The complete list of thyristor family members include diac (bidirectional diode thyristor), triac (bidirectional triode thyristor), SCR (silicon controlled rectifier), Shockley diode, SCS (silicon controlled switch), SBS (silicon bilateral switch), SUS (silicon unilateral switch) also known as com­plementary SCR or CSCR, LASCR (light activated SCR), LAS (light activated switch) and LASCS (light activated SCS).

1. UNIT 3
INTRODUCTION TO POWER
ELECTRONICS
DEE 402
Thyristor Family
by
VINOD SRIVASTAVA
Department of Electrical Engineering
University Polytechnic
IFTM University, Moradabad

2. CONTENTS
1. Programmable UJT (PUT)
2. Silicon Unilateral Switch (SUS)
3. Silicon Control switch (SCS)
4. Light Activated Thyristor
5. Static Induction Thyristor (SITH)
6. DIAC
7. TRIAC
8. Important Questions
9. MCQs
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3. Programmable UJT (PUT)
Programmable unijunction transistor or PUT is a close
relative of the thyristor family.
It has a four layered construction just like the thyristors and
have three terminals named anode(A), cathode(K) and
gate(G) again like the thyristors.
Yet some authors call it a programmable UJT just because its
characteristics and parameters have much similarity to that
of the unijunction transistor.
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4. Programmable UJT (PUT)
It is called programmable because the parameters like
intrinsic standoff ratio (η), peak voltage(Vp) etc can be
programmed with the help of two external resistors.
In a UJT, the parameters like Vp, η etc are fixed and we
cannot change it.
The main application of programmable UJT are relaxation
oscillators , thyristor firing, pulse circuits and timing circuits.
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5. SUS
Silicon Unilateral Switch
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Because of the presence of diode, SUS turns on for a fixed anode to
cathode voltage.
SCR’s trigger voltage and/or current vary widely with changes in
ambient temperature.

6. SUS
Silicon Unilateral Switch
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SUS is mainly used in Timing, logic and trigger circuits.
Its largest ratings are about 20 v and 0.5 A.

7.  An SCS is a tetrode. It is a four electrode thyristor.
 It has two gates, one anode gate (AG) like a PUT and
another cathode gate (KG) like an SCR.
 An SCS is a four layer, four terminal pnpn device with
anode A, cathode K, anode gate AG and cathode gate KG.
 An SCS can be turned on by either gate.
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SCS(Silicon Controlled Switch)

8. SCS(Silicon Controlled Switch)
When a negative pulse is applied to gate AG, junction J1 is forward
biased and SCS is turned on.
A positive pulse at AG will reverse bias junction J1 and turns off the
SCS.
When a positive pulse is applied to gate KG, junction J2 is forward
biased and SCS is turned on. (Just like an SCR)
A negative pulse at KG will reverse bias junction J2 and turns off the
SCS. (Just like a G.T.O.)
Its ratings are about 100 v and 200 mA.
It can be operated like an OR gate.
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9. Light Activated Thyristor
 LASCR or light activated SCR is a semiconductor device which turns
ON when it is exposed to light.
The constituent element of SCR is silicon, and it works like a rectifier,
and thus, it is termed as Silicon Controlled Rectifier.
The LASCR is a type of thyristor which is triggered by photons present
in the light rays.
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10. Light Activated Thyristor
It is a three terminal device, consists
of cathode, anode and gate terminal.
The gate terminal is used when the electrical triggering is
supplied to the LASCR.
The advantage of using triggering of the thyristor by light is
prevention from electrical noise disturbances.
Thus, LASCR is considered to be one of the best devices.
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11. Light Activated Thyristor
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Applications of the Light Activated SCR
• Low Power Applications
• Motor Control
• Computer Applications
• Optical light Controls
• Solid State Relay

12. STATIC INDUCTION
THYRISTOR(SITH)
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 The static induction thyristor (SITh) or field-controlled diode (FCD) was first
introduced by Teszner in the 1960’s.
 This device is capable of conducting large currents with a low forward voltage
and turn-off quickly.
 It is a self-controlled GTO-like on-off device that was commercially
introduced by Toyo Electric Co. (Toyo Denki) of Japan in 1988.
 It belongs to a family of Static Induction Device and is a high power high
frequency power semiconductor device.
 It is essentially a p+nn+ diode with a buried p+ grid like gate structure.The
device structure is analogous to SIT except that a p+layer has been added to the
anode side.

