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.
Describe and Analyze:
When emitter is forward biased, channel switches from high resistance to low resistance.
A PUT is actually a thyristor that acts like a UJT.
Its breakover point is set by a voltage divider.
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
UJT oscillator supplies trigger pulses.
This device can be triggered OFF as well as ON.
A low-power device similar to a GTO.
Behaves like an SCR that triggers itself. Vbo is low.
AC version of a Shockley Diode.
A TRIAC is the AC equivalent of an SCR.
Trigger polarity changes to match AC polarity.
Trigger timing controlled by RC time constant.
Rule 1: BE CAREFUL!