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Chapter 20:
pnpn and Other Devices
Robert Boylestad
Digital Electronics
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Part I: pnpn Devices
Slide 1
1.
2.
3.
4.
5.
6.
7.
Robert Boylestad
Digital Electronics
SCR – Silicon-Controlled Rectifier
SCS – Silicon-Controlled Switch
GTO – Gate Turn-Off Switch
LASCR – Light-Activated SCR
Shockley Diode
Diac
Triac
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Slide 2
1. SCR – Silicon-Controlled Rectifier
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 07458
All rights reserved.
Slide 3
SCR Basic Operation
Turning - on
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 07458
All rights reserved.
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 07458
All rights reserved.
Slide 5
SCR Basic Operation
Turning - off
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 07458
All rights reserved.
Slide 6
SCR Basic Operation
Commutation
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 07458
All rights reserved.
Slide 7
SCR characteristics
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 07458
All rights reserved.
Slide 8
SCRs and Temperature
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 07458
All rights reserved.
Slide 9
SCR Terminal Identification
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 07458
All rights reserved.
Slide 10
SCR Applications
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 halfwave rectifier.
Robert Boylestad
Digital Electronics
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Slide 11
SCR Applications (cont’d)
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 07458
All rights reserved.
Slide 12
SCR Applications (cont’d)
• 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 07458
All rights reserved.
Slide 13
2. SCS – Silicon-Controlled Switch
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 07458
All rights reserved.
Slide 14
SCS Basic Operation
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 07458
All rights reserved.
Slide 15
Comparison of SCR and SCS
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 07458
All rights reserved.
Slide 16
SCS Applications
• Pulse generator
• Voltage sensor
• Alarm circuits
Robert Boylestad
Digital Electronics
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Slide 17
Robert Boylestad
Digital Electronics
SCS Pin Identification
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Slide 18
3. GTO – Gate Turn-Off Switch
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 07458
All rights reserved.
Slide 19
Comparison of GTO and SCR
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 07458
All rights reserved.
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 07458
All rights reserved.
Slide 21
Robert Boylestad
Digital Electronics
GTO Pin Identification
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Slide 22
4. LASCR – Light-Activated SCR
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 07458
All rights reserved.
Slide 23
Robert Boylestad
Digital Electronics
LASCR Pin Identification
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Slide 24
LASCR Applications
• 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 07458
All rights reserved.
Slide 25
5. Shockley Diode
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 07458
All rights reserved.
Slide 26
Shockley Diode Basic Operation
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 07458
All rights reserved.
Slide 27
Shockley Diode Application
• Trigger switch for an SCR
Robert Boylestad
Digital Electronics
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Slide 28
6. Diac
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 07458
All rights reserved.
Slide 29
Diac Basic Operation
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 07458
All rights reserved.
Slide 30
Diac Applications
• Trigger circuit for the Triac
• Proximity sensor circuit
Robert Boylestad
Digital Electronics
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Slide 31
7. Triac
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 07458
All rights reserved.
Slide 32
Triac Basic Operation
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 07458
All rights reserved.
Slide 33
Robert Boylestad
Digital Electronics
Triac Pin Identification
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Slide 34
Triac Applications
AC power control circuits
Robert Boylestad
Digital Electronics
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Part II: Other Devices
Slide 35
1.
2.
3.
4.
Robert Boylestad
Digital Electronics
UJT – Unijunction Transistor
Phototransistors
Opto-Isolators
PUT – Programmable UJT
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Slide 36
1. UJT – Unijunction Transistor
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 07458
All rights reserved.
Slide 37
UJT Basic Operation
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 07458
All rights reserved.
Slide 38
UJT Characteristic Curve
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 07458
All rights reserved.
Slide 39
UJT Characteristic Curves (cont’d)
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 07458
All rights reserved.
Slide 40
UJT Vp Voltage
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 07458
All rights reserved.
Slide 41
Robert Boylestad
Digital Electronics
UJT Pin Identification
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Slide 42
UJT Applications
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 07458
All rights reserved.
Slide 43
UJT SCR Trigger Waveform
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 07458
All rights reserved.
Slide 44
2. Phototransistor
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 07458
All rights reserved.
Slide 45
Robert Boylestad
Digital Electronics
Phototransistor Pin Identification
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Slide 46
Phototransistor Applications
• 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 07458
All rights reserved.
Slide 47
3. Opto-Isolators
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 07458
All rights reserved.
Slide 48
Varieties of Opto-Isolators
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 07458
All rights reserved.
Slide 49
4. PUT – Programmable UJT
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 07458
All rights reserved.
Slide 50
PUT Characteristics
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 07458
All rights reserved.
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.
VG 
RB1
VBB  ηVBB
RB1  RB2
[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 07458
All rights reserved.
Slide 52
Applications of the PUT
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 07458
All rights reserved.