120v light dimming

Professorwiz

I was trying to dim a strand of Christmas lights with a triac setup similar to the sprinkler setup in this post
http://forums.parallax.com/showthread.php?p=667859
I was trying to use the PWM command, but it's not working.· Is it that the triac is not turning all the way off quick enough for that to work?

Russ

Bean

Russ,
The PWM command won't work, because a triac stays on until the AC goes·to zero. So if the PWM output goes off on just after the zero point, the triac will still stay on until the AC reaches zero again.

Basically the PWM needs to monitor the AC zero crossing.

Bean.

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Post Edited (Bean (Hitt Consulting)) : 1/3/2008 9:13:52 PM GMT

JonnyMac

Attached you'll find the [noparse][[/noparse]updated] code and schematic for a project I originally did for the November issue of Nuts & Volts. To Terry's point, you have to monitor the zero-cross and then turn on the triac some time after that -- the longer you wait, the more dim the light will be. The process is explained in the article.

Professorwiz

Thanks, I will get the article. I appriciate it very much.
Russ