Scientech Technologies 2703 User manual

SCR TriggeringTechniques
Scientech2703
LearningMaterial
Ver1.1
AnISO9001:2008 company
ScientechTechnologiesPvt. Ltd.
94, ElectronicComplex, Pardesipura, Indore-452 010 India,
[phone], : [e-mail] , : www.ScientechWorld.com

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SCR TriggeringTechniques
Scientech2703
TableofContents
1. SafetyInstructions4
2. Introduction5
3. Features6
4. TechnicalSpecifications7
5. Theory8
6. Experiments
Experiment112
ToStudythetriggering ofSCR using UJT
Experiment214
ToStudythetriggering ofSCR using IC555
7. Datasheet17
8. Warranty23
9. ListofAccessories23

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SafetyInstructions
Readthefollowing safetyinstructionscarefullybeforeoperatingtheinstrument.To
avoidanypersonalinjuryordamagetotheinstrumentoranyproductconnectedtothe
instrument.
Donotoperatetheinstrumentifyoususpectanydamagewithin.
Theinstrumentshouldbeservicedbyqualifiedpersonnelonly.
Foryoursafety:
UseproperMainscord: Useonlythemainscorddesignedforthis
instrument.Ensurethatthemainscordis
suitableforyourcountry.
Ground theInstrument: Thisinstrumentisgroundedthroughthe
protectiveearthconductorofthemainscord.
Toavoidelectricshockthegrounding
conductormustbeconnectedtotheearth
ground.Beforemaking connectionstothe
inputterminals,ensurethattheinstrumentis
properlygrounded.
ObserveTerminalRatings: Toavoidfireorshockhazards,observeall
ratingsandmarksontheinstrument.
UseonlytheproperFuse: Usethefusetypeandratingspecifiedforthis
instrument.
UseinproperAtmosphere: Pleaserefertooperatingconditionsgivenin
themanual.
1. Donotoperateinwet/dampconditions.
2. Donotoperateinanexplosive
atmosphere.
3. Keeptheproductdustfree,cleanand
dry.

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Introduction
Scientech2703 isaplatformwhereStudentscanunderstandthevariousthyristor
firing techniquesbyusing IC555 andUJT. Thisisveryusefulforunderstanding of
pulsegeneration.2703 isprovidedwithinbuiltpowersupplies,socketsformaking
interconnectioninthecircuitandexhaustivelearningmaterial.

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Features
•InbuiltPowerSupply
•Easytooperateand understand
•Twotriggeringcircuitsonsingleboard
•Testpointstoobserveoutputpulses
•Socketstomakedifferentconnections
•OnboardDC sourcesof5Vand12 V

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TechnicalSpecifications
OnboardAC source: 18 V-0V-18 V
OnboardDC Supply: +5V, +12V
Onboardtriggeringcircuits:
555 ICtriggeringcircuit
UJTtriggering circuit
Interconnection: 2mm socket(Goldplated)
SCR : SCRsTYN616, 600V/16 A
Testpoints: 4nos(Goldplated)
Dimensions(mm): W420 xD255 xH100
PowerSupply: 110V-260VAC, 50/60Hz
Weight: 1Kg. (approximately)
OperatingConditions: 0-400C,80%RH
Learningmaterial: CD(Theory, procedure,referenceresults, etc),
Online(optional)

