1. ENT 162 ANALOG ELECTRONICS
Thyristors
HASIMAH ALI
SCHOOL OF MECHATRONIC ENGINEERING.
EXT: 5205,

2. CONTENT PNPN and Special Devices Introduction Four-Layer diode
Silicon controlled switch (SCR)
Silicon Control Switch (SCS)
Gate Turn-Off Switch (GTO)
DIAC and TRIAC
Unijunction transistor (UJT)
Programmable unijunction transistor (PUT)

3. Introduction
Four semiconductor layer (pnpn) devices with a control mechanism are known as thyristors.
This include silicon controlled rectifier (SCR), diac, triac, silicon controlled switch (SCS).
They act as open circuits capable of withstanding a certain rated voltage until they trigger.

4. Introduction
When trigger, they turn on and become low resistance current path and remains so, even after the trigger is removed, until the current is reduced to a certain level or until they are trigger off.
The devices are mainly used in industrial applications where power control and switching are needed such as lamp dimmers, motor speed control, ignition systems and charging circuits.

5. Introduction
Programmable unijunction transistor (PUT) and unijunction transistor (UJT) are also used as trigger devices for thyristors and in oscillators and timing circuits.

6. The Four-Layer Diode (Shockley Diode)
4-layer diode or Shockley diode or SUS is a thyristor and is constructed of four alternating layers of p and n materials forming two pn junctions or pnp Q1 and npn Q2.
BE junction Q1 is pn1, BE junction Q2 is pn3. Base collector junction of both Q1 and Q2 correspond to junction 2.
When positive voltage applied to anode respect to cathode, BE of Q1 and Q2 is fwd-bias, and BC junction is reverse-bias.

7. The Four-Layer Diode (Shockley Diode)
Construction and Symbol

8. The Four-Layer Diode (Shockley Diode)
At low-bias levels, IA is little, and thus it is in off state or fwd-blocking region. e.g if VAK = 20V and IA = 1 µA, so RAK = 20V/1µA = 20MΩ
Region prior to breakover is called forward blocking, which device has a high fwd-resistance (ideally open) and it is in off state.
It exist from VAK = 0V to VBR(F) (Fwd-breakover voltage).

9. The Four-Layer Diode (Shockley Diode)
VAK increase, IA increase untill equal to switching current, IS, where VAK = VBR(F), and internal transistor structure become saturated.
Then VAK suddenly decrease, and diode enter fwd-conduction region, and on state.
When it on, it will continue conduct until
IA < IH (holding current), where it rapidly switch off and enter fwd-blocking region.

10. The Four-Layer Diode (Shockley Diode)
Switching current IS is IA value when it switch from off to on and always less than IH.

11. The Four-Layer Diode (Shockley Diode)
Ex : If VAK is 20 V and IA is 1 µA, find the RAK.
RAK = VAK/IA = 20 V/1 µA = 20 MΩ
Ex : Voltage at anode, VA = 0.8V, find IA.
VRS = Vbias – VA = 110V – 0.8V = 109.2V
IA = VRS/RS = 109.2V/1KΩ = mA

12. The Four-Layer Diode (Shockley Diode)
APPLICATION
It’s rarely used now in design. An application of 4-layer diode is a relaxation oscillator.
When switch closed, capacitor charge through R until reach VBR(F) and it switch to on, capacitor rapidly discharge through diode.
The discharge continue until IA < IH, where diode will switch back to off and capacitor begin to charge again.

13. The Four-Layer Diode (Shockley Diode)
APPLICATION
Relaxation oscillator
Voltage waveform across C

14. Silicon-Controlled Rectifier
SCR is a switching device for high voltage and current operations.
It’s a four layer device with three terminals, anode, cathode, and gate.
In off state, it act ideally as an open circuit between A and K, and high resistance.
In on state it’s act as short between A and K and small forward resistance.
Some application are motor control, time delay, heater control, relay control and phase control.

