piclist-bounces@mit.edu wrote:
> So can anyone else offer any advice on PWM on 120VDC before I
> just abandon the idea? I really really don't want to give up on it...

I missed the beginning of this thread, so maybe there is a crucial point
I'm missing, but in theory seems easy enough (OTOH the devil is in the
details)

so 120V dc I assume is generated from 120V ac, fed through a bridge and
suitable cap. then there are a bunch of series LED (probably a current
limiting resistor also, but maybe some other method is used)

If the above is true, then a common IRF840 should do nicely. Using a
PIC or other micro, you'll need to translate the PWM signal to
around 12V-15V or so, as was already stated. 5V won't be enough unless 
you use a logic-level FET. I'm not sure if a logic-level FET is readily
available for higher voltages but I'm not familiar with logic level FETs

so I can't recommend. It's easy enough to level shift in any case with 
some transistors and resistors, or just use a MOSFET driver like 
Microchip's TC4427. Similar drivers are available from TI, International

Rectifier and others.  Add a small series resistor + zener before the
gate. 
For driving just LEDs, you might get away without any drain protection, 
but adding an RC snubber or TVS would be advisable unless you know
better.

Of course to control brightness on LEDs, an adjustable current source
would be the way to go, IMO. There are plenty of examples of current
sources/sinks on the web. You could use filtered PWM to generate a 
voltage reference to the current source/sink. Have the current sink be 
on the low side so you have the LEDs drop the voltage before you get 
to the current sink (also easy for ground reference).



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