec i r c ui t ,f or c i ngc ur r ent t hr ought heSCRt ohal t ,t husf or c i ngi tt ot ur noff( l owc ur r entdr opout ) .

I ft heSCRf ai l st ol at c h,t hepr obl em ma ybewi t ht hel oadandno tt heSCR.Ac er t ai n mi ni mum amountofl oadcur r enti sr equi r edt ohol dt heSCRl at c hedi nt he" o n"s t at e.Thi s mi ni mum c ur r entl ev el i sc al l edt hehol .Al oadwi t ht oogr eatar es i s t anc ev al ue di ngcur r ent ma yno td r a we no ug hc u r r e ntt ok ee pa nSCRl a t c h edwh enga t ec ur r e ntc e as e s ,t h us gi v i ngt hef al s ei mpr es si onofabad( unl at c habl e)SCRi nt het es tc i r c ui t .Hol di ngc ur r ent v al uesf ordi ff er entSCRsshoul dbea v ai l abl ef r om t hemanuf ac t ur er s .T y pi c al hol di ng c ur r entv al uesr angef r om 1mi l l i ampt o50mi l l i ampsormor ef orl ar geruni t s . Fort het es tt obef ul l ycompr ehens i v e,mor et hant het r i gger i ngac t i onneedst obet es t ed. Thef or war dbr eak o v erv ol t agel i mi toft heSCRc ou l dbet es t edb yi nc r eas i ngt heDC v ol t ages uppl y( wi t hnopus hbut t onsac t uat ed)unt i l t heSCRl at c hesal l oni t sown.Bewar e t h atabr ea k ov e rt e s tma yr eq ui r ev e r yhi g hv o l t a ge :ma nypo we rSCRsha v eb r ea k ov e r v ol t ager at i ngsof600v ol t sormor e!Al s o,i fapul s ev ol t agegener at ori sav ai l abl e,t he c r i t i c al r at eofv ol t ager i s ef ort heSCRc oul dbet es t edi nt hes ameway :s ubj ec ti tt opul s i ng s uppl yv ol t agesofdi ff er entV/ t i mer at eswi t hnopus hbut t ons wi t c hesac t uat edands ee wheni tl at c hes . I nt hi ssi mpl ef or m,t heSCRt es tc i r c ui tc oul ds uffic easas t ar t / s t opc ont r ol c i r c ui tf oraDC mot or ,l amp,orot herpr ac t i c al l oad:( Fi gur ebel ow)

DCmot ors t ar t / s t opc ont r ol c i r c ui t

Ano t herpr ac t i c al us ef ort heSCRi naDCc i r c ui ti sasacr   dev i c ef orov er v ol t age owbar pr ot ec t i on.A" c r owbar "c i r c ui tc ons i s t sofanSCRpl ac edi npar al l el wi t ht heout putofaDC powers uppl y ,f orpl ac i ngadi r ec ts hor t c i r c ui tont heout putoft hats uppl yt opr ev ent e x c es s i v ev ol t a gef r o mr e ac hi n gt h el o ad .Da ma get ot h eSCRa ndpo wers up pl yi s pr ev ent edb yt hej udi c i ouspl ac ementofaf us eors ubs t ant i al s er i esr es i s t anc eaheadoft he SCRt ol i mi ts hor t c i r c ui tc ur r ent :( Fi gur ebel ow)

