1. Chapter 20: pnpn and Other Devices
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

2. Part I: pnpn Devices Slide 1 1. SCR – Silicon-Controlled Rectifier
2. SCS – Silicon-Controlled Switch
3. GTO – Gate Turn-Off Switch
4. LASCR – Light-Activated SCR
5. Shockley Diode
6. Diac
7. Triac
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

3. 1. SCR – Silicon-Controlled Rectifier
Slide 2
The SCR is a switching device for high voltage and current operations.
Schematic Symbol:
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

4. SCR Basic Operation Turning - on
Slide 3
To turn the SCR on:
1. forward bias the anode-cathode
2. apply sufficient gate voltage (Vgate) and Gate current (IGT)
Once the SCR is turned it remains latched on, even if the gate signal is removed. The SCR like a diode only conducts in one direction.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

5. SCR Basic Operation Forced - on
Slide 4
An SCR can be forced on by
•Excessively high voltage from anode to cathode
• High frequency signal from gate to cathode
• High temperatures
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

6. SCR Basic Operation Turning - off
Slide 5
Removing the gate voltage cannot turn off an SCR. It is latched on.
To turn the SCR off:
• Remove the power source from anode to cathode
• Reverse bias the anode-cathode
Both of the above can be accomplished with commutation circuitry.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

7. SCR Basic Operation Commutation
Slide 6
Commutation circuitry is simply a switching device connected in parallel with the SCR.
A control signal activates the switching circuitry and provides a low impedance bypass for the anode to cathode current. This momentary loss of current through the SCR will turn it off.
The switching circuitry can also apply a reverse bias voltage across the SCR, which also will turn the SCR off.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

8. SCR characteristics Slide 7
The SCR has a characteristic horizontal voltage swing. The voltage across the SCR (VF) is high before it fires, but then it drops significantly once it begins conducting. The SCR only conducts in one direction.
The SCR will “fire “ (turn on) if the voltage from anode to cathode is greater or equal to the forward breakover voltage (V(BR)F). In this instance the gate current (IG) can be 0.
As more gate current is applied (IG1, IG2), less forward voltage (VF1, VF2, VF3) is required.
Holding current (IH) is the minimum required current from anode to cathode.
Reverse breakdown voltage is the maximum reverse bias voltage for the SCR.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

9. This means that at higher temperatures the SCR may fire by mistake!!
SCRs and Temperature
Slide 8
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!!
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

10. SCR Terminal Identification
Slide 9
The SCR has three terminal Anode (A), Cathode (K), and the Gate (G).
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

11. SCR Applications Slide 10
The gate voltage can be set to fire the SCR at any point in the AC cycle. Remember the SCR is only a switch!
In this case the SCR fires as soon as the AC cycle crosses 0. Therefore it acts like a half-wave rectifier.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

12. SCR Applications (cont’d)
Slide 11
In this circuit, the SCR fires later in the cycle.
As you can see when the SCR fires provides more information than at what gate voltage.
Therefore SCR firing is indicated by angle; i.e. at what degree in the AC cycle.
In this circuit the SCR fires at 90, therefore it conducts for 90.
The half-wave rectifier example fires at 0;therefore it conducts for 180.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

13. SCR Applications (cont’d)
Slide 12
• Battery-charging regulator
• Temperature controller circuit
• Emergency-lighting system
In these applications the SCR gate circuit is used to monitor a situation and trigger the SCR to turn on a portion of the circuit.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

14. 2. SCS – Silicon-Controlled Switch
Slide 13
This device is like an SCR except that it has two gates: Cathode gate and an Anode gate.
Schematic Symbol:
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

15. SCS Basic Operation Slide 14
Either gate can fire the SCS. A positive pulse or voltage on the Cathode gate or a negative pulse or voltage on the Anode gate will fire the SCR. The SCS only conducts in one direction.
The gates can also turn the SCS off. A negative pulse or voltage on the Cathode gate or a positive pulse or voltage on the Anode gate will fire the SCR.
The difference between the gates: The Anode gate requires higher voltages than the Cathode gate.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

16. Comparison of SCR and SCS
Slide 15
The SCS has a much lower power capability compared to the SCR.
The SCS has faster switching times than the SCR.
The SCS can be switched off by gate control.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

17. SCS Applications Slide 16 • Pulse generator • Voltage sensor
• Alarm circuits
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

18. SCS Pin Identification
Slide 17
SCS Pin Identification
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

19. 3. GTO – Gate Turn-Off Switch
Slide 18
GTO is similar to the SCR, except that the gate can turn the GTO on and off. It only conducts in one direction.
Schematic Symbol:
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

20. Comparison of GTO and SCR
Slide 19
GTO is a low power device.
The gate signal necessary to fire the GTO is larger than the SCR gate signal.
The gate signal necessary to turn the GTO off is similar to that of SCS.
The switching rate for turning the GTO off is much faster than the SCR.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

21. GTO Applications Slide 20 • Counters • Pulse generators • Oscillators
• Voltage regulators
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

