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I've been coming up to speed on thyristors, and triacs in particular. I've found detailed material on them a little hard to find, in particular on the device physics. Horowitz & Hill is silent on them, Sedra & Smith as well, not even a mention.

I suppose I get a vague idea that thyristors belong a bit to a past era, but they're still a key component in solid state relays (SSR) for example, so I thought I'd be able to dig up more.

Anyway, one of the questions that's still open for me is why a triac triggers with gate current in either direction (in/out). That seems a bit odd so definitely aroused my curiosity.

enter image description here

Does anyone understand how that bit works?

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    \$\begingroup\$ The Wiki page has a decent start on this, but the explanation of Q4 is a bit lacking. Try to avoid Q4- either drive with the same polarity on the gate as MT2, or always drive it with negative current . \$\endgroup\$ – Spehro Pefhany Apr 6 '16 at 11:31
  • \$\begingroup\$ Oooh, not sure how I missed that resource on search, it looks like just the ticket, thanks Spehro! :) \$\endgroup\$ – scanny Apr 6 '16 at 18:29
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You can consider the triac as two thyristors connected back to back with their gate connections commoned.

enter image description here

On each half cycle the appropriate 'thyristor' is fired. The downside is that the triggering voltages are not symmetric. To improve this a DIAC is used at the gate input.

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