Why aren't many / most LED light bulbs dimmable?

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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?
They're crazy-priced as is. Not being able to dim them makes them even less desirable as a replacement for incandescent bulbs.
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This is new, last 3 years. They had to develop a ballast that was adjustable. CFL's and LEDS are not a good comparison.

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.
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On 12/24/2010 07:25 AM, G. Morgan wrote:

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.
nate
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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.
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wrote:

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.
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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.
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wrote:

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.
--
Tegger

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wrote:

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.
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On Fri, 24 Dec 2010 15:02:02 -0500, snipped-for-privacy@snyder.on.ca 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.
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On Fri, 24 Dec 2010 15:32:11 -0600, " snipped-for-privacy@att.bizzzzzzzzzzzz"

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.
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On Fri, 24 Dec 2010 18:24:52 -0500, snipped-for-privacy@snyder.on.ca wrote:

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

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.
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On Fri, 24 Dec 2010 17:35:37 -0600, " snipped-for-privacy@att.bizzzzzzzzzzzz"

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 too narrow.
See: http://www.gardasoft.com/uploads/APP930%20Overdriving%20LEDs.pdf 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-vision-imaging-processing/display/vsd-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/products/overdrive-series and http://www.freepatentsonline.com/7639219.html and http://www.parameter.se/products/Default.aspx?ID1 &ID20&ID32&ID45
Are just a very few references for you to look at re: (commercial applications of) pulsed overdrive applications for high luminence LED applications.
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On Sat, 25 Dec 2010 13:30:47 -0500, snipped-for-privacy@snyder.on.ca 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 *WRONG*.
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On Sat, 25 Dec 2010 19:54:58 -0600, " snipped-for-privacy@att.bizzzzzzzzzzzz"

I refuse to argue with an idiot
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On Sun, 26 Dec 2010 00:38:39 -0500, snipped-for-privacy@snyder.on.ca wrote:

As usual, when you're shown to be wrong you run away with fingers in your ears rather than admitting it and learning something. You *are* an idiot.
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wrote:

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.
--
Jim Yanik
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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.
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<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:
http://members.misty.com/don/ledp.html

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 efficacy.

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 second.

For cameras, frame grabbers and machine vision.
<minor snip from here>
--
- Don Klipstein ( snipped-for-privacy@misty.com)

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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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wrote:

Actually, phase control, not PWM. Similar to, but not to be confused with. With a nonlinear load, like an LED, it's quite different.

Completely clueless.
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