On Sun, 26 Dec 2010 11:29:15 -0600, Jim Yanik wrote:

" wrote in
:

On Sat, 25 Dec 2010 13:30:47 -0500, wrote:

On Fri, 24 Dec 2010 17:35:37 -0600, "
wrote:

On Fri, 24 Dec 2010 18:24:52 -0500, wrote:

On Fri, 24 Dec 2010 15:32:11 -0600, "
wrote:

On Fri, 24 Dec 2010 15:02:02 -0500, wrote:

On Fri, 24 Dec 2010 06:25:05 -0600, G. Morgan
wrote:

Home Guy wrote:


I'm seeing more LED lightbulbs turning up on store shelves.

I don't think I've seen one yet that is ok to use with a dimmer
switch.

I can understand why CFL's can't be put on a dimmer - but why
not LED bulbs?

This is new, last 3 years. They had to develop a ballast that
was adjustable. CFL's and LEDS are not a good comparison.

They're crazy-priced as is. Not being able to dim them makes
them even less desirable as a replacement for incandescent
bulbs.


L.E.D.S. Are going to difficult (impossible) to dim. Remember
they are DIODES that only need .7V to illuminate. AFAIK ---
LED's are not dim-able.
Try 3. something volts to light a white LED. The only way to
"dim"
LEDs is to PWM them with variable pulse width/duty cycle. The
dimming range is quite narrow.

Not true at all. Using PWM, or a variable current, you can get a
very substantial dimming range (with less change in color than an
incandescent). It's just a PITA and a phase-control (Triac) wall
dimmer ain't going to do it.

The same can be done for "overdriving" an LED. Shourt duration
pulses can significantly increase the visible light output without
overheating the junction.

No, it doesn't increase the light output at all. You may be able
to see it with less output because a flashing light catches the
eye, but as long as it's a "constant" light output (i.e. not
visibly blinking) the light output of an LED is pretty much a
linear function of the *average* current through it. Flashing of an
LEDm above the eye's critical fusion frequency does not increase
efficiency, rather the opposite. The efficiency of an LED goes
down, at high currents, as it heats.
MANY high output led applications are pulsed "overdrive"
applications, and believe me, they DO put out a LOT more light.

No, they don't, for any reasonable reading of that sentence. The
physics doesn't allow it (the opposite, in fact).

Driving
them steady at those currents would blow them in a matter of
minutes, but pulsed at 15-20% duty cycle at up to 4 or 5 times rated
current they still deliver almost rated lifespan, and, if I remember
correctly,over 5 times the rated light output.

The *average* current is all that matters. The average also does the
heating, so it's a no win to pulse them, other than it's the easier
way to dim them.


As usual you are not "completely right". I won't argue and say you
are wrong in your assertions - but my UNDERSTANDING is that PEAK
current controlls the visible light output, and average current
affects lifespan (due to junction heating). It is not totally linear.

Nope. Current controls the light output. Average current controls
the average light (which the eye detects, integrated over the
"critical fusion frequency"). Yes, peak current controls the peak
light output, if you're detecting peaks, this might be important. It
is certainly *not* if you're looking at it. Above the CFF, human eyes
average the light intensity. Having high peaks with long spaces does
*nothing* to aid perception and in fact reduces efficiency; LEDs are
LESS efficient at high currents. Pulsing LEDs is a lose-lose
proposition.

My experience is obviously different than yours.
As in many other cases, I need to say that just because you haven't
seen it, don't make it wrong or impossible. Just means your scope is
too narrow.

No, you're just wrong. It's not the first time.

See: http://www.gardasoft.com/uploads/APP...ing%20LEDs.pdf

Did you actually *READ* that app note? An overdrive factor of *6*
will produce only 3 times the light (efficiency drops by half).

"The average current must be kept below the current rating for the
LED."

IOW, you can't overdrive it for long.

The table "High Power LEDs" indicates that you can drive the LED up to
5x current for 2ms, with a 10% duty cycle. A 5X current you get 2.5X
the light or ONE HALF the average light output as you would have
gotten if you'd just driven it at 100%, DC. IOW, a loser.

also http://www.lunaraccents.com/educational-LED-driver.html
and http://www.light-speed-tech.com/ltleds.htm
and
http://www.optoiq.com/index/machine-...sing/display/v
sd-article-display/351674/articles/vision-systems-design/daily-product/
intelligent-strobe-driver-safely-controls-led-intensity.html and
http://www.gardasoft.co.uk/ and
http://www.smartvisionlights.com/pro...erdrive-series and
http://www.freepatentsonline.com/7639219.html and
http://www.parameter.se/products/Def...30&ID3=142&ID4
=155

Are just a very few references for you to look at (commercial
applications of) pulsed overdrive applications for high luminence LED
applications.

As I've shown with the first article, pulsing LEDs is a loser. You're
simply *WRONG*.


the high power Cree XR-E LEDs I used for my homemade bike light are spec'd
at 228 lumens at 1 amp,but emit half that(~114 lumens) at only .35amps.
So,they are more efficient at the lower current.

Tell that to "free-lunch" Clare. The datasheet he linked had a 2:1 efficiency
reduction (4x current for only 2x lumens) rather than 30%, but that was by
overdriving them above their average current spec.