15. Silicon-Controlled Rectifier

16. Silicon-Controlled Rectifier
SCR Equivalent Circuit

17. Silicon-Controlled Rectifier
Turning the SCR on
When IG = 0 V, the device acts as a 4-layer diode in the off state.
When pulse IG is positive, both Q on (anode must more positive then cathode). IB2 turns on Q2, provide path for IB1 out of the Q2 collector, thus turning on Q1. IC of Q1 provides additional IB for Q2 so that Q2 stay conduct after IG is zero.
Then Q2 sustains the saturated conduction of Q1 by providing a path for IB1, in turn Q1 sustains the saturated conduction of Q2 by providing a path for IB2.

18. Silicon-Controlled Rectifier
Turning the SCR on

19. Silicon-Controlled Rectifier
SCR Characteristics
SCR has a horizontal voltage swing. Voltage across SCR VF is high before it fires, but then it drops significantly once it begins conducting. SCR only conducts in one direction.
SCR will on if voltage anode to cathode >= forward breakover voltage V(BR)F. In this instance the gate current IG can be 0.
More IG1, IG2 is applied, less V(BR)F1, V(BR)F2, V(BR)F3 is required.

20. Silicon-Controlled Rectifier
SCR Characteristic
Holding current, IH is min current from anode to cathode.
Reverse breakdown voltage is the max reverse bias voltage.

21. Silicon-Controlled Rectifier
SCR Characteristic
SCR can be forced on;
Without triggering, by inc VAK > VBR(F).
VBR(F) dec as IG inc above 0V and IG is reach at which SCR trigger at very low VAK.
Apply sufficient VG and IG
Excessively high VAK
High frequency signal from gate to cathode
High temperatures
Once the SCR is turned it remains latched on, even if VG is removed. SCR it’s only conducts in one direction. Removing VG cannot turn off SCR. It is latched on.

22. Silicon-Controlled Rectifier
SCR Characteristic
To turn SCR off:
Anode current interruption
Forced commutation
SCR turn-off by anode current interruption

23. Silicon-Controlled Rectifier
SCR Characteristic
As temperature increases, the SCR requires less forward voltage and gate current to fire.
This means that at higher temperatures the SCR may fire by mistake!!.

24. Silicon-Controlled Rectifier
Application – On-Off Control of Current
Assuming the SCR is initially off. SW1 close, provide a pulse of current into the gate. SCR on so it conduct current to load. Remain in conduction even after the momentary conduct of SW1 is removed if the IA =>than IH.

25. Silicon-Controlled Rectifier
Application – On-Off Control of Current
When SW2 is momentary closed, IA reduced to below IH. SCR off.
In this circuit SW1 is pressed momentarily to turn the SCR on and SW2 is pressed momentarily to turn it off.

26. Silicon-Controlled Rectifier
Application – Half Wave Power Control
Application in lamp dimmer, electric heater, electric motor.
Vac are applied across terminal A and B. RL represents the resistance of load (heating element or lamp element). R1 limits the current. R2 is potentiometer (it sets the trigger level for the SCR).

27. Silicon-Controlled Rectifier
Application – Half Wave Power Control
By adjusting R2, SCR can be made to trigger at any point on the positive half cycle of ac waveform (0 to 900)
When trigger at beginning, it conducts for approximately and maximum power is delivered to load.

28. Silicon-Controlled Rectifier
Application – Half Wave Power Control
When trigger at near peak of positive half cycle, it conducts for approximately 900 and less power is delivered to load.
When input goes negative, SCR off and diode is used to prevent negative ac voltage from being applied to the gate of SCR.

29. Silicon-Controlled Rectifier
Application: Over -Voltage protection Circuit –Crow Bar Circuit
Vout dc from regulator is monitor by zener, D1 and resistive voltage divider R1 and R2. The Vout max is set by zener voltage, if this voltage exceeded, D1 conducts and voltage divider produces an SCR trigger voltage. SCR on and connected across the line voltage. SCR current causes the fuse to blow, thus disconnecting the line voltage from power supply.

