This document provides an overview of thyristors, optical devices, and power control fundamentals. It introduces thyristors like SCRs and triacs that are used for switching and phase control. Triggering devices like DIACs and UJTs are described. Optical devices discussed include LEDs, photodetectors, optocouplers, and semiconductor lasers. Power control using phase control and delayed triggering of thyristors is also summarized.

1. Chapter 33
Thyristors and Optical
Devices

2. 2
Introduction to Thyristors
• Thyristors
– Switch
– On-state, off-state
– Unilateral or bilateral
– Latching
– High power

3. 3
Introduction to Thyristors
• Thyristors
– Sinusoidal
• Firing angle
• Conduction angle

4. 4
Triggering Devices
• Used to pulse switching devices
• Diac
– 3-layer
– Bi-directional conduction
– Breakover voltage
– Blocking region
Symbols

5. 5
Triggering Devices
• Unijunction Transistor (UJT)
– 3-terminal device
– Intrinsic standoff ratio
P
n
E
B2
B1
B2
E
B1
Symbol
1 1
1 2
B B
B B BB
R R
R R R
  


6. 6
Triggering Devices
• UJT
– 0.5 < η < 0.9
– Emitter region heavily doped
– VE – B1 = 0, p-n junction reverse biased
– Increase VE – B1, reach peak point (maximum
current)

7. 7
Triggering Devices
• UJT
– Continue increase, reach valley point
– Further increase VE – B1, UJT is saturated

8. 8
Triggering Devices
• UJT relaxation oscillator
+VBB
___
__
_
vout
RE
CE
+
-
1
ln
1
1
E E
T R C
f
T

 
  

 


9. 9
Silicon Controlled Rectifiers (SCRs)
• 4-layer device, p-n-p-n
• Anode (A)
– Cathode (K)
– Gate (G)
• Unidirectional

10. 10
Silicon Controlled Rectifiers (SCRs)
• High-power (I up to 2500 A, V up to 2500 V)
• Phase control
• Small VAK when On

11. 11
SCRs

12. 12
SCRs
• Operation
– IG = 0, no anode current
– IG > IGT → regenerative feedback → high IAK
– IAK < IH → turn off → IAK = 0

13. 13
SCRs
• Can cause SCR turn-on
– High temperature
– High ∆V/∆t (noise)
– Radiation

14. 14
SCRs
• Specifications
– VDRM or VRRM Peak Repetitive Off-state
Voltage
– IT(RMS) On-State RMS current (maximum)
– ITSM Peak Non-Repetitive Surge current
– IGT Gate trigger current
– IL Latching current
– IH Holding current

15. 15
SCRs
• SCR phase control

16. 16
SCRs
• Small R1
– Short RC time constant
– SCR turns on rapidly, close to 0°
• Large R1
– long RC time constant
– SCR turns on slowly, close to 180°

17. 17
SCRs
• Too large R1
– SCR does not turn on

18. 18
Triacs
• 3-terminal switch
• Bi-directional current
• Symbol
• Gate trigger may be either + or – pulse
G
MT1 or Anode (A)
MT2
I

19. 19
Triacs
• Characteristics
– Direct replacement for mechanical relays
– Trigger circuit for full-wave control
– 4 modes
– Remains on in either direction until I < IH
– Blocking region, I ≈ μamps
– Small voltage across Triac when On

20. 20
Triacs
• Specifications
– Similar to SCR
– PGM Peak Gate Power
– PG(AV) Average Gate Power
– VGM Peak Gate Voltage
– VGT Gate trigger voltage
– tgt Turn-On Time

21. 21
Triacs
• Phase control light dimmer

22. 22
Triacs
• Circuit operation
– Turn-off due to small load current
– Capacitor charges/discharges through load
– DIAC is bi-directional
– RC time constant → 0° to 180° turn on in
each direction

23. 23
Power Control Fundamentals
• Review equations
• Control
– Lamp intensity
– Heat from a resistive heater
– Speed of a motor

24. 24
Power Control Fundamentals
 
2
2
0
( ) ( )
( )
2
2
rms
T
rms
rms FW rms HW
V
P
R
V t dt
V
T
V V
V V


 


25. 25
Power Control Fundamentals
• Delayed turn-on,
full-wave signal
• Delayed turn-on,
half-wave signal
 
 
2
( )
2
( )
sin
sin
2
F
F
T
rms FW
rms HW
V d
V
V d
V



 

 






26. 26
Power Control
Fundamentals
• V and P curves for
full-wave control

27. 27
Introduction to Optical Devices
• Opto-electronic devices λ =
wavelength
– Current → light
– Light → current
– c = speed of light in a vacuum
– c = 3 x 108 m/s
f
c

λ

28. 28
Introduction to Optical Devices
• Electromagnetic spectrum
– Visible (380 < λ(nm) < 750)
– Infrared region (750 < λ(nm) < 1000)
1 Å = 1  10–10 m = 0.1 nm

29. 29
Introduction to Optical Devices
• LED is a diode
– When forward biased
– Electron-hole recombination energy
– Photons released: E = hf , h is Planck’s
constant
– h = 6.626  10–34 Joules∙seconds
– High energy → visible spectrum
– Lower energy → IR spectrum

30. 30
Introduction to Optical Devices
• LED advantages
– Low voltage
– Rapid change in light output with input V
change
– Long life
– LED output can be matched to photodetector

31. 31
Introduction to Optical Devices
• LED disadvantages
– Easily damaged
– Brightness dependent on temperature
– Chromatic dispersion
– Inefficient compared to LCDs

32. 32
Photodetectors
• R varies with light intensity
– Photoresistors
• Voltage or current varies with light intensity
– Photodiodes
– Phototransistors
– Light-Activated SCRs (LASCRs)

33. 33
Photodetectors
• Photodiodes
– Reverse biased
– Low ambient light → very small current, ID
(small leakage current)
– High ambient light → increased current, ID
(increase in minority carriers)

34. 34
Photodetectors
• Photodiodes
– Symbol

35. 35
Photodetectors
• Phototransistor
– Base open
– Light on reverse-biased CB junction
– Increase minority carriers
– Increase IC

36. 36
Photodetectors
• Phototransistor
– Usually used as a switch
• Off → IC = 0
• On → IC > 0

37. 37
Photodetectors
• LASCR
– Light-Activated SCR
or photo-SCR
– Symbol
– Light turns LASCR on
– Open gate or resistor
on gate to control
sensitivity

38. 38
Optocouplers
• Couple two circuits
– LED and Photodetector in single circuit
• Electrical isolation
– Medical equipment
– High voltage circuit to digital circuit

39. 39
Optocouplers
• Use as
– Linear device
– Digital buffer

40. 40
Optocouplers
• Phototransistor
optocoupler

41. 41
Optocouplers
• Current transfer ratio
• 0.1 < CTR < 1
CTR C
F
I
I


42. 42
Optocouplers
• Operation
– High diode current in input circuit yields
– High diode light output which yields
– High collector current in output circuit

43. 43
Semiconductor LASERs
• Light Amplification through Stimulated
Emission of Radiation
• Operation
– Similar to LEDs
– Monochromatic (same frequency)
• Coherent (same phase) output
– Small pulse dispersion