On Mon, 27 Dec 2010 07:17:59 -0600, Jim Yanik wrote:

" wrote in
:

On Sun, 26 Dec 2010 15:17:19 -0500, wrote:

On Sun, 26 Dec 2010 11:49:47 -0600, Jim Yanik
wrote:

LEDs can pop very fast when overdriven.(as the rheostat guy will
discover)

Since the rheostat will be in series with the proper "full load"
ballast resistor, overdriving is not an issue. Current is going to be
dropping as a square of the change in resistance and so will the heat.
As long as the rheostat is rated as high as the ballast resistor,
where is the possibility of a problem?

High power rheostats aren't all that common, or cheap, these days.

I suppose the open question is, how big is the resistor in a
commercial lamp?

I haven't seen a lamp with a rheostat is decades.

In my case it doesn't matter at all since I will be driving mine from
a wall wart. I am tempted to just take a cheap 4.5v LED flashlight
apart (a couple bucks at H.D) . That is probably cheaper than I can
buy 9 LEDS and for my purposes, it provides plenty of light. I just
want to spread them out a bit. I even have a 4.5v wall wart.

If you're powering this thing with a DC wall wart, simply PWM the
thing. For less than a buck or two in parts, you're all done, and you
can forget about the rheostat and waste heat.

his wallwart may burn them out,if it can supply more current than the LEDs
can handle;there's no limiting resistor.

Just gotta control the PWM better. ;-) A PWM current source is typically
used.