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.
– 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)
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)
A PUT is actually a thyristor that acts like a UJT.
Its breakover point is set by a voltage divider.
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
UJT oscillator supplies trigger pulses.
Gate-Turnoff SCR (GTO)
This device can be triggered OFF as well as ON.
Silicon Controlled Switch:SCS
A low-power device similar to a GTO.
Shockley Diodes
Behaves like an SCR that triggers itself. Vbo is low.
DIACS
AC version of a Shockley Diode.
TRIACs
A TRIAC is the AC equivalent of an SCR.
TRIACs
Trigger polarity changes to match AC polarity.
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
Trigger timing controlled by RC time constant.
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.