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

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

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.

Hi Roy,

Constant current LEDs have been around since the late 70s. They were
marginally common then, but I rarely see them now.

As for the spectrum of LEDs, the all require different battery
voltages to bring them to full output. Light power is always
expressed as a function of current, not voltage however. The voltage
is somewhat likened to the forward conduction knee.

IR LEDs have the lowest knee voltage, roughly at the same potential as
common diodes: 0.6V. Blue LEDs have the highest knee voltage, roughly
3V to 3.5V. The colors Red, Orange, Yellow, Green, Aquamarine, Blue,
UV exhibit the order of ascending voltage (and in the conventional
order of wavelength for a rainbow). The White LED is simply a Blue or
UV LED coated with phosphor (a solid state fluorescent light).

73's
Richard Clark, KB7QHC

On Fri, 29 Apr 2005 00:37:12 -0700, "John Smith"
wrote:

I wonder if a LED is not "strobed", either occuring as a natural property of
the LED itself, or circuitry incorporated on the LED chip, itself, which
"strobes" it?

Hi Brett,

The voltage supplied to the LED elevates the electron out of one
orbital to the conduction band. When it falls back, a photon is
emitted. The wavelength of the emitted photon is the path length the
electron follows spiraling from one orbital to the other (DeBroglie
wave).

There are no lasers that are pumped in a cyclic sense except those
that emit a pulse like the old CO2 UV lasers. I had a buddy who built
one that used plate glass and aluminum foil to build the high voltage
charge used to excite the gas to lasing (you still need an optically
resonant chamber to build the intensity).

This design, from the pages of Scientific American's Amateur Scientist
column exhibited a very high peak power because of the extremely short
pulse duration. His knowing this instilled the caution to aim it out
the window into free space for its inaugural firing. When he pulsed
it he cracked the window. Window glass is not perfectly transmissive,
and the high peak power and short interval conspired to create a very
hot dislocality - the glass couldn't shed the heat fast enough and it
cracked.

73's
Richard Clark, KB7QHC

Phosphor? Wow, that fact had missed me totally...
The lifetime rating on those white LEDs are incredible!!!
From seeing crt tubes degrade over time, I'd expect there to be some sort of
problem with the phosphor over such a long lifetime? A crt fails/dims due
to air entering the tube, possibly?

regards,
john

Richard Clark wrote:
The wavelength of the emitted photon is the path length the
electron follows spiraling from one orbital to the other (DeBroglie
wave).

Actually, I think it's more of a quantum thing - a function of the band
gap energy E, which is equal to hv (read h nu) where h is Planck's
constant. Wavelenth is of course related to v by a constant, c (speed
of light). Lambda = c/v. So wavelength = hc/E. If it was a DeBroglie
thing, wavelength would depend on the momentum of the particle as h/p.
The band gap energy is not determined by the energy carried by the
particle, (or by the length of its path), but by the atomic nature of
the material comprising the LED, and the difference in energy between
the conduction band and the valence band of that material.

ac6xg

Probably... and those of us who have taken calculus and spent years of our
life closely working with mathematicians (or, those truck drivers who had an
uncanny interest in math and have studied it in great detail--just for
self-satisfaction) grasp the general concept--however, a wise-intelligent
individual tries to express the complex in terms where the greatest numbers
can make use of the knowledge, especially, when debates/arguments and proofs
are offered in such a public forum as usenet--here, your explaination
fails... I would venture that to most--this looks like nothing more than you
stroking your ego...

Regards,
John

On Fri, 29 Apr 2005 09:05:27 -0700, "John Smith"
wrote:

A crt fails/dims due
to air entering the tube, possibly?

Hi Brett,

I've seen air enter the tube - perhaps, but there are more
conventional explanations. Those episodes of "seeing" it enter were
slightly less than catastrophic (accidentally broke the final seal,
which, as it happens, is the right way to de-activate them).
Principally it is the oxidation of the cathode (one reason why tubes
have scavengers) from what air had remained from the production cycle.

Back when I did TV repair as a teen, and then later in the Navy as
part of a Christmas charity project, we used what was called a
"rejuvenator."

This piece of gear would apply several hundred to a thousand volts
between the cathode and the first grid (usually a cylindrical shield
capped with a plate with a pinhole in it) and hold the voltage there
for a couple of seconds (manufacturer's recommended time). This would
produce a current that would rip the surface of the cathode open
revealing a new emitting area and the tube would come to life for
another 6 months.

Well, during the extended project in the Navy, we found any number of
tubes that refused this Lazarus treatment. We simply held the button
down for 30-60 seconds as they sizzled. Who cared? It often
convinced the tube that there was more to life than consignment to the
dumpster (the guys loved to give them the heave-ho to thundering
explosion). The only downside to this aggressive treatment was that
in ripping open the surface, part of the current was a debris stream
towards the grid that would occlude the pin hole. Sort of self
defeating when you think about it (and good reason for the recommended
couple of seconds instead of our heavy thumb).

All rather remote possibilities for the White LED. LEDs suffer other
problems in commercial service. Usually with aging where the
half-life is quicker than expected. Another problem is with what is
called sun bleaching (usually the problem of LED traffic lights). I
would suspect either, or both, to be issues with vendors and poor QA
or selection. There are a world of "ultra-bright" specifications that
wander the map.

73's
Richard Clark, KB7QHC

On Fri, 29 Apr 2005 12:47:18 -0700, Jim Kelley
wrote:
Richard Clark wrote:
The wavelength of the emitted photon is the path length the
electron follows spiraling from one orbital to the other (DeBroglie
wave).

Actually, I think it's more of a quantum thing

Hi Jim,

The De Broglie [my spelling error] wave is a quantum thing.

In fact Louis de Brolie presented in 1924 that MATTER was composed of
discrete wavelengths (quanta). His was the identical treatment of
Einstein's proposition with light. Both he and Einstein had revealed
the duality of nature, and De Broglie's assertions were proven in
1927. The effect of his work left Schrodinger with the starting point
for his development of quantum mechanics.

73's
Richard Clark, KB7QHC

Damn...
That is all silly...
But, hey, does that light bulb in the refrig really go out when I close the
door? Or, is it too subject to the quantum laws, and it only depends on an
observer? grin

Regards,
John

On Fri, 29 Apr 2005 14:30:01 -0700, "John Smith"
wrote:
But, hey, does that light bulb in the refrig really go out when I close the
door? Or, is it too subject to the quantum laws
Only if Schrodinger's cat was inside to watch.