CHAPTER 17: Thyristors (4-Layer Devices) doc

– Use low input power to control large load currents.. • Unijunction transistors UJTs are not thyristors, but are commonly used with SCRs... Unijunction Transistors UJTsUJTs • Before lo

CHAPTER 17

Thyristors (4-Layer Devices)

Describe and Analyze:

• SCRs & Triacs

• Shockley diodes & Diacs

• Other 4-Layer Devices

• UJTs

• Troubleshooting

• 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

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

Once it is ON, it conducts until current is interrupted

Switches from OFF to ON instantly when triggered

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

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 V bo is low

AC version of a Shockley Diode

A TRIAC is the AC equivalent of an SCR

Trigger polarity changes to match AC polarity

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

• 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