Cr o wba rc i r c u i tu se di nDCp owe rs u pp l y

Somede vi c eorc i r c ui ts ens i ngt heout putv ol t agewi l l bec onnec t edt ot hegat eoft heSCR, s ot hatwhenanov er v ol t agec ondi t i onoc c ur s ,v ol t agewi l l beappl i edbet weent hegat eand c at hode,t r i gger i ngt heSCRandf or c i ngt hef us et obl ow.Theeff ec twi l l beappr o xi mat el y t hes ameasdr oppi ngas ol i ds t eel c r owbardi r ec t l yac r os st heout putt er mi nal soft hepower s uppl y , henc et henameoft hec i r c ui t . Mos tapp l i c at i onsoft heSCRar ef orACpo werc ont r ol ,des pi t et hef ac tt hatSCRsar e i nher ent l yDC( uni di r ec t i onal )dev i c es .I fbi di r ec t i onal c i r c ui tc ur r enti sr equi r ed,mul t i pl e SCRsma ybeu s ed ,wi t ho neo rmo r ef a c i n ge ac hdi r e ct i ont oh an dl ec u r r e ntt h r o ug hb ot h hal f c y c l e soft heACwa v e.Thepr i mar yr eas onSCRsar eus edatal l f orACpo werc ont r ol appl i c at i onsi st heuni quer es pons eofat hy r i s t ort oanal t er nat i ngc ur r ent .Aswes aw,t he t h yr at r ont ube( t heel ec t r ont ubev er s i onoft heSCR)andt heDI AC,ah ys t er e t i cde vi c e t r i gger edondur i ngapor t i onofanAChal f c y cl ewi l l l at c handr emai nont hr oughoutt he r emai nderoft hehal f c y cl eunt i l t heACc ur r entdec r eas est oz er o,asi tmus tt obegi nt he nex thal f c yc l e.J us tpr i ort ot hez er oc r os sov erpoi ntoft hec ur r entwav ef or m,t het hy r i s t or wi l l t ur noffduet oi ns uffic i entc ur r ent( t hi sbehav i ori sal s ok nownasnat ur al c ommut at i on) andmus tbefi r edagai ndur i ngt henex tc y cl e.Ther es ul ti sac i r c ui tc ur r entequi v al entt oa " c h op pedu p"s i n ewa v e.Forr e v i e w,h er ei st h eg r a pho faDI AC' sr e sp on s et oanAC v o l t a gewh os ep eakex c e ed st h eb r e ak o v e rv o l t a geo ft h eDI AC:( Fi g ur ebel o w)

DI ACbi di r ec t i onal r es pons e

Wi t ht heDI AC,t hatbr eak o v erv ol t agel i mi twasafi x edquant i t y . Wi t ht heSCR,weha v e c ont r ol o vere x ac t l ywhent hede vi c ebec omesl at c hedbyt r i gger i ngt hegat eatan ypoi nti n t i meal ongt hewa v ef or m.Byc onnec t i ngas ui t abl ec ont r ol c i r c ui tt ot hegat eofanSCR,we c an" c hop"t hes i newa v eatan ypoi ntt oal l owf ort i mepr opor t i onedpo werc ont r ol t oal oad. T ak et hec i r c ui ti nFi gur ebel o w  asane xampl e.Her e,anSCRi spos i t i onedi nac i r c ui tt o c on t r o lp owe rt oal o adf r om anACs ou r c e.

SCRcont r olofACpower

Bei ngauni di r ec t i onal ( onewa y)de vi c e,atmos twec anonl ydel i v erhal f wa v epo wert ot he l oad,i nt hehal f c y cl eofACwher et hes uppl yv ol t agepol ar i t yi spos i t i v eont het opand negat i v eont hebot t om.Howev er ,f ordemons t r at i ngt hebas i cconc eptoft i mepr opor t i onal c ont r ol ,t hi ss i mpl ec i r c ui ti sbet t ert hanonec ont r ol l i ngf ul l wav epower( whi c hwoul dr equi r e t woSCRs ) . Wi t hnot r i gger i ngt ot heg at e,andt heACs our c ev o l t agewel l bel o wt heSCR' sbr eak o v er v ol t ager at i ng,t heSCRwi l l ne v ert ur non.Connec t i ngt heSCRg at et ot heanodet hr ougha s t andar dr ec t i f y i ngdi ode( t opr ev entr ev er s ec ur r entt hr ought hegat ei nt heev entoft he SCRc ont ai ni ngabui l t i ngat ec at hoder es i s t or ) ,wi l l al l owt heSCRt obet r i gger edal mos t i mmedi at el yatt hebegi nni ngofev er ypos i t i v ehal f c yc l e:( Fi gur ebel ow)

Gat ec onnec t eddi r ec t l yt oanodet hr oughadi ode;near l yc ompl et ehal f wa v ec ur r ent t hr oughl oad.

Wec andel a yt het r i gger i ngoft heSCR,howev er ,b yi ns er t i ngsomer es i s t anc ei nt ot hegat e c i r c ui t ,t husi nc r eas i ngt heamountofv ol t agedr opr equi r edbef or eenoughgat ec ur r ent t r i gger st heSCR.I not herwor ds ,i fwemak ei thar derf orel ec t r onst ofl owt hr ought hegat e byaddi ngar es i s t anc e,t heACv ol t agewi l l hav et or eac hahi gherpoi nti ni t scy c l ebef or e t her ewi l l beenoughgat ec ur r entt ot ur nt heSCRon.Ther es ul ti si nFi gur ebel ow.