22. GTO Pin Identification
Slide 21
GTO Pin Identification
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

23. 4. LASCR – Light-Activated SCR
Slide 22
This is an SCR that is fired by a light beam striking the gate to cathode junction or by applying a gate voltage.
Schematic Symbol:
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

24. LASCR Pin Identification
Slide 23
LASCR Pin Identification
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

25. • Optical light controls • Relays • Phase control • Motor control
LASCR Applications
Slide 24
• Optical light controls
• Relays
• Phase control
• Motor control
• Computer applications
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

26. 5. Shockley Diode
Slide 25
The Shockley diode conducts once the breakover voltage is reached. It only conducts in one direction.
Schematic Symbol:
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

27. Shockley Diode Basic Operation
Slide 26
The Shockley diode must be forward biased, and then once the voltage reaches the breakover level it will conduct. Like an SCR it only conducts in one direction.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

28. Shockley Diode Application
Slide 27
• Trigger switch for an SCR
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

29. 6. Diac Slide 28 The Diac is also a breakover type device.
Schematic Symbol:
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

30. Diac Basic Operation Slide 29
Once the breakover voltage is reached the Diac conducts. The Diac, though, can conduct in both directions. The breakover voltage is approximately symmetrical for a positive and a negative breakover voltage.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

31. Diac Applications Slide 30 • Trigger circuit for the Triac
• Proximity sensor circuit
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

32. 7. Triac Slide 31 The Triac is like a Diac with a gate control.
Schematic Symbol:
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

33. Triac Basic Operation Slide 32
When fired by the gate or by exceeding the breakover voltage, the Triac conducts in both directions.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

34. Triac Pin Identification
Slide 33
Triac Pin Identification
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

35. Triac Applications Slide 34 AC power control circuits
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

36. Part II: Other Devices Slide 35 1. UJT – Unijunction Transistor
2. Phototransistors
3. Opto-Isolators
4. PUT – Programmable UJT
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

37. 1. UJT – Unijunction Transistor
Slide 36
The UJT is also basically a switching device.
Schematic Symbol:
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

38. UJT Basic Operation Slide 37
Even though the UJT is a switching device it works very differently from the SCR variety of devices.
The equivalent circuit indicates that the UJT is like a diode and a resistive voltage divider circuit. The resistance exhibited by RB1 is variable; it is dependent on the value of current IE.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

39. UJT Characteristic Curve
Slide 38
A voltage is applied across the UJT (VBB) and to the Emitter input (VE). Once VE reaches a peak value (Vp) the UJT begins to conduct. At the point where VE = Vp, the 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 phenomenon occurs because the internal resistance labeled RB1 in the equivalent circuit decreases as the UJT conducts more and more. This is called negative resistance.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

40. UJT Characteristic Curves (cont’d)
Slide 39
As VBB increases so does the VE threshold voltage that is necessary to put the UJT into conduction.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

41. UJT Vp Voltage
Slide 40
The value of VE necessary to put the UJT into conduction is called Vp.
Vp can be calculated:
Vp = VBB
Where  is the intrinsic stand-off ratio, and its value is available from the specification sheet of the UJT.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

42. UJT Pin Identification
Slide 41
UJT Pin Identification
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

43. UJT Applications Slide 42
The UJT is used almost exclusively as a trigger circuit for SCRs.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

44. UJT SCR Trigger Waveform
Slide 43
The UJT waveform to the SCR gate is almost a sawtooth like oscillator output. Hence this trigger circuit is sometimes called a relaxation oscillator. The rate at which the waveform repeats depends on the capacitor value, the external resistor, and .
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

45. 2. Phototransistor Slide 44
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.
Schematic Symbol:
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

46. Phototransistor Pin Identification
Slide 45
Phototransistor Pin Identification
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

47. Phototransistor Applications
Slide 46
• Punch-card readers
• Lighting control
• Level indication
• Relays
• Counting system
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

48. 3. Opto-Isolators
Slide 47
This is a package that can be integrated on an IC that contains both an infrared LED and a photodetector.
Pinout of an IC opto-isolator IC:
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

49. Varieties of Opto-Isolators
Slide 48
Opto-isolator circuits can be diodes, diode-Darlington pair, or diode-SCR:
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

50. 4. PUT – Programmable UJT Slide 49
The PUT is more like an SCR in some of its operating characteristics, than a UJT.
Schematic Symbol:
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

51. PUT Characteristics Slide 50
Like the UJT, the PUT has a negative resistance region. But this region is unstable in the PUT. The PUT is operated between the on and off states.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

52. PUT Operation
Slide 51
The gate voltage required to turn the PUT on is determined by external components, instead of by specifications of the device as in the  value for the UJT.
[Formula 20.19]
Reducing or removing the gate voltage will not turn the PUT off. Instead, like an SCR, the Anode to Cathode voltage must drop sufficiently to reduce the current below a holding level.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.

53. Applications of the PUT
Slide 52
The PUT is used as a trigger device for an SCR. Like the UJT, a relaxation oscillator circuit is used to trigger the PUT, which then fires the SCR.
Robert Boylestad Digital Electronics
Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved.