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

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On Sat, 25 Dec 2010 22:29:03 -0500, snipped-for-privacy@aol.com wrote:

Because it does?

The resistor is a current regulator not a voltage regulator.
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On Sat, 25 Dec 2010 21:59:15 -0600, snipped-for-privacy@att.bizzz wrote:

<sorry, new netbook and keyboard/trackpad is driving me nuts>
No, that would assume an ideal diode. They are not ideal, the I-V curve is not a vertical line. The more diodes you put in series and the lower the voltage across the resistor, the more pronounced this becomes. From some recent work, a blue LED at about 5mA drops about 3V. At 20mA the drop is closer to 3.3V. Now, put thirty of these in a string and the difference is 10V. You only have 20V across the resistor - it's changed 50%. ...and this is quite nonlinear.

A resistor regulates nothing. R==V/I. If you say that I is constant because V is constant and V is constant because I is constant, you're getting nowhere. ;-).
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On Sat, 25 Dec 2010 22:15:14 -0600, snipped-for-privacy@att.bizzz wrote:

Lets get to the bottom line. Are you trying to say a dimmed LED (by increasing the size of the resistor) draws more current than one at full brightness?
You may have a more efficient way of dimming a LED lamp but how many decades would it take to get your money back over just using a rheostat? Sometimes KISS is the best rule.
I like doing experiments like this and I will be back as soon as my high intensity LEDs arrive because this is my plan. We will see. I have no problem admitting when I am wrong but I want to see it.
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On Sat, 25 Dec 2010 23:25:40 -0500, snipped-for-privacy@aol.com wrote:

Of course not. I'm saying that it is NOT LINEAR and NOT EFFICIENT.

A reostat is a BAD idea. The worst, in fact. It's easy enough to perform this electronically, but the results aren't good. Doing the job right is somewhat more difficult.

Go for it. I do this stuff all the time, though with indicators, not for illumination.
I just did one where I used current sources instead of resistors to eek out the last couple of tenths of a volt on the supply. It was an application where we were using red and green LEDs but the owner like blue but the product is LiIon powered so we have to operate from 3.6V to 5V (when charging we run off the input). The current source allowed a constant intensity over this range (3.6V is pushing it) while having suffiient drive at the low end.
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On Sat, 25 Dec 2010 22:36:45 -0600, snipped-for-privacy@att.bizzz wrote:

I guess the point is LEDs are so efficient to start with and dimming them will cut the draw even more, why make things hard on yourself to squeeze out an extra few pennies a year.
If you are replacing an incandescent any thing you do has to be better.
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On Sun, 26 Dec 2010 00:52:51 -0500, snipped-for-privacy@aol.com wrote:

I could say the same thing about replacing incandescents in the first place. Why, for a few mennies a year. In fact, that's what I do.

Not necessarily true. You still haven't come up with a good scheme to dim them.
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On Sun, 26 Dec 2010 00:09:54 -0600, snipped-for-privacy@att.bizzz wrote:

The difference between an LED and an incandescent is far more than pennies and when you dim the LED you are still reducing the power you use. Garden variety triac dimmers are not that efficient either but we still say they save money. Have you ever looked at the lumens per applied power number on a dimmed incandescent? They are horrible. Bear in mind they are still drawing power when they are virtually too dim to see.
The application I am making right now will be low voltage so my little experiment is really a prototype. I will just need to make the necessary adjustments for the high intensity LEDs I bought.
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On Sun, 26 Dec 2010 02:10:31 -0500, snipped-for-privacy@aol.com wrote:

Sure they're efficient. They have a fairly well fixed 1.4V across them, times the duty cycle of the brightness setting. At half-power that's well less than a watt. Your rheostat will easily produce that much heat.

