1	Chapter 33 Thyristors and Optical Devices
2	Introduction to Thyristors Thyristors Switch On-state, off-state Unilateral or bilateral Latching High power
3	Introduction to Thyristors Thyristors Sinusoidal Firing angle Conduction angle
4	Triggering Devices Used to pulse switching devices Diac 3-layer Bi-directional conduction Breakover voltage Blocking region
5	Triggering Devices Unijunction Transistor (UJT) 3-terminal device Intrinsic standoff ratio
6	Triggering Devices UJT 0.5 < η < 0.9 Emitter region heavily doped V_E – B₁ = 0, p-n junction reverse biased Increase V_E – B₁, reach peak point (maximum current)
7	Triggering Devices UJT Continue increase, reach valley point Further increase V_E – B₁, UJT is saturated
8	Triggering Devices UJT relaxation oscillator
9	Silicon Controlled Rectifiers (SCRs) 4-layer device, p-n-p-n Anode (A) Cathode (K) Gate (G) Unidirectional
10	Silicon Controlled Rectifiers (SCRs) High-power (I up to 2500 A, V up to 2500 V) Phase control Small V_AK when On
11	SCRs
12	SCRs Operation I_G = 0, no anode current I_G > I_GT → regenerative feedback → high I_AK I_AK < I_H → turn off → I_AK = 0
13	SCRs Can cause SCR turn-on High temperature High ∆V/∆t (noise) Radiation
14	SCRs Specifications V_DRM or V_RRM Peak Repetitive Off-state Voltage I_T(RMS) On-State RMS current (maximum) I_TSM Peak Non-Repetitive Surge current I_GT Gate trigger current I_L Latching current I_H Holding current
15	SCRs SCR phase control
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	SCRs Too large R₁ SCR does not turn on
18	Triacs 3-terminal switch Bi-directional current Symbol Gate trigger may be either + or – pulse
19	Triacs Characteristics Direct replacement for mechanical relays Trigger circuit for full-wave control 4 modes Remains on in either direction until I < I_H Blocking region, I ≈ μamps Small voltage across Triac when On
20	Triacs Specifications Similar to SCR P_GM Peak Gate Power P_G(AV) Average Gate Power V_GM Peak Gate Voltage V_GT Gate trigger voltage t_gt Turn-On Time
21	Triacs Phase control light dimmer
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	Power Control Fundamentals Review equations Control Lamp intensity Heat from a resistive heater Speed of a motor
24	Power Control Fundamentals
25	Power Control Fundamentals Delayed turn-on, full-wave signal Delayed turn-on, half-wave signal
26	Power Control Fundamentals V and P curves for full-wave control
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
28	Introduction to Optical Devices Electromagnetic spectrum Visible (380 < λ(nm) < 750) Infrared region (750 < λ(nm) < 1000)
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	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	Introduction to Optical Devices LED disadvantages Easily damaged Brightness dependent on temperature Chromatic dispersion Inefficient compared to LCDs
32	Photodetectors R varies with light intensity Photoresistors Voltage or current varies with light intensity Photodiodes Phototransistors Light-Activated SCRs (LASCRs)
33	Photodetectors Photodiodes Reverse biased Low ambient light → very small current, I_D (small leakage current) High ambient light → increased current, I_D (increase in minority carriers)
34	Photodetectors Photodiodes Symbol
35	Photodetectors Phototransistor Base open Light on reverse-biased CB junction Increase minority carriers Increase I_C
36	Photodetectors Phototransistor Usually used as a switch Off → I_C = 0 On → I_C > 0
37	Photodetectors LASCR Light-Activated SCR or photo-SCR Symbol Light turns LASCR on Open gate or resistor on gate to control sensitivity
38	Optocouplers Couple two circuits LED and Photodetector in single circuit Electrical isolation Medical equipment High voltage circuit to digital circuit
39	Optocouplers Use as Linear device Digital buffer
40	Optocouplers Phototransistor optocoupler
41	Optocouplers Current transfer ratio 0.1 < CTR < 1
42	Optocouplers Operation High diode current in input circuit yields High diode light output which yields High collector current in output circuit
43	Semiconductor LASERs Light Amplification through Stimulated Emission of Radiation Operation Similar to LEDs Monochromatic (same frequency) Coherent (same phase) output Small pulse dispersion