Hi,
I forgot to mention in this thread that the main reason the dimmer circuit works with only 'one' wire (no neutral) is that the load looks like a very low resistance compared to the trigger circuit, so BEFORE the triac turns on almost all of the line voltage appears across the trigger circuit, giving it plenty of power to operate on. Once the triac turns on, it shorts out the trigger circuit so it no longer gets much voltage, but then it does not need power anymore because AFTER the triac is turned on it stays on and only turns off when the line current goes through zero. So this makes it very clear why the neutral wire is not needed.
If the firing angle is adjusted too long however, the sine line voltage again approaches zero (greater than 90 degrees) so there is not enough line voltage to trigger the triac, so the cheapie circuits wont be able to trigger the triac. That and the fact that they need perhaps 30v to overcome the diac voltage means some firing angles wout be possible.
But the main reason it works at all is because of the former paragraph, the triac stays on once triggered and so does not need power anymore after that to keep it turned on for the duration of the half cycle.