Res i s t anc ei ns er t edi ngat ec i r c ui t ;l es st hanhal f wav ec ur r entt hr oughl oad.

Wi t ht heha l f s i newa v ec hopp edupt oagr eat erdegr eeb yde l a y edt r i gger i ngoft heSCR, t hel oadr ec ei v esl es sav er agepower( poweri sdel i v er edf orl es st i met hr oughoutac y cl e) . Bymak i ngt hes er i esgat er es i s t orv ar i abl e,wec anmak ea dj us t ment st ot het i mepr opor t i onedpower :( Fi gur ebel ow)

I nc r eas i ngt her es i s t anc er ai s est het hr es hol dl ev el ,c aus i ngl es spowert obedel i v er edt o t hel oad.Dec r eas i ngt her es i s t anc el o wer st het hr es hol dl e v el ,c aus i ngmor epowert obe del i v er edt ot hel oad.

Unf or t unat el y ,t hi scont r ol s chemehasas i gni fi cantl i mi t at i on.I nus i ngt heACs our c e wav ef or mf orourSCRt r i gger i ngs i gnal ,wel i mi tc ont r ol t ot hefi r s thal foft hewav ef or m' s hal f c y cl e.I not herwor ds ,i ti snotpos s i bl ef orust owai tunt i laf   t hewa ve' speakt ot r i gger t er t h eSCR.Th i sme an swec ant u r ndo wnt h ep owe ro nl yt ot h ep oi n twh er et h eSCRt u r n son att hev er ypeakoft hewa v e:( Fi gur ebel ow)

Ci r c ui tatmi ni mum po wers et t i ng

Rai s i ngt het r i ggert hr es hol danymor ewi l l c aus et hec i r c ui tt onott r i ggeratal l ,s i nc enot e v ent hepeakoft heACpo werv ol t agewi l l been ought ot r i ggert heSCR.Ther es ul twi l l be n op owe rt ot h el o ad .

Ani ngeni ouss ol ut i ont ot hi sc ont r oldi l emmai sf oundi nt headdi t i onofaphas es hi f t i ng c apac i t ort ot hec i r c ui t :( Fi gur ebel ow)

 Addi t i onofaphas es hi f t i ngc apac i t ort ot hec i r c ui t

Thesma l l erwa v ef or ms ho wnont hegr aphi sv ol t ageac r os st hec apa ci t or .Fort hes ak eof i l l us t r at i ngt hephas es hi f t ,I ' m as sumi ngacondi t i onofmax i mum c ont r ol r es i s t anc ewher e t heSCRi snott r i gger i ngatal l wi t hnol oadc ur r ent ,s av ef orwhatl i t t l ec ur r entgoest hr ough t hec ont r ol r es i s t orandc apac i t or .Thi sc apac i t orv ol t agewi l l bephas es hi f t edany wher e f r o m 0o  t o90o  l a gg i n gb eh i n dt h ep owe rs o ur c eACwa v ef or m.Wh ent h i sp has e s h i f t e d v ol t ager eac hesahi ghenoughl e vel ,t heSCRwi l l t r i gger . Wi t henoughv ol t ageac r os st hec apac i t ort oper i odi c al l yt r i ggert heSCR,t her es ul t i ngl oad c ur r entwav ef or m wi l l l ooksomet hi ngl i k eFi gur ebel ow)

Phas es hi f t eds i gnal t r i gger sSCRi nt oc onduc t i on.

