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Understanding Thyristors: SCR, Triacs, and More in Industrial Applications

Learn about 4-layer thyristors including SCR, Triacs, UJTs, Shockley Diodes, Diacs, and troubleshooting steps on how to analyze and control large load currents using low input power in industrial power and motor control applications.

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Understanding Thyristors: SCR, Triacs, and More in Industrial Applications

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  1. CHAPTER 17 Thyristors (4-Layer Devices)

  2. Objectives Describe and Analyze: • SCRs & Triacs • Shockley diodes & Diacs • Other 4-Layer Devices • UJTs • Troubleshooting

  3. 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.

  4. 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.

  5. UJTs When emitter is forward biased, channel switches from high resistance to low resistance.

  6. Programmable UJT (PUT) A PUT is actually a thyristor that acts like a UJT. Its breakover point is set by a voltage divider.

  7. Silicon Controlled Rectifiers Two-transistor model of an SCR

  8. SCRs Once it is ON, it conducts until current is interrupted.

  9. SCRs Switches from OFF to ON instantly when triggered.

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

  11. 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.

  12. SCR Motor Control UJT oscillator supplies trigger pulses.

  13. Gate-Turnoff SCR (GTO) This device can be triggered OFF as well as ON.

  14. Silicon Controlled Switch:SCS A low-power device similar to a GTO.

  15. Shockley Diodes Behaves like an SCR that triggers itself. Vbo is low.

  16. DIACS AC version of a Shockley Diode.

  17. TRIACs A TRIAC is the AC equivalent of an SCR.

  18. TRIACs Trigger polarity changes to match AC polarity.

  19. TRIACs TRIACS: • Small ones used in light dimmer circuits. • Can control series-wound DC motors. • Commutation is “provided free” by the AC current.

  20. Light Dimmer using a TRIAC Trigger timing controlled by RC time constant.

  21. 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.

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