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-...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...D3=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*.