On Fri, 29 Apr 2005 02:43:31 -0700, Roy Lewallen
wrote:


There is a letter on this very subject in the Pease Porridge column of
the latest issue of "Electronic Design."

Now, that got my attention. I haven't kept up well with LED technology,
but I haven't heard of any LEDs that have a built-in regulator,
switching or otherwise. Out of curiosity, I tested the LED lights I have
with an AM radio, and found the following:

1. A three-LED headlamp with no obvious electronics, fixed intensity,
runs from 3 AAA cells: Quiet, no RFI.
2. Two different LED headlamps, one with 3 and the other 4 LEDs, with
several intensity settings, run from 3 AAA cells: Loud audio whine.
3. Two small UV LED lights, run from 3 button cells: Quiet, no RFI.
4. Newly purchased one-LED flashlight, runs from a single AA cell:
Generates an RF hiss.

This is about what I expected. The #2 headlamps switch the lights on and
off with varying duty cycle to get the different intensities. This can
be seen by rapidly moving the light while on. The switching isn't
visible at the maximum brightness setting but apparently the switcher is
still on. It might be doubling as a regulator. High intensity LEDs seem
to require about 4.5 volts for full brightness, and one cell is nowhere
near enough to light one up without a DC-DC converter. So the #4 light
has to have one. That explains its RFI. Built-in converters seem to be
getting more common; without one, batteries last an incredible length of
time, but at the expense of light intensity that falls dramatically as
the battery discharges, especially during the first part of the
discharge period.

So unless there's some sort of LED defect that can cause RF noise, my
money's still on your having some kind of converter or switch hidden in
your flashlight somewhere. All the lights I have which don't have either
are quiet.

Roy Lewallen, W7EL

John Smith wrote:
Well, you might be right at that (and contrary to how you took my posts--I
DO recognize you as an authority which has been around the block many more
times than I), however, I have had that flashlight apart a half-dozen times,
at least, it cannot be found!

But, it is quite possible the current limiter is right on the chip with the
LEDS and potted with it in the LED lenses... all five LEDS run in
parallel....


Warmest regards,
John