John Deans wrote:
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Some years ago I designed a stage lighting dimmer (I still use them). I found that at the end and beginning of the mains cycle the load current was too low to hold the tric on


That is rather the point, ANY triac turns off at zero current - its how they work ! The triac will conduct in both directions, but only when it is triggered. An SCR won't, since it must block in one direction.

There are always miniumum load specs on electronically commutated things like this.
Look at all the X10 stuff.

Steve