Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River,...

Robert BoylestadDigital Electronics

Copyright ©2002 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458

All rights reserved.

Chapter 20: pnpn and Other Devices




Part I: pnpn Devices

1. SCR – Silicon-Controlled Rectifier2. SCS – Silicon-Controlled Switch3. GTO – Gate Turn-Off Switch4. LASCR – Light-Activated SCR5. Shockley Diode6. Diac7. Triac




1. SCR – Silicon-Controlled Rectifier

The SCR is a switching device for high voltage and current operations.

Schematic Symbol:




SCR Basic OperationTurning - 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.




SCR Basic OperationForced - on

An SCR can be forced on by

•Excessively high voltage from anode to cathode• High frequency signal from gate to cathode• High temperatures




SCR Basic OperationTurning - 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.

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.

Slide 6




SCR Basic OperationCommutation




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.




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!!




SCR Terminal Identification

The SCR has three terminal Anode (A), Cathode (K), and the Gate (G).




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 half-wave rectifier.




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.




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.




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:




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.




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.




SCS Applications

• Pulse generator• Voltage sensor• Alarm circuits




SCS Pin Identification




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:




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.




GTO Applications

• Counters• Pulse generators• Oscillators• Voltage regulators




GTO Pin Identification




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:




LASCR Pin Identification




LASCR Applications

• Optical light controls• Relays• Phase control• Motor control• Computer applications




5. Shockley DiodeThe Shockley diode conducts once the breakover voltage is reached. It only conducts in one direction.

Schematic Symbol:




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.




Shockley Diode Application

• Trigger switch for an SCR




6. Diac

The Diac is also a breakover type device.

Schematic Symbol:




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.




Diac Applications

• Trigger circuit for the Triac• Proximity sensor circuit




7. Triac

The Triac is like a Diac with a gate control.

Schematic Symbol:




Triac Basic Operation

When fired by the gate or by exceeding the breakover voltage, the Triac conducts in both directions.




Triac Pin Identification




Triac Applications

AC power control circuits




Part II: Other Devices

1. UJT – Unijunction Transistor2. Phototransistors3. Opto-Isolators4. PUT – Programmable UJT




1. UJT – Unijunction Transistor

The UJT is also basically a switching device.

Schematic Symbol:




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.




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.




UJT Characteristic Curves (cont’d)

As VBB increases so does the VE threshold voltage that is necessary to put the UJT into conduction.




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.




UJT Pin Identification




UJT Applications

The UJT is used almost exclusively as a trigger circuit for SCRs.




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 .




2. PhototransistorThis 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:




Phototransistor Pin Identification




Phototransistor Applications

• Punch-card readers• Lighting control• Level indication• Relays• Counting system




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:




Varieties of Opto-Isolators

Opto-isolator circuits can be diodes, diode-Darlington pair, or diode-SCR:




4. PUT – Programmable UJT

The PUT is more like an SCR in some of its operating characteristics, than a UJT.

Schematic Symbol:




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.




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.

BBBBB2B1

B1G ηVV

RR

RV




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,...

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