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A Three Terminal Semiconductor Device
Invented in 1957 Used as a controlled switch to perform
rectification, inversion and regulation of powerflow.
Can handle currents up to 1000 A and voltagesmore than 1 KV
Converts AC into DC & also controls theamount of power fed to the load.
Hence, combines the features of a rectifier anda transistor
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Leakage current in Si is very small when
compared to Ge
Device is used as switch minimum leakagecurrent.
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PN Junction + junction transistor = SCR pnpndevice
3 terminal device
Anode from outer p region(A)Cathodefrom outer n region(K)Gate base of transistor section
Also called THYRISTOR as it is the solid
state equivalent of a thyratron.
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In SCR, the load is connected in series withthe anode
Anode is always kept at a positive potentialw.r.t cathode
The working can be understood under 2conditions:
1. When Gate is open
2. When gate is positive w.r.t cathode
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No voltage is applied to the gate . J2 is FB andJ1 & J3 are RB.
No current flow through SCR because J2 is RBSCR is cut off
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When the supply voltage V exceeds a certainthreshold, the junction J2 breaks down.
The SCR now conducts heavily and is in theO state.
Breakover Voltage the voltage at which theSCR conducts heavily without gate voltage.
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The device can be made to conduct heavily byapplying a small positive potential at the gate
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J3 is FB and J2 is RB
Electrons from the n type material move across J3
towards left Holes from p type move towards right
Electrons from J3 are attracted across J2 gatecurrent
As J2 is now conducting, anode current flowsthrough SCR
J2 breaks down SCR is ON anode currentdoesnt decrease even when gate voltage is
removed To make SCR OFF reduce the applied voltage to
zero.
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1. SCR has two states - either it does not conduct orit conducts heavily. There is no intermediatestate. Hence SCR behaves like a switch
2. There are 2 ways to turn ON the SCR
(i) with Gate open and increasing the supplyvoltage,
(ii) with a positive Gate trigger. (gate triggershould be >10mA)
3. The SCR is turned OFF only by reducing thesupply voltage to zero, thereby reducing theanode current
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The curve between V & I when anode is +vew.r.t cathode
When supply voltage is increased from zero,suddenly the SCR starts conducting => breakover voltage
Voltage drops at this point suddenly asshown by the dotted line.
If proper gate current is made to flow, thenSCR can close at smaller supply voltage.
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The curve between V & I when anode is -ve w.r.t
cathode
If reverse voltage is increased gradually, at first,
the anode current remains small (leakage current)
Beyond a particular reverse voltage, the SCR starts
massive conduction (avalanche) => Reverse
breakdown voltage.
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1. Supply voltage < Break over voltage2. SCR is turned on by passing an appropriate
gate current (few mA) and not by break overvoltage
3. Peak Reverse Voltage should not exceed thereverse breakdown voltage
4. When SCR needs to be turned OFF , the
anode current should be reduced to theholding current
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Switch
Controlled Rectification
Power control of load
Speed Control of dc shunt motor
Over light detector
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