30. Silicon-Controlled Switch (SCS)
SCS is similar to the SCR in construction with the exception being the SCS has two gates.
It can be turned on and off by using either terminal. Normally the SCR is available in power ratings lower than SCR and has faster switching time than SCR.

31. Silicon-Controlled Switch (SCS)
Either gate can fire SCS. To start, assume that Q1 and Q2 are off and not conducting.
A positive pulse on the cathode gate drive Q2 into conduction and provide a path for Q1 IB. When Q1 on, its IC provide IB to Q2, thus sustaining the on state of SCS. SCS only conducts in one direction.

32. Silicon-Controlled Switch (SCS)
SCS can also be turned on with negative pulse on anode gate. This drives Q1 into conduction, in turn provide IB for Q2. Once Q2 on, it provide a path for Q1 IB, thus sustaining the on state.
To turn off, a positive pulse is applied to anode gate. This rev-bias BE junction of Q1 and turn it off. Q2 in turn cut-off and SCS cease conduction. It also can turn off using negative pulse on cathode gate.

33. Silicon-Controlled Switch (SCS)
Another method for turn off SCS is using switching method, to reduce anode current below holding value. BJT acts as a switch to interrupt the anode current.
SCS is being used in digital application such as counter, register and timing circuits.

34. DiAc and Triac
Diac and triac unlike the SCR will conduct in both directions making it ideal for ac applications. Diac has two terminals, while triac has a third terminal, which is the gate for triggering.
Diac function basically like two parallel 4-layer diode turned in opposite direction. The triac function basically like two parallel SCR turned in opposite directions with a common gate terminal.
Diac turns on when breakover voltage is reached in either direction.

35. Diac
Diac is also a breakover type device. It’s has two terminals A1 and A2. When breakover voltage reach conduction occur with either polarity across the two terminals.

36. Diac
Once breakover occurs, current direction depending on the polarity of the voltage across the terminal. The device turn off when the current drops below the holding value. The breakover voltage is approximately symmetrical for a positive and a negative breakover voltage.

37. Diac
When Diac is biased, the pnpn structure from A1 to A2 (positive direction) provide the same operation as 4-layer diode. In equivalent circuit Q1 and Q2 are fwd-bias, Q3 and Q4 are rev-bias. The other way around if Diac is biased from A2 to A1.

38. Triac
Triac is basically a diac with a gate terminal. Triac can be turned on by a pulse at the gate and does not require breakover voltage to initiate conduction, as Diac.
Basically triac can be though as two SCR connected in parallel and in opposite directions with a common gate terminal.
Unlike SCR, triac can conduct current in either direction when it is trigger on, depends on the polarity of the voltage across A1 and A2 terminals.

39. Triac

40. Triac
Breakover potential decrease as the gate current increase (as SCR).
Triac cease to conduct when IA drop below specified value of IH. The only way to turn off the triac is to reduce the current to a sufficiently low level. If A1 is biased positive respect to A2 and positive pulse at gate, triac is on (Q1 and Q2 on).

41. Triac
When fired by the gate or by exceeding the breakover voltage, the Triac conducts in both directions.
Triac being used in AC power control circuits.

42. Triac Application- Phase Control
Here R1 controls the trigger point at which the triac turns on for each half of the cycle.
The off time is called delay angle and the on time is called the conduction angle.

43. Triac Application- Phase Control
D1 is used to provide trigger pulses to triac gate and conduct during positive half at which the triac trigger. A1 and G are positive with respect to A2.
D2 conduct during negative half cycle and R1 set the trigger point. A2 and G are positive with respect to A1.

44. Unijunction Transistor
UJT does not belong to thyristor family because it’s does not have a four-layer type of construction and term unijunction refer to it’s has one pn junction.
UJT being used mainly as a triggering device or switch in thyristor circuits and can also be used in oscillator circuits.

