Transcript Fundamentals of Linear Electronics Integrated & Discrete
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.