Sorry to come into this thread a little late, but I'm about 225
threads behind on my PICList email.

Anyway, I'm going to be getting into a similar project. Basically I
have some LED ropelight that I want to dim (and control for effects).
Internally the rope is setup as series strings of LEDs (with a current
limiting resistor). Each series string receives 120VDC (the 120VAC
line is fed through a full wave bridge rectifier).

The controller that came with the string is just a cheap christmas
light controller that is then fed through a bridge rectifier. It's
doing phase angle control to accomplish dimming. While it works in
this situation, I'd like to implement PWM as it's a bit easier to
handle computationally than phase control (for me anyways).

However, I have a few issues.
1. I've never done PWM on 120V before. Can anyone recommend what
type/value of semiconductor to use? I'd assume that a bipolar or
darlington transistor won't be the best choice. A reference circuit
would be awesome if someone can point me to something.
2. The individual LED strings are designed to work on 120V, and I
can't change that. Ideally I'd be able to get a source of 120VDC
(steady DC, not oscillating), but I'm not sure of an easy way to get
that. If I just do a simple rectification/cap filtering I'll end up
with a voltage that's much too high.
3. If I indeed have to stick with a 120VDC oscillating source, will my
PWM on it be effective? From my thinking it should be ok as long as my
PWM frequency is high enough.

Thoughts?

Thanks!

Josh
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