45. Unijunction Transistor
The symbol is almost similar to a JFET. Note the angle of the emitter. The other terminals are called B1 and B2. The characteristics are quite different than any other transistor.

46. Unijunction Transistor
Even though UJT is a switching device it works very differently from SCR variety of devices.
The equivalent circuit indicates that UJT is like a diode and a resistive voltage divider circuit.

47. Unijunction Transistor
The resistance exhibited by rB1 is variable; it is dependent on the value of current IE.
Voltage is applied across UJT, VBB and to emitter input, VEB1.
Once VEB1 reaches a peak value (Vp) the UJT begins to conduct (pn junction forward biased).
At point where VE = Vp, current IE is at minimum. This is the threshold value of VE that puts the UJT into conduction.
Once conducting, IE increases and VE decreases. This is called negative resistance.

48. Unijunction Transistor
Beyond the valley point (VE= VV and IE = IV), the device is in saturation and VE increases very little with an increasing IE.
UJT characteristics curve for a
fixed value of VBB

49. Unijunction Transistor
The resistive equivalent circuit of a UJT shown makes it easier to understand it’s operation.
r’B1 represent the internal dynamic resistance between emitter and base 1, B1 and r’B2 represent the dynamic resistance between emitter and base 2, B2.
Total resistance or interbase resistance,
r’BB = r’B1 + r’B2.

50. Unijunction Transistor
r’B1 varies inversely with IE with value from several thousand ohm to tens ohm. The voltage across resistance,
r’B1, Vr’B1 = (r’B1/r’BB)VBB

51. Unijunction Transistor
Standoff Ratio
Standoff ratio, (eta) = r’B1/ r’BB.
Value of VE necessary to put the UJT into conduction is called Vp. Vp = VBB +Vpn
Vpn is barrier potential of the pn junction.
Where  is the intrinsic stand-off ratio, and its value is available from the specification sheet of the UJT.

52. Unijunction Transistor
UJT Application – Trigger Circuit
UJT is used almost exclusively as a trigger circuit for SCRs.

53. Unijunction Transistor
UJT Application – Trigger Circuit
UJT can be used as a trigger device for SCR and triac (other application : nonsinusoidal oscillator, sawtooth generator, phase control)
To ensure turn on, R1 must not limit IE at peak point to less than IP. To ensure this, VR1 at peak should be greater than IPR1.
VBB – VP > IPR1 or R1 < (VBB – VP)/IP

54. Unijunction Transistor
UJT Application – Trigger Circuit
To ensure turn-off, of UJT at valley point, R1 must be large enough that IE can decrease below the specified value of IV. So VR1 at valley point must be less than IVR1. So for turn off ;
VBB – VV < IVR1
R1 > (VBB – VV)/IV

55. Unijunction Transistor
UJT Application – Trigger Circuit

56. Programmable Unijunction Transistor
PUT structure is more similar to an SCR. Gate is connected to the n region adjacent to anode and is used to turn PUT off and on.
Gate always biased positive respect to cathode. A PUT trigger into conduction when VA exceed VG.
When VA exceeds VG by approx 0.7V, pn junction is fwd-bias, and PUT turn on.
It’s stay on until VA fall back below this level, then it’s turn off.

57. Programmable Unijunction Transistor

58. Programmable Unijunction Transistor
Gate can be biased to a desired voltage with an external voltage divider, instead of by specifications of the device as in the  value for the UJT.
So that when the anode voltage exceeds this programmed level, PUT turns on. Like UJT, PUT has a negative resistance region.
But this region is unstable in PUT. PUT is operated between on and off states.

59. Programmable Unijunction Transistor
This yields a curve similar to a UJT therefore it can used in oscillator circuits like UJT.