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On Sun, 26 Dec 2010 11:32:25 -0600, " snipped-for-privacy@att.bizzzzzzzzzzzz"

I would really like to see the math on that. If I reduce current by 50%, I am reducing power by 75% (the 2 in I2R) In my little string I was producing a total of 0.18 watts at full brightness (15ma) and that dropped to .036w at 3ma. Even if all of that was all in the rheostat, so what? The reality is you are still dropping most of the voltage on the LEDs and that is where the lions share of the heat will be.,
Since we are talking LEDs, what do you know about a 10 mm -130,000 mcd Intensly Bright Blue LED? That is one of the types I ordered but I am not sure about the specs.
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On Sun, 26 Dec 2010 15:07:26 -0500, snipped-for-privacy@aol.com wrote:

I normalized current to an amp (100W). 1A * 1.4V * .5(duty cycle - dimmed by half power) is .7W. Scale that percent, give or take, for your choice of lights.
I'd have to know what circuit you're intending to use, but a ballast resistor for LED lighting can easily get there. Dimming makes it worse.

Overall, you will waste more, percentage wise, at lower settings. The rheostat will dissipate more (V^2/R).

You aren't lighting anything with your little string. Scale that up to something useful.

I can tell you that they'll be about 3.3-3.6V each at 30mA, or so. More at higher current. It's a blue thing. ;-) If you give me a P/N I can probably look them up.
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On Sat, 25 Dec 2010 22:36:45 -0600, snipped-for-privacy@att.bizzz wrote:

OK Here you go
These are garden variety indicator LEDs (what I had handy) There is a 270 ohm in series with 4 LEDs and a 1k pot wired as a rheostat. With the pot set to 0 ohms get a tad over 15M/A and the LEDs are as bright as you can expect from these, may even be overdriven. I al dropping 4.11v over the 270 ohm resistor. Turning the pot down toward 1k ohms dims the LEDs quite smoothly At 1K ohms plus the 270 I get 3.7 MA or so and I am dropping 4.8 volts across the resistors.
http://gfretwell.com/electrical/LED%20Dimming%20experiment.jpg
I agree it is not exactly linear but I also do not understand how this small variance makes any real world difference since most of the voltage is still being dropped across the LED string. You certainly would have a hard time seeing it on your electric bill.
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snipped-for-privacy@aol.com wrote:

There are a bunch of issues. Yes, LED's are VERY dimmable. The questions are: Can you build a reliable, affordable, LED lamp that works with the EXISTING dimmer mounted on your wall. Can you build an affordable dimmer that works with CFL, LED and incandescent lamps. If you're buying a dimmer to dim a 20W LED, will you pay for the extra cost to dim a 300W incandescent load. Not much of a premium for old-school dimmers. For one that works with LED, the premium can be significant. Vendors don't want to warranty replace a dimmer you blew up by plugging in an incandescent.
And dimming is not the only problem. Lots of the X10 home automation stuff uses SCR circuits that are not CFL or LED friendly. Ditto for motion sensor lights...but that's for another thread.
Everybody reading this probably feels comfortable opening the box on the wall and changing the dimmer. But Suzie homemaker may be faced with this option. A)Go buy a 57W incandescent for 25-cents on sale and screw it in. B)Go buy a $40 LED lamp and a $40 dimmer and PAY somebody to install it. The stuff doesn't exist (at reasonable prices) because the demand isn't there. The demand isn't there because the cost is too high because the demand isn't there and one might argue that the technology isn't there either.
Basing your decision on the STATED lifetime numbers is ill-advised. My budget can survive a failed 25-cent incandescent. I'm much more sensitive to a failed $40 LED whatever the failure mode. I have NEVER had a CFL fail because the bulb quit. I've had many fail because the electronics exploded. Don't think I ever got 8000 hours out of one.
The dimmer on your wall doesn't work because it requires a load to charge the cap that provides the phase shift that delays the turn-on and dims the incandescent. Sure you can design one better, but 1) That one ain't on your wall 2) It will initially cost so much that you won't put one on your wall.
As for the LED lamps. You've got a 160V peak or twice that depending on where you live coming out of the wire. Your LED is 3V or so. It was mentioned that you can stack them up to 95% of the volts. Not so. There are voltage variations between LEDs and temperature variations. When you stack a bunch of them up, that variation can figure significantly into the means you use to limit current.
Putting 40 of them in series drops the reliability by a factor of 40 or so. Yes, I expect some statistician will take issue, but the factor ain't small.
If you use a resistor to limit the current with only small headroom, the current becomes very voltage sensitive. You don't want your lights to dim every time the line voltage sags a volt. And there's also the reliability issue. You don't want your bulbs to burn out every time your air conditioner compressor kicks off and spikes the line. And you don't have room or $$ for a line filter in each bulb.
Another way to limit current is to use a series capacitor. It's much more efficient, but is critically dependent on having a sinewave input. Line transients can pop your lamps. And the dimmer on your wall puts a HUGE transient into the lamp every half cycle. And the caps aren't cheap.
Then there are the MUCH MORE EXPENSIVE active ways to manage the current.
Unless you're designing a new lighting system, you don't have control of both ends of the system. For currently installed systems, Suzie homemaker is gonna go to home depot and buy whatever is on sale and works with what's currently installed.
At today's prices, the only rational compromise is CFL and stick with incandescent where you absolutely must have dimming. YMMV
As an experiment, I did buy some of the lights-of-america 1.5W 20-led lights for cheap at a garage sale. Make great night lights, but after a year, they're getting dimmer. Wish I'd measured the initial light output. And they waste over half of that 1.5W in the dropping resistor. So much for efficiency.
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On Sun, 26 Dec 2010 01:54:30 -0500, snipped-for-privacy@aol.com wrote:

