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As part of my 2330 lv3 am required to know about thyristors. I just want to confirm my understanding is correct. A thyristor is basically similar to a transistor, only it stays switched on when it's trigged for as long as there isn't a break in the circuit it's switching. Am I correct?

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Pretty much correct.

It's not while there is no break in the circuit, but while the thyristor is forward biased.

So, while there is a positive voltage potential across the Anode/Cathode terminals.

That means that if an AC signal is fed through it the thyristor will switch on when the gate is driven high (as long as the AC signal is currently in the positive phase), and stay on until the end of the current positive phase of the signal - when the potential drops to zero or below - thus creating a zero-crossing switch-off.

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You are basically right, but as Majenko said, it's not about there being a break in the circuit but rather whether current keeps flowing.

Here is a simple way to think of how a SCR works:

A SCR isn't just two transistors hooked up like this, but this isn't bad as a mental picture of how one operates. You can even build this and have it work like a SCR as long as you keep the currents low. Note that all current in this circuit ultimately flows thru one of the bases. These don't have much current capability in normal transistors. A SCR is a sandwich of P and N regions such that the "bases" are intended to handle the current.

Anyway, think about this circuit or even build it. Once current starts flowing in one transistor somehow, it turns on the other transistor, which keeps the first transistor on. Once on, this thing will stay on until there is too little current to keep it on.

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Wikipedia has a detailed yet simple explanation of what a thyristor is.

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    \$\begingroup\$ Just a link is not a good answer. \$\endgroup\$
    – stevenvh
    Apr 2 '12 at 15:01

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