On Mon, 27 Dec 2010 16:52:33 -0600, Jim Yanik wrote:

" wrote in
:

On Sun, 26 Dec 2010 20:25:53 -0500, wrote:

On Sun, 26 Dec 2010 17:21:21 -0600, "
wrote:

You keep talking about this waste heat like we were dissipating
hundreds of watts. I only say rheostat to define a single ended
pot. If the ballast resistor is 1/2w or even 1/4w why would the pot
need to be any bigger?

You're not talking about illumination at these power levels.

I suppose that depends on what kind of illumination you are talking
about. If you mean replacing a couple hundred watts of incandescent, I
tend to agree, absent of more knowledge but if you are talking about
accent lighting, what you likely want to dim, I am not sure you need a
whole lot more than the equivalent of a few of these flashlights.

I thought you wanted task lighting under kitchen cabinets. That has
to be quite bright, even in a well lit room. A couple of watts isn't
going to cut it.

the usual "bright white" LED found in these flashlights is a ~0.1W light.
~3.5 @20ma.

I don't know how a 120v bulb works but I can guess based on my
flashlight investigation.

Series strings of LEDs with a ballast.

These things are very bright at 26.6ma in this flashlight.

26ma seems a bit high.

If you had
about 25 of them in series (dropping ~112.5v) and a resistor 300-350
ohms you would be dropping around a quarter of a watt across it with a
3.1w total package. Putting a 1k pot in series with this is not going
to be that wasteful even if it does offend your engineering
sensibilities. You don't like the flashlight either but there are
millions of them out there working just fine.

don't forget that 120VAC rectifies out to ~170 VDC,not 120 VDC.

Only with a capacitor filter. It'll still have a 170V peak but the RMS will
still be ~120V. The current waveform will be messed, though.

for that string and a 350 ohm resistor,your LEDs are not going to last more
than a millisecond.
BTW,the "bright white" T1-3/4 LEDs work out to ~3.5v forward voltage.

I don't like them because some day someone is going to put the wrong
battery in the things.

not gonna happen;only 3 AAA cells will fit,there's a plastic battery holder
inside the flashlight barrel.

In *your* freebie HF flashlight, perhaps. In the case of the HF flashlights,
the zinc cells may be the worst case. I don't see evidence that this is the
case in every flashlight out there.