60. Programmable Unijunction Transistor
Application
Trigger device and it’s replace UJT in many applications.
A relaxation oscillator circuit is used to trigger PUT.
Gate voltage required to turn the PUT on is determined by external components, R2 and R3.
When dc power is applied, PUT is off and capacitor charge toward 18V through R1.

61. Programmable Unijunction Transistor
Application
When VC reach VG + 0.7V, PUT turn on and capacitor rapidly discharge through low on resistance of PUT and R4. Voltage spike is develop across R4 during discharge. As soon capacitor discharge, PUT turn off and charging cycle start again.

62. Programmable Unijunction Transistor
Application
Reducing or removing VG will not turn the PUT off. Instead, like an SCR, VAK voltage must drop sufficiently to reduce the current below a holding level.

63. Phototransistor
Phototransistor is a BJT with the light sensitive collector base junction exposed to light through a window.
Light intensity controls the collector current.
IC = βDCIλ

64. Phototransistor
With no light present the transistor is off with exception to the small amount leakage current. This is called dark current.
This transistor is biased on by a light beam, which produces a base current.
The greater the intensity of the light beam, the higher the resulting base and collector currents.
It is sometimes called a photodetector.

65. Phototransistor
There are many applications for this device, light activated switching being one. They come with or without the base connection.
The curve illustrates collector current with different light intensities.

66. Phototransistor Application- Light Operated Relay Circuit
Phototransistor, Q1 drives the BJT Q2. When there is sufficient incident light on Q1, transistor Q2 is driven into saturation, and IC through relay coil energizes the relay. Diode D1 is to control back EMF when Q2 turns off.

67. Phototransistor Application- Darkness Operated Relay Circuit
When there is insufficient light, Q2 is biased on, keep the relay energized. When there is sufficient light, Q1 turns on, this pulls base Q2 low, thus turn off Q2 reenergizes the relay. Diode D1 is to control back EMF when Q2 turns off.

68. Phototransistor Application- Light Interruption Alarm Circuit
Q1 normally on, holding SCR gate low. When the light is interrupted, Q1 turn off. The rise time pulse trigger SCR and sets off the alarm mechanism. SW1 is for reset the alarm. It being used as smoke detection or intrusion detection.

69. The light-activated scr (lascr)
LASCR is basically an SCR that is triggered on by light beam striking the gate to cathode junction or by applying a gate voltage. It’s most sensitive to light when gate is open and if necessary a resistor from G to K being used to reduce the sensitivity.
In app. below, input source which is electrically isolated, turn on lamp, and the light trigger LASCR. IA energise the relay and close contact.

70. Opto-Couplers
Opto-Couplers provide isolation between the input and output circuit to provide protection high-voltage transients, surges, and noise that could cause problems. This basic optical coupling device shows an LED and a phototransistor. Fiber optic lines can provide a path for light to travel between the devices.

71. Opto-Couplers
When Vin fwd-bias LED, light transmitted to phototransistor turns it on, producing current through external load.
a. Darlington transistor coupler can be used when inc output current capability is needed more than photodiode capability, bur it’s switching time is slower than phototransistor.
b. LASCR output coupler being used such as when a low-level input is required to latch a high voltage relay for activating electromechanical devices.
c. Phototriac output coupler is for applications that require isolated triac triggerring, such as switching a 110V ac line from a low-level input.

72. Opto-Couplers
d. Optical isolated ac linear coupler. It’s converts an input current variation to an output voltage variation.
Output consist of amp and photodiode across its input terminals.
Light variation emitted from LED are pick-up by photodiode, providing input signal to amp.
Output of amp is buffered with emitter-follower stage. It’s can be used for telephone, peripheral equipment isolation and audio application.

73. Opto-Couplers
e. Digital Output coupler. It’s consist of high speed detection circuit follower by a transistor buffer stage.
When there is current through input LED, detector is light activated and turn on output transsitor, so that collector switch to low voltage level.
If there is no current through LED, output is at high-voltage level.

74. Opto-Couplers

75. Thank you for listening