Now, scale that to your 120V application.

It will *not* be a small variance when you add another 30 or so in series and a smaller ballast resistor. Add in the sine wave excitation and you're going to find your results don't match your experiment very well.
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On Dec 26, 2:54am, snipped-for-privacy@aol.com wrote:

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
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On Sun, 26 Dec 2010 20:27:58 -0800 (PST), JIMMIE

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.
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On Dec 26, 11:51pm, snipped-for-privacy@aol.com wrote:

The yellow lights didnt look good at all to me, of course this may have something to do with the color of my counter top too. I have been thinking of experimenting with combining different color LEDs in the same cluster. Replacing one of the LEDs with a yellow one might not be a bad idea. LEDs arent ideal for task lighting so why use them for that. I used to have some light fixtures that had fluorescent, 60 watt incandescent and night light type bulbs all in one fixture. I can see where combining fluorescent and LED in an under-cabinet light would have some advantages.
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snipped-for-privacy@att.bizzz wrote in wrote:

when I worked at Tektronix,I had a semiconductor CURVE TRACER that plotted out the V-I curve on the CRT screen. for a LED,it's not a sharp bend,it's more like a hockey stick,a "knee" and then a sloping current increase as voltage increased. heating of the LED chip causes the angle to change. you could measure the V drop at the desired current,and not risk destroying the LED by overcurrent or overheating. LEDs can pop very fast when overdriven.(as the rheostat guy will discover) for high power LED's,it will be an expensive lesson. My 3Watt Cree XR-E's cost $5.50 apiece. (from DealExtreme)
--
Jim Yanik
jyanik
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wrote:

Since the rheostat will be in series with the proper "full load" ballast resistor, overdriving is not an issue. Current is going to be dropping as a square of the change in resistance and so will the heat. As long as the rheostat is rated as high as the ballast resistor, where is the possibility of a problem? I suppose the open question is, how big is the resistor in a commercial lamp? In my case it doesn't matter at all since I will be driving mine from a wall wart. I am tempted to just take a cheap 4.5v LED flashlight apart (a couple bucks at H.D) . That is probably cheaper than I can buy 9 LEDS and for my purposes, it provides plenty of light. I just want to spread them out a bit. I even have a 4.5v wall wart.
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On Sun, 26 Dec 2010 15:17:19 -0500, snipped-for-privacy@aol.com wrote:

High power rheostats aren't all that common, or cheap, these days.

I haven't seen a lamp with a rheostat is decades.

If you're powering this thing with a DC wall wart, simply PWM the thing. For less than a buck or two in parts, you're all done, and you can forget about the rheostat and waste heat.
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On Sun, 26 Dec 2010 16:24:20 -0600, " snipped-for-privacy@att.bizzzzzzzzzzzz"

... 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?
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