Bec aus et hec apac i t orwav ef or mi ss t i l lr af t ert hemai nACpowerwavef or m has i s i ng  r eac hedi t speak ,i tbec omespos s i bl et ot r i ggert heSCRatat hr es hol dl ev el be y ondt hat peak ,t husc hoppi ngt hel oadc ur r entwa v ef ur t hert hani twaspos s i bl ewi t ht hes i mpl er c i r c ui t .I nr eal i t y ,t hec apac i t orv ol t agewav ef or mi sabi tmor ec ompl e xt hatwhati ss hown her e,i t ss i nus oi dal s hapedi s t or t ede ver yt i met heSCRl at c heson.Howe ver ,whatI ' mt r y i ng t oi l l us t r at eher ei st hedel ay edt r i gger i ngac t i ongai nedwi t ht hephas es hi f t i ngRCnet wor k ; t hus ,as i mpl i fi ed,undi s t or t edwa vef or ms er v est hepur pos ewel l . SCRsmayal s obet r i gger ed,or" fi r ed, "bymor ec ompl exc i r c ui t s .Whi l et hec i r c ui tpr ev i ous l y s howni ssuffic i entf oras i mpl eappl i c at i onl i k eal ampc ont r ol ,l ar gei ndus t r i al mot orc ont r ol s of t enr el yonmor es ophi s t i c at edt r i gger i ngmet hods .Somet i mes ,pul s et r ans f or mer sar e us edt oc oupl eat r i gger i ngc i r c ui tt ot hegat eandc at hodeofanSCRt opr ov i deel ec t r i c al i s ol at i onbet weent het r i gger i ngandpowerc i r c ui t s :( Fi gur ebel o w)

T r ans f or merc oupl i ngoft r i ggers i gnal pr ov i desi s ol at i on.

Whenmul t i pl eSCRsar eu sedt oc ont r ol po wer ,t hei rc at hode sar eof t ennot   el ec t r i c al l y c ommon,mak i ngi tdi ffic ul tt oc onnec tas i ngl et r i gger i ngc i r c ui tt oal l SCRsequal l y .An ex ampl eoft hi si st hec   s h owni nFi g ur ebel ow. ont r ol l edbr i dger ec t i fi er

Cont r ol l edbr i dger ec t i fi er

I nanybr i dger ec t i fi erc i r c ui t ,t her ec t i f y i ngdi odes( i nt hi sex ampl e,t her ec t i f y i ngSCRs ) mus tc onduc ti noppos i t epa i r s .SCR1  an dSCR3  mu s tb efi r e ds i mu l t a ne ou s l y ,a ndSCR2 and SCR4  mus tbefi r edt oge t herasapai r .Asy ouwi l l no t i c e,t hough,t hes epa i r sofSCRsdono t s har et hes amec at hodec onnec t i ons ,meani ngt hati twoul dnotwor kt os i mpl ypar al l el t hei r r es pec t i v egat ec onnec t i onsandc onnec tas i ngl ev ol t agesour c et ot r i ggerbot h: ( Fi gur ebel ow)

Thi ss t r at egywi l l notwor kf ort r i gger i ngSCR an dSCR4  asapai r .  2 

Al t hought het r i gger i ngvol t ages our c es hownwi l l t r i ggerSCR4,i twi l l nott r i gger SCR2  pr o pe r l ybe ca us et h et wot h y r i s t o r sdon ots h ar eac o mmo nc at ho dec on ne c t i o nt o r ef er enc et hatt r i gger i ngv ol t age.Pul s et r ans f or mer sconnec t i ngt het wot hy r i s t orgat est oa c ommo nt r i g ge r i n gv o l t a ges ou r c ewi   wor k ,howev er :( Fi gur ebel ow) l l

T r ans f or merc oupl i ngoft hegat esal l owst r i gger i ngofSCR 2  an dSCR4 .

Beari nmi ndt hatt hi sci r c ui tonl yshowst hegat ec onnec t i onsf ort wooutoft hef ourSCRs . Pul s et r ans f or mer sandt r i gger i ngs our c esf orSCR1an dSCR3,aswel l ast hedet ai l soft he pul s es our c est hems el v es ,ha v ebeenomi t t edf ort hes ak eofs i mpl i c i t y .

Cont r ol l edbr i dger ec t i fi er sar enotl i mi t edt os i ngl ephas edes i gns .I nmos ti ndus t r i al c ont r ol s y s t ems ,ACpo weri sav ai l abl ei nt hr eepha sef or mf orma xi mu m effic i enc y ,ands ol i ds t at e c ont r ol c i r c ui t sar ebui l tt ot ak eadv ant ageoft hat .At hr eephas ec ont r ol l edr ec t i fi erc i r c ui t bui l twi t hSCRs ,wi t houtpul s et r ans f or mer sort r i gger i ngc i r c ui t r yshown,woul dl ookl i k e Fi gur ebel o w.

