What is Optothyristors

Chapter 7.7 Optothyristors

Lessons In Electric Circuits Volume III – Semiconductors Book
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Lessons In Electric Circuits Volume III – Semiconductors Book

  • 346CHAPTER 7. THYRISTORS• TRIAC controls are more often seen in simple, low-power circuits than complex, high-power circuits. In large power control circuits, multiple SCRs tend to be favored.• When used to control AC power to a load, TRIACs are often accompanied by DIACs con-nected in series with their gate terminals. The DIAC helps the TRIAC fire more symmet-rically (more consistently from one polarity to another).• Main terminals 1 and 2 on a TRIAC are notinterchangeable.• To successfully trigger a TRIAC, gate current must come from the main terminal 2(MT2)side of the circuit!7.7OptothyristorsLike bipolar transistors, SCRs and TRIACs are also manufactured as light-sensitive devices,the action of impinging light replacing the function of triggering voltage.Optically-controlled SCRs are often known by the acronym LASCR, or Light ActivatedSCR. Its symbol, not surprisingly, looks like Figure 355,7.44.Light Activated SCRLASCRFigure 7.44:Light activated SCROptically-controlled TRIACs don’t receive the honor of having their own acronym, but in-stead are humbly known as opto-TRIACs. Their schematic symbol is shown in Figure 355,7.45.Opto-TRIACFigure 7.45:Opto-TRIACOptothyristors (a general term for either the LASCR or the opto-TRIAC) are commonlyfound inside sealed ”optoisolator” modules.7.8The Unijunction Transistor (UJT)Unijunction transistor: Although a unijunction transistor is not a thyristor, this device cantrigger larger thyristors with a pulse at base B1. A unijunction transistoris composed of a bar