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Thyristors
25 questions By Tony R. Kuphaldt
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Question 7 of 25
Explain what must be done to the SCR to make it turn on and send power to the light bulb:
Next, explain what must be done to turn the SCR off so that the light bulb de-energizes.
Reveal answerI’ll let you research the answers to these questions!
Notes:Let students explain (or perhaps even demonstrate) their answers. It is extremely important for students to realize that SCR’s are thyristors, “latching” on and off with transient stimuli. They differ significantly from transistors in this regard.
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Question 8 of 25
When an SCR latches “on,” it drops very little voltage between anode and cathode. Explain why this is, and what advantage this gives SCRs over transistors when conducting heavy load currents.
Reveal answerThe constituent transistors of an SCR become driven heavily into saturation in its conductive state, with a bare minimum of drive (gate) current necessary.
Follow-up question: how does the internal operation of an SCR explain its very fast turn-on time, in addition to explaining its low conducting voltage drop?
Notes:The key to fully answering why an SCR drives itself so hard during conduction is found in the principle of positive feedback. Discuss this principle with your students if they have not already studied it. If they have already studied it, use this question as an opportunity for review.
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Question 9 of 25
Explain what a TRIAC is, and how it is at once similar and different from an SCR. What applications might a TRIAC be used in that an SCR would be inappropriate for?
Reveal answerA “TRIAC” functions as two reverse-parallel-connected SCR units, so as to be able to control AC and not just DC.
Follow-up question: draw the equivalent circuit for a TRIAC.
Notes:A popular application for TRIACs is lamp dimmer controls, for line-powered (50 or 60 Hz) incandescent lamps. If time permits, discuss with your students how these lamp dimmer circuits control power to the lamp in a manner reminiscent of PWM (Pulse-Width Modulation).
Can someone please explain why do we need C1 capacitor in question 22 ? Circuit behaves exactly the same without it and I am confused about it’s purpose.