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Theory
TriggeringofSCR usingUJT
TheUJTis oftenusedasatriggerdevice forSCR’sandTRIAC’s. Themostcommon
UJT circuitinusetodayistherelaxationoscillator,whichisshowninfigure1.
Figure1
Thediode-resistance,resistance,resistance-capacitanceandthediode-resistance-
capacitance circuitproduce prolongedpulses, sopowerdissipationismoreatthegate.
Thepowerloss canbelimitedbytheuseofthisUJTinthefiring circuit.Pulse
triggeringispreferredasitoffersseveralmeritsoverRandRCtriggering.Gate
characteristicswidespread;pulses canbeadjustedeasilytosuitsuchawidespectrum
ofgatecharacteristics.Thepowerlevelinpulsetriggeringislowasthegatedriveis
discontinuous;pulsetriggering is thereforemoreefficient. Theabovefigure1iscalled
therelaxationoscillator.Theresistorandcapacitorconnectedtotheemitterforman
RC timing circuit. Normally, thevalueofcapacitorisfixedandthevalueofresistoris
ofpotentiometertype.Thecharging rateofthecapacitordependsonthevalueofthe
resistorandsince theresistorisvariabletheRCtimeconstantcanbecontrolled.
Whenthevoltageacross thecapacitorisequaltomorethanthepeakvoltageVPofthe
UJT, itstartsconducting.Since theUJThasanegativeresistance,itsvoltagestarts
decreasing up tothevalleyvoltage,andthecapacitordischargesup tothevalley
voltage.
Thisrepetitiveprocess producingatrainofpulsesatitsoutputisshowninfigure2.
Fromtheoutputvoltagewaveformitisclearthattheoutputpulseshasaverysmall
widthandthatalongrelaxationtimeexistsbetweenthetwopulses.Thereforeitis
saidthatthedeviceisrelaxedinthisdurationandiscalledtherelaxationoscillator.

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Figure2
Animportantdesignconsiderationinthistypeofcircuitconcernspremature
triggeringofSCR.Inthefiring circuitt1=R1C1aretimeconstantforcharging circuit
andt2=Ra3C1aredischarging timeconstant. Heret2 ismuchsmallerthant1. Resistance
R3shouldbesufficientlysmallsothatnormalleakagecurrentdropacross R3,when
UJT isoff,isnotabletotriggertheSCR. Inotherwords,
VBB.R3/RBB+R2+R3<SCR triggervoltage
WHERE,
RBB =RB1+RB2 (INTERNALRESISTANCE OFUJT BASES)
THEOUTPUTPULSEFROMUJTISCONNECTEDTOTHEGATE OFSCR.BY
USING APOTTHEGATE CURRENTCANBECONTROLLEDAND MONITOR
THEANODETOCATHODECURRENT. ITWILLSHOWATWHICHPOINT
THESCR ISGETTING TRIGGERED.

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TriggeringofSCR usingIC555:
TheAstableandMonostablecircuitsaresocommonlyrequiredthespecialmonolithic
ICcalledICtimers,havebeenmadeavailable.The555 IChasgainedwide
acceptance intermsofcostandversatility.SometypicalapplicationsareMonostable
andAstableMultivibrators,dc-dcconverters, digitallogicprobes,waveform
generators, Analogfrequencymetersandtachometers, temperaturemeasurementand
control,infraredtransmitters,burglarandtoxicgasalarms,voltageregulators,etc.
Thedevice 555 isamonolithictiming circuitthatcanproduce accurateandhighly
stabletimedelaysoroscillations.The555 ICisusedfortriggeringofSCR inbothdc
andac circuits.AnAstabletimeroperationisachievedbyaddingresistorRBtofigure
3andconfiguringasshowninfigure3.IntheAstableoperation,thetriggerterminal
andthethresholdterminalareconnectedsothataself-triggerisformed,operatingas
aMultivibrator.Whenthetimeroutputishigh,itsinternaldischargingTr.turnsoff
andtheVC1increasesbyexponentialfunctionwiththetimeconstant(RA+RB)*C.
WhentheVC1,orthethresholdvoltage,reaches2Vcc/3,thecomparatoroutputon
thetriggerterminalbecomeshigh,resettingtheF/Fandcausing thetimeroutputto
becomelow.ThisinturnturnsonthedischargingTr.andtheC1dischargesthrough
thedischargingchannelformedbyRB andthedischargingTr.WhentheVC1falls
belowVcc/3,thecomparatoroutputonthetriggerterminalbecomeshighandthe
timeroutputbecomeshighagain.ThedischargingTr.turnsoff andtheVC1rises
again.
Intheaboveprocess,thesectionwherethetimeroutputishighisthetimeittakesfor
theVC1torisefromVcc/3to2Vcc/3, andthesectionwherethetimeroutputislowis
thetimeittakesfortheVC1todropfrom2Vcc/3toVcc/3.Whentimeroutputis
high, theequivalentcircuitforchargingcapacitorC1isasfollows:
Figure3
Figure4