I'm planning on building this little device to reduce output voltage
on 18v LCD power packs down to 12v.
http://www.national.com/mpf/LM/LM350.html#General Description
What wattage rating do I need for those two resistors?
They are just programming resistors, with high enough resistance that
they pull neglible current.
1/4 watt ought to work fine.
But you know, have you considered learning switching supplies?
(instead of linear).
Pretty soon, you won't even be able to sell linear widgets in
California (USA)
The problem is they waste power due to their inherent low efficiency.
Note: Several power-plant's worth with enough linear devices
considered!!!
So why not bite the bullet and tackle a switching design?
I suggest the LM25576, which you can design online here:
http://www.national.com/pf/LM/LM25576.html
If you can settle for 1-amp (instead of 3A output), the LM2575T is a
very simple design.
For 12 volts, they even have one that has the programming resisitors
already set inside the package. But for the variable one, you would
still add the 2 programming resistors as above.
Anyway, I think the LM2575T is ideal if you want to learn about
switching supplies.
At 52 kHz, it's easy enough to scope, and layout on a PCB without too
much noise trouble. You'll need a coil and a couple filter caps. And
a fast Shottkey diode, but that's about it.
Good luck.
-mpm
