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
They're crazy-priced as is. Not being able to dim them makes them even
less desirable as a replacement for incandescent bulbs.
sure they are, I had a LED dash light kit in my old 944. It didn't dim
linearly like the incandescents though, so a slightly different dimmer
would be required. I thin kthe difficulty is with dimming with AC.
replace "roosters" with "cox" to reply.
Diodes have two states, off and on. I couldn't see a practical
way to dim them, without fooling the brain by frequency of the
light. That won't work well either, I may be able to see 3000
fps with my eyes, but you would need a high-speed camera and light
to control constantly changing lighting environments.
You probably had an adjustable current device <potentiometer> that
would limit DC current... That would explain the non-linear
aspect side of the theory. Perhaps there were more LED's that you
knew about. I could easily see Porsche make a multi-diode lamp.
You need to do more research on diodes, especially the LED types. They do
not have two states like you mentioned. They are non linear devices. Also
LED are not dropping .7 volts as you mentioned. The .7 volts is a nominal
voltage for most silicon diodes only. Most LEDs drop differant voltages.
They range from about 1.5 to 4.5 volts. LEDs are current dependant and
not so much voltage. The more current through them, the brighter they are.
The current must be limiated to prevent burn out.
If a resistor is placed in series with a led, the voltage can be raised and
lowered to change the brightness. This is because the resistor is in part
controlling the current.
They can be pulse controled also. The human eye is not fast enough to
respond to fast changing lights. It will tend to average the brightness.
That is the way the moving pictuers work.
LEDs arent dimable in the normal since of the word. The current to
them can be pulse width modulated to produce what we percive as a
dimming of the light. This is accomplished by switching the diode off
and on at a high enough rate that our eyes cant see the flicker.
Dimming is controled by changing the ratio of off to on time. You may
have noticed that LED Christmas tree light are a good bit dimmer than
the typical LED lamp found on an appliance like the computer you are
working at now. This is because the computer supplies a constant DC
voltage and the LED is on 100% of the time while the Christmas tree
lights are powered by a varying AC voltage that will only turn them on
for about 40% of the time.
I suppose you could simulate dimming by adjusting the pulse rate, the way
some automakers do it with their LED tail/brake lights. But that would
probably cause flickering at the lower rates.
Could you simply have a dimmer illuminate more or fewer LEDs as the dimmer
is turned up or down? Perhaps the voltage from the dimmer switch could be
used as a signal by the LED assembly, which would interpret it as a command
to turn-on or turn-off LEDs.
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.
The same can be done for "overdriving" an LED. Shourt duration pulses
can significantly increase the visible light output without
overheating the junction.
On Fri, 24 Dec 2010 15:02:02 -0500, firstname.lastname@example.org wrote:
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.
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.
On Fri, 24 Dec 2010 15:32:11 -0600, " email@example.com"
MANY high output led applications are pulsed "overdrive"
applications, and believe me, they DO put out a LOT more light.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.
On Fri, 24 Dec 2010 17:35:37 -0600, " firstname.lastname@example.org"
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.
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
and http://www.gardasoft.co.uk /
Are just a very few references for you to look at re: (commercial
applications of) pulsed overdrive applications for high luminence LED
On Sat, 25 Dec 2010 13:30:47 -0500, email@example.com wrote:
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.
No, you're just wrong. It's not the first time.
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.
As I've shown with the first article, pulsing LEDs is a loser. You're simply
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.
<Significantly edited for space>
<SNIP back-and-forth leading to this>
There is a major myth about peak rather than average light output of
LEDs determines how bright they appear to humans, even when pulsed rapidly
enough to appear continuously on.
I mention its origins and the truth in:
Shows efficiency decreasing with overdrive.
Effectiveness of combining overdriving and pulsing is for machine vision
applications where strobing is suitable.
Mentions usefulness of pulsing, but not for increasing visual luminous
Mentions short pulse overdrive - apparently for strobing, single-pulse
or machine vision applications.
Machine vision is in the name of the link.
Mentions for machine vision.
Stated to be for machine vision lighting.
That is for a visibly strobing application - mentions 10 flashes per
For cameras, frame grabbers and machine vision.
<minor snip from here>
- Don Klipstein ( firstname.lastname@example.org)
Standards light dimmers use PWM, not variable voltage. They should work
great with LEDs if not for the fact that LED's have a power
supply that converts AC power to low voltage DC and which will result
in the same low voltage DC based on the peak voltage in, the same for
1% as for 100%.
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