13. 13
DIAC
The DIAC is a bi-directional semiconductor switch that can
be switched on in both polarities.
The full form of the name DIAC is diode alternating current.
Diac is connected back to back using two zener diodes and
the main application of this DIAC is, it is widely used to
help even activating of a TRIAC when used in AC switches,
dimmer applications and starter circuits for florescent lamps.

14. DIAC
Construction and Operation of DIAC
 Basically, the DIAC is a two terminal device; it is a combination of parallel
semiconductor layers that allows activating in one direction.
 This device is used to activating device for the triac. The basic construction of
diac consist of two terminals namely MT1 and MT2.
 When the MT1 terminal is designed +Ve with respect to the terminal MT2, the
transmission will take place to the p-n-p-n structure that is another four layer
diaode.
 The diac can be performing for both the direction.
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15. DIAC
Characteristics
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16. TRIAC
A Triac device comprises of two thyristors that are connected in opposite
direction but in parallel but, it is controlled by the same gate.
Triac is a 2-dimensional thyristor which is activated on both halves of the
i/p AC cycle using + Ve or -Ve gate pulses.
The three terminals of the Triac are MT1; MT2 & gate terminal (G).
Generating pulses are applied between MT1 and gate terminals. The ‘G’
current to switch 100A from triac is not more than 50mA or so.
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17. TRIAC
Construction and Operation of
DIAC
Traic is a three terminal device and
the terminals of the triac are MT1,
MT2 and Gate.
Here the gate terminal is the control
terminal. The flow of current in the
triac is bi directional that means
current can flow in both the
directions.
In the above structure, the MT1 and
gate terminals are near to each other.
When the gate terminal is open, the
triac will obstruct the both the
polarities of the voltage across the
MT1 & MT2.
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18. TRIAC
Characteristics
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19. 19
IMPORTANT QUESTIONS
Q.1. Describe briefly the following members of thyristor
family (i) DIAC (ii) TRIAC
Q.2. Explain PUT.
Q.3 Explain SUS and SCS in detail.
Q.4. Explain the following members of Thyristor family- (a)
Static induction Thyristor (b) TRIAC
Q.5.Explain Light activated Thyristor.

20. MULTIPLE CHOICE QUESTIONS
Q1. A triac is a
1. 2 terminal switch 2. 2 terminal bilateral switch 3. 3 terminal
bilateral switch 4. 3 terminal bidirectional switch
Q.2. In PUT, Gate terminal is connected to
i) Inner n-layer ii) Inner p- layer iii) Upper p- layer
iv) Lower n- layer
Q3. A Triac has three terminals viz ………………
1. Drain, source, gate 2.Two main terminal and a gate terminal
3. Cathode, anode, gate 4. None of the above
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21. MULTIPLE CHOICE
QUESTIONS
Q4. A triac is equivalent to two SCRs …………..
1. In parallel 2. In series 3. In inverse-parallel 4.None of
the above
Q5. A triac is a …………. switch
1. Bidirectional 2. Unidirectional 3. Mechanical
4. None of the above
Q6. A triac can pass a portion of …………… half-cycle through the load
1. Only positive 2. Only negative 3.Both positive and negative
4. None of the above
Q7. A diac has ………….. terminals
1. Two 2.Three 3. Four 4.None of the above
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22. MULTIPLE CHOICE
QUESTIONS
Q8. A triac has …………….. semiconductor layers
1. Two
2. Three
3. Four
4. Five
Q9. A diac has …………… pn junctions
1. Four
2. Two
3. Three
4. None of the above
Q10. The device that does not have the gate terminal is ……………….
1. Triac 2. FET 3. SCR 4. Diac
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23. THANK YOU
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