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

I've tried all of the ones available - at least four different brands - at my local stores. (at significant cost, I might add.) They ranged from unacceptable (really weird colors when dimmed, made dimmer hum, only dimmed to maybe 50% brightness) to didn't even come close to working as advertised (e.g. instead of dimming they would flicker and hum and eventually shut off - no perceptible difference between them and regular non-dimmable CFLs) This with several year old standard, off the shelf Lutron dimmers.

I've heard tell of one particular dimmable CFL that is supposed to be used with a special dimmer, I forget who makes it, sylvania maybe? haven't cared enough to order them, and they're not readily available in stores. In any case, the combination would be significantly more expensive, if it did work (ever wonder why the demo displays in the home centers don't simply show a CFL on a dimmer so you can try them? If someone had something like that I would consider taking the plunge) than simply shoving an incandescent in the fixture and replacing as necessary. Wouldn't save enough in electricity or lifespan to come close to paying for itself.

nate

... but you are talking about incandescents.

You keep talking about this waste heat like we were dissipating hundreds of watts. I only say rheostat to define a single ended pot. If the ballast resistor is 1/2w or even 1/4w why would the pot need to be any bigger?

Other than toys, I haven't seen an LED lamp, either.

You're not talking about illumination at these power levels.

I've tried all sorts of things for seeing around with little light. My experience says to make use of night vision. A cool white LED can illuminate a room to extent that I can walk around and see everything, using a couple to a few percent as much light as if I used red light.

On a civil note I will point out the problem with phase control is the voltage will drop below the amount to bias the diodes before you get a lot of dimming.

This is what one looks like on a scope.

That's interesting and a bit counterintuitive. I can see fine around the bedroom, at night, with only the backlight of my XM radio but it's quite bright. I guess it's not enough to trigger the iris but still make use of the higher sensitivity wavelengths. OTOH, for astronomical viewing (and submarines ;) one uses red lights.

I suppose that depends on what kind of illumination you are talking about. If you mean replacing a couple hundred watts of incandescent, I tend to agree, absent of more knowledge but if you are talking about accent lighting, what you likely want to dim, I am not sure you need a whole lot more than the equivalent of a few of these flashlights.

I don't know how a 120v bulb works but I can guess based on my flashlight investigation. These things are very bright at 26.6ma in this flashlight. If you had about 25 of them in series (dropping ~112.5v) and a resistor 300-350 ohms you would be dropping around a quarter of a watt across it with a

I am going to get one of these bulbs the next time I see one and we will rewind the experiment at 120v. I think they are at the HD/Lowes. My immediate interest is if they have some kind of voltage regulation. My line voltage cruises in the 123-124 range. I bet they are very bright if they are not regulated. Of course if it is regulated, the pot won't really work until I get down to the regulated voltage but that regulator is wasting power too. If it has a switcher power supply in it, all bets are off. Nothing will really dim it. I bet I can see a switcher on a scope. They are not going to have that good a filter in them. CFL currents are really ugly on a scope. This is a current transformer, looking at a CFL

I was taught (in college) that once break-through voltage is equaled, that's it, on or off. Now I have to learn WTF went wrong in my imaginary circuit if that is true.

KRW is right that there is some difference in the voltage drop across the junction as the current rises., It is not a big difference but it is a difference.(1.79 - 1.96 in the ones I tested over a range of 3ma to 15ma) It could just be heat doing it at the high end. I did notice in my flashlight tests that have no internal esistor that a very small series resistance starts dropping the current very fast so they may be using this "wall" of a heated junction to regulate the current, similar to a nichrome wire.

The only way to get an LED to work is put a resistor in series. Are you sure the (bulb, lamp, LED) was not a 'package'? They sell them both ways, people who want only the LED (specialized), and people who just want it to work. The later is what I think is causing this confusion.

You should see the PWM method. brightness is variable from next to nothing to full bright. They also retain their color while white lights turn yellow using your method.

Jimmie

resistors.http://gfretwell.com/electrical/LED%20Dimming%20experiment.jpg>>

I think most people find the color change to a softer color is part of the reason they want to dim the light. You usually dim accent lights or to turn task light to accent light. Bear in mind in my house I have a bunch or rope lights and 15w sign bulbs for our normal lighting and we use task lighting where we are reading or doing some kind of work. That is why my under cabinet light will probably be the 9 LEDs from a $2 flashlight and on a dimmer. I just want enough light to mix a drink. ;-) The light follows you wherever you go in the house but it is generally fairly low level light. (rope light under toe kicks and overhead in crown moldings) on motion sensors. Outside most of my motion lighting is 15w sign bulbs but there are lots of them. I hate that "Stallag 13" look you get with Par 38s.

They sure look like garden variety LEDs to me. It is a clear package and all I see is the normal standoffs with the junction on top. The drop off when I started adding resistance was virtually instant from the full brightness then it seemed fairly linear after that but I really needed a smaller pot to see the first few ohms going in. I have some real low resistance pots in my junk box but I didn't try it.

Thanks for joining in. I hope it is more joy than aggravation. I take it that the failure is with the phosphors and not with the junction generating the UV? Interesting as most of us had been thinking of LEDs as being forever. I have noticed that many of the new LED traffic lights have sections out. I don't know whether that is a circuitry break or whether an LED actually failed. In the environment they are in I would think connection failure from thermal cycling. Either internal or external.

Jeff

I know of one LED nightlight "bulb" that does make

" snipped-for-privacy@att.bizzzzzzzzzzzz" wrote in news: snipped-for-privacy@4ax.com:

I meant several series strings -in parallel-.

curiously,some free Harbor Freight 9 LED flashlights I have have all 9 LED's in parallel,and no current limiting resistor,depending instead on the internal resistance of the 3 AAA cells.

" snipped-for-privacy@att.bizzzzzzzzzzzz" wrote in news: snipped-for-privacy@4ax.com:

hey,they were FREE,including the batteries. ;-) HF gives them away free with a coupon from their many ads.

of course,they don't use the batteries efficiently.

" snipped-for-privacy@att.bizzzzzzzzzzzz" wrote in news: snipped-for-privacy@4ax.com:

don't forget the meter resistance,too.

" snipped-for-privacy@att.bizzzzzzzzzzzz" wrote in news: snipped-for-privacy@4ax.com:

his wallwart may burn them out,if it can supply more current than the LEDs can handle;there's no limiting resistor.

Oh yes, I do remember my stretch of time when I was into astronomy.

Red lights were used to see things other than stars, such as star maps, so as to see in high resolution (from photopic vision) with light that does not overload and reduce sensitivity of scotopic vision.

I would think the requirements of nightlights are usually different. I don't see the need to be able to read a newspaper - only to recognize it, to be able to read the name of the newspaper. There is also the fact that I don't mind having my night vision attenuated a bit by using a light that makes use of it for this purpose. I have some green and blue LEDs that can illuminate a largish living room that well to me with maybe .1 milliamp (and full dark adaptation), though I would only count on low current performance good enough to do that with .25 milliamp.