Chapter 20 pnpn and Other Devices Robert Boylestad

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 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 (IG 1, IG 2), less forward voltage (VF 1, VF 2, VF 3) 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

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

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

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

Slide 34 Triac Applications AC power control circuits Robert Boylestad Digital Electronics Copyright ©

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 RB 1 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 RB 1 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

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

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.

Slide 51 PUT Operation 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 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.