You can turn on a triac or scr for phase angle control but forced commutation to turn off at other than the zero crossing can be a pain. I have done a lot with this over the years for temperature controllers and then as a power controller for a high powered CO2 laser system. 3 phase 480 volt input to a transformer with a triac controller to vary the power output (20KV at 1.5 amps) to adjust beam current.
Larry

> 
> From: "microsoftwarecontrol" <microsoftwarecontrol@yahoo.ca>
> Date: 2005/12/09 Fri AM 09:47:54 EST
> To: "Microcontroller discussion list - Public." <piclist@mit.edu>
> Subject: Re: [EE] Triac as relay slave, production worries
> 
> I worked with triac quite some.
> If I need triac to open at some angle, then I never
> get a stable performance. I guess
> it is due to drive by AC line.
> 
> stabel way to get power percentage control
> still is 0 cross sensoring + pwm driving.
> 
> 

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