Thr e ep ha s eb r i d geSCRc on t r o lo fl o ad •

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  REVI EW:

ASi ,orSCR,i ses sent i al l yaShoc kl eydi odewi t hanex t r a l i c onCont r ol l edRec t i fi er t er mi nal added.Thi sex t r at er mi nal i scal l edt h e at ti sus edt ot   t he  g e,andi r i gger dev i c ei nt oc onduc t i on( l at c hi t )byt heappl i c at i onofas mal l v ol t age.

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Tot r i gger ,orfi nSCR,v o l t a gemu s tb ea pp l i e db et we ent h eg at ean dc at h od e, r e,a p os i t i v et ot h eg at ea ndn eg at i v et ot h ec at h od e.Wh ent e s t i n ga nSCR,amo me nt a r y c onnec t i onbet weent hegat eandanodei ss uffic i enti npol ar i t y ,i nt ens i t y ,and dur at i ont ot r i ggeri t .

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SCRsma ybefi r edbyi nt ent i onal t r i gger i ngoft hegat et er mi nal ,e x ces s i v ev ol t age ( b r e ak do wn )b e t we ena no dean dc at ho de ,o re x c es s i v er a t eo fv o l t ag er i s eb et we en a no dean dc a t h od e.SCRsma ybet u r n edoffb yan od ec u r r e ntf a l l i n gb el o w t h ehol ( l owc ur r entdr opout ) ,orby" r ev er s efi r i ng"t hegat e di ngc ur r entv al ue  ( appl y i nganegat i v ev ol t aget ot hegat e) .Rev er s efi r i ngi sonl ys omet i meseffec t i v e, andal wa ysi n vol v eshi ghgat ec ur r ent .

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Av ar i antoft heSCR,c al l edaGat eT ur nOfft h yr i s t or( GTO) ,i ss pec i fi c al l ydes i gned t obet ur nedoffb ymeansofr e ver s et r i gger i ng.Ev ent hen,r ev er s et r i gger i ng r equi r esf ai r l yhi ghc ur r ent :t y pi c al l y20% oft heanodec ur r ent .

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SCRt er mi nal sma ybei dent i fi edb yacont i nui t ymet er :t heonl yt wot er mi nal s s ho wi ngan yc ont i n ui t ybe t weent hem atal l s hou l dbet hegat eandc at hode.Gat e andc at hodet er mi nal sc on nec tt oaPNj unc t i oni n si det heSCR,s oac ont i nui t y me t ers houl dobt ai nadi odel i k er eadi ngbe t weent hes et wot er mi nal swi t ht her ed ( +)l eadont hegat eandt hebl ac k( )l eadont hec at hode.Bewar e,t hough,t hat s omel a r g eSCRsh av eani n t er na lr e si s t o rc on ne ct e dbe t wee ng at ean dc a t h od e, whi c hwi l l aff ec tan yc ont i nui t yr eadi ngst ak enb yamet er .

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SCRsar et r uer heyonl yal l owc ur r entt hr ought hem i nonedi r ec t i on.Thi s ec t i fi er s:t me an st h eyc an no tbeu s eda l o nef o rf ul l wa v eACp owe rc on t r o l .

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I ft hedi odesi nar ec t i fi erc i r c ui tar er epl ac edb ySCRs ,y ouha vet hemak i ngsof ac r ec t i fi erc i r c ui t ,wher ebyDCpowert oal oadma ybet i mepr opor t i oned ont r ol l ed  b yt r i g ge r i n gt h eSCRsatd i ff er e ntp oi n t sal o ngt h eACp owe rwa v ef o r m.

Circuit &iagra#

'rocedure

CR Settings

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 Time () A2is : 3ms4div 5olta"e (6) A2is : 054div -robe : 1:10 ,' -robe is suitable one.

!"peri#ent Steps • •

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 The connections are made as #er the circuit dia"ram "iven above.  The ' 7oad must be /0 bulb. hen the load is 'heostat% then it must be in ma2imum #osition.

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 Turn on the + and observe the wave!orm across between 8191

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 Turn * +1 and observe the wave!orm across the ' load.

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;easure the $a2is time interval o! * time o! 7oad 5olta"e and <<  Time o! 7oad 5olta"e.

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Adjust the value o!
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'e#eat the ste# /.

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'e#eat the e2#eriment !or di!!erent values o! = and note down 5o.

Taulation SCR S.  1 2 / 3

$lp+a( in &egree) * ti#e (#s) /

01

-o(-olts)

4odel 5rap+

Total Ti#e 'eriod

T+e $6erage utput -oltage