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Thyristors
25 questions By Tony R. Kuphaldt
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Question 4 of 25
Silicon-controlled rectifiers (SCRs) may be modeled by the following transistor circuit. Explain how this circuit functions, in the presence of and absence of a “triggering” voltage pulse at the gate terminal:
Reveal answerThe positive feedback intrinsic to this circuit gives it hysteretic properties: once triggered “on,” it tends to stay on. When “off,” it tends to stay off (until triggered).
Notes:Have students demonstrate the positive feedback “latching” action of this circuit by drawing directions of current on a diagram for the class to see (on the whiteboard, in view of everyone). Ask your students why the circuit “waits” until a triggering pulse to turn on, and why it “latches” on once triggered.
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Question 5 of 25
Shown here is an illustration of a large “stud mount” type of SCR, where the body is threaded so as to be fastened to a metal base like a bolt threads into a nut:
With no test instrument other than a simple continuity tester (battery and light bulb connected in series, with two test leads), how could you determine the identities of the three terminals on this SCR?
Hint: The threaded metal base of the SCR constitutes one of the three terminals.
Reveal answerThe smallest terminal (on top) is the gate. The identities of cathode and anode may be determined by connecting one test lead to the gate terminal, and touching the other test lead to either of the other terminals.
Notes:Ask your students how they know the gate terminal is the smallest one. Why would it be the smallest? Does it have to be the smallest terminal? Why? Also, ask them what continuity indication would distinguish cathode from anode in the continuity test described in the answer.
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Question 6 of 25
Explain what happens in each of these circuits when the pushbutton switch is actuated and then released:
Reveal answerThe SCR circuit’s lamp will energize when the switch is actuated, and remain on after the switch is released. The TRIAC circuit’s lamp will energize when the switch is actuated, and immediately de-energize when the switch is released.
Follow-up question: explain why these circuits do not behave identically. Aren’t SCRs and TRIACs both thyristor (hysteretic) devices? Why doesn’t the TRIAC remain in the “on” state after its triggering voltage is removed?
Notes:This question addresses a very common misunderstanding that students have about TRIACs in AC circuits. Students often mistakenly think that TRIACs will latch AC power just like an SCR latches DC power, simply because the TRIAC is also a hysteretic device. However, this is not true!
One might be inclined to wonder, of what benefit is the TRIAC’s hysteresis in an AC circuit, then? If latching is impossible in an AC circuit, then why have TRIACs at all? This is a very good question, and its answer lies in the operation of a TRIAC within the timespan of an AC power cycle, which is much faster than human eyes can see.
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.