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CHAPTER 17. Thyristors (4-Layer Devices). Objectives. Describe and Analyze: SCRs & Triacs Shockley diodes & Diacs Other 4-Layer Devices UJTs Troubleshooting. Introduction. Thyristors: Are 4-layer silicon semiconductors. Use low input power to control large load currents.

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Chapter 17

CHAPTER 17

Thyristors

(4-Layer Devices)


Objectives
Objectives

Describe and Analyze:

  • SCRs & Triacs

  • Shockley diodes & Diacs

  • Other 4-Layer Devices

  • UJTs

  • Troubleshooting


Introduction
Introduction

  • Thyristors:

    • Are 4-layer silicon semiconductors.

    • Use low input power to control large load currents.

    • Are very common in industrial power & motor control.

    • Are inherently nonlinear devices.

    • Have two states: ON and OFF.

  • Unijunction transistors (UJTs) are not thyristors, but are commonly used with SCRs.


Unijunction transistors ujts
Unijunction Transistors (UJTs)

UJTs

  • Before looking at SCRs, we will look at UJTs. A UJT is a “one trick pony”: its only common application is to provide trigger pulses to SCRs.

  • A simple relaxation oscillator can be made with a UJT, a capacitor, and a potentiometer.


UJTs

When emitter is forward biased, channel switches from high resistance to low resistance.


Programmable ujt put
Programmable UJT (PUT)

A PUT is actually a thyristor that acts like a UJT.

Its breakover point is set by a voltage divider.


Silicon controlled rectifiers
Silicon Controlled Rectifiers

Two-transistor model of an SCR


SCRs

Once it is ON, it conducts until current is interrupted.


SCRs

Switches from OFF to ON instantly when triggered.


SCRs

Sizes range from I < 1A to I > 1000A.


SCRs

SCR Motor Control

  • A major application of SCRs is to control DC motors.

  • SCRs, like all thyristors, need to be “commutated”, meaning interrupting the flow of current.

  • An SCR motor control typically uses a full-wave rectifier without filtering the DC.

  • When the pulsating DC goes to zero, the SCR turns off until it’s triggered again.

  • SCRs conduct current in one direction only: they are DC devices.


Scr motor control
SCR Motor Control

UJT oscillator supplies trigger pulses.


Gate turnoff scr gto
Gate-Turnoff SCR (GTO)

This device can be triggered OFF as well as ON.


Silicon controlled switch scs
Silicon Controlled Switch:SCS

A low-power device similar to a GTO.


Shockley diodes
Shockley Diodes

Behaves like an SCR that triggers itself. Vbo is low.


Diacs
DIACS

AC version of a Shockley Diode.


Triacs
TRIACs

A TRIAC is the AC equivalent of an SCR.


Triacs1
TRIACs

Trigger polarity changes to match AC polarity.


Triacs2
TRIACs

TRIACS:

  • Small ones used in light dimmer circuits.

  • Can control series-wound DC motors.

  • Commutation is “provided free” by the AC current.


Light dimmer using a triac
Light Dimmer using a TRIAC

Trigger timing controlled by RC time constant.


Troubleshooting
Troubleshooting

Rule 1: BE CAREFUL!

  • Industrially, thyristors are used in high-power circuits, often with high voltages. SCRs and TRIACs commonly use a 110 Volt or 220 Volt AC main’s power without an isolation transformer.

  • Check to see if the trigger circuit is providing pulses properly.

  • If trigger pulses do not look right, disconnect them from thyristor and look again.

  • If the trigger pulses look good, the thyristor may be defective.


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