This pdf shows how to trigger a triac in all four modes of operation. Page 5 shows a drawing of a dc controlled triac. Page 2 says that it is easier to trigger a triac if the trigger current is in the same direction as the MAIN TERMINAL current. (That's what I was calling "K", and yes, getting old sucks.)

I searched for this because some people might not believe I built a DC controlled triac circuit 30 years ago, and it's still working. It was a 723 chip used as a thermostat and dc from the op-amp supplied current to the gate of a triac that energized a 120 volt AC socket where I could plug a heater in.

ps, nice circuit Cdrive. The capacitor sizes will do well at keeping noise off the gate of Q1.

There are a dozen other ways to do this, a differential pair with a capacitor to slowly make it flip states, a 555 timer used as a window comparator, an op-amp instead of a differential pair, and several very imaginative discrete designs. Anything with a switching point and a capacitor to slow it down.

 

This pdf shows how to trigger a triac in all four modes of operation. Page 5 shows a drawing of a dc controlled triac. Page 2 says that it is easier to trigger a triac if the trigger current is in the same direction as the MAIN TERMINAL current. (That's what I was calling "K", and yes, getting old sucks.)

Thanks for posting this. I'm going to have to study and possibly model that concept to get a better understanding.

Yes it does!

 

I really hate to call someone on a mistake. They sometimes want to argue, and I hate to do that even more than I hate pointing out a mistake. Thanks for taking this graciously.

I figured out the DC method by reading datasheets meticulously and repeatedly, and I surely couldn't just tell you to "read the fine procedure", so I finally found a document that explains it better than I could. Happy learning. It keeps happening to me, too.

 

Well I'm not too proud to say I was wrong and that I'm in a state of shock. I made a few spice runs and the results seem to indicate that the polarity between MT1 and the Gate is not relevant. The voltage only needs to exceed its threshold. It doesn't seem to care if it's positive or negative. How could I have missed this for so, so many years???

 

Just think about the number of relays you won't have to use to get a DC circuit to power an AC load!