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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 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.