DIYbanter - View Single Post - Why aren't many / most LED light bulbs dimmable?

Jim Yanik

" wrote in
:

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

" wrote in
m:

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.

Well,you KNOW this guy is not going to use a PWM circuit with his
wallwart,just a pot(rheostat).

--
Jim Yanik
jyanik
at
localnet
dot com