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

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On Sun, 26 Dec 2010 18:13:03 -0500, snipped-for-privacy@aol.com wrote:

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

You're not talking about illumination at these power levels.
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On Sun, 26 Dec 2010 17:21:21 -0600, " snipped-for-privacy@att.bizzzzzzzzzzzz"

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 3.1w total package. Putting a 1k pot in series with this is not going to be that wasteful even if it does offend your engineering sensibilities. You don't like the flashlight either but there are millions of them out there working just fine. I understand the engineering frame of mind but I also understand simple is good. The reality is the waste heat drops as the dimming starts.because that "I2" thing is falling off very fast as the "R" goes up.
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
http://gfretwell.com/electrical/CT%20fun/CFL%20wave.jpg
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On Sun, 26 Dec 2010 20:25:53 -0500, snipped-for-privacy@aol.com wrote:

I thought you wanted task lighting under kitchen cabinets. That has to be quite bright, even in a well lit room. A couple of watts isn't going to cut it.

Series strings of LEDs with a ballast.

I don't like them because some day someone is going to put the wrong battery in the things.

Agreed, but too simple is not. ...primarily because people are. ;-)

The *waste* heat may go *up* as dimming starts. The waste heat is V^2/R.

They don't. Voltage regulation would serve little purpose.

Right.
You should easily be able to see a switcher's signature. If you have a scope, you can also see the effect of heating by driving the LEDs from an AC source.

Yep, a few million of them will really screw with the power factor. ;-) You'll see some oddities with LED lamps, too.
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On Mon, 27 Dec 2010 13:58:41 -0600, " snipped-for-privacy@att.bizzzzzzzzzzzz"

If the voltage drop across the LEDs is somewhat fixed at a hundred and sumpin and your ballast is picking up the slack, that slack can vary quite a bit with the normal tolerance in power. A LED string that works in the browned out north (108v or so) is going to be running pretty hot here with my nominal 124v. That ballast needs to give you a fairly reasonable current over a wide swing. I almost have to believe they have real solid state current or voltage regulation, not just a resistor. If so, that is the problem with dimming them.
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On Mon, 27 Dec 2010 15:48:03 -0500, snipped-for-privacy@aol.com wrote:

But there is your mistake. It is *NOT*.

Why? All they need to do is cover the maximum, so it doesn't burst into flame.

~:-/
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wrote:

120VAC is RMS voltage,not PEAK voltage. multiply by 1.414 if you want to find the DC voltage after rectification.
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26ma seems a bit high.

don't forget that 120VAC rectifies out to ~170 VDC,not 120 VDC. for that string and a 350 ohm resistor,your LEDs are not going to last more than a millisecond. BTW,the "bright white" T1-3/4 LEDs work out to ~3.5v forward voltage.

not gonna happen;only 3 AAA cells will fit,there's a plastic battery holder inside the flashlight barrel.
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Only with a capacitor filter. It'll still have a 170V peak but the RMS will still be ~120V. The current waveform will be messed, though.

In *your* freebie HF flashlight, perhaps. In the case of the HF flashlights, the zinc cells may be the worst case. I don't see evidence that this is the case in every flashlight out there.
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his wallwart may burn them out,if it can supply more current than the LEDs can handle;there's no limiting resistor.
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wrote:

This is the LED flashlight guts. You can see there are no resistors.
http://gfretwell.com/electrical/LED%20flashlight.jpg
I would always use a resistor in anything I made, even if it was fairly low resistance.
I broke one of those LED lights that were all the rage on TV for a while. The ones that respond to a magnet. There are 2 LEDs, 2 transistors, a reed switch and 4 SMT resistors. The 2 transistors are in a darlington and 2 of the resistors are in series with the 2 LEDs. This seems to be a fairly well thought out design.
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Just gotta control the PWM better. ;-) A PWM current source is typically used.
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Well,you KNOW this guy is not going to use a PWM circuit with his wallwart,just a pot(rheostat).
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Well, he's not going to PWM the wall wart (voltage source), either.
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bullshit. LEDs have a voltage drop of 1.7V. Silicon diodes by the way have a voltage drop of 0.7V.
Put a meter across a lit LED some time if you don't believe me.
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On Sun, 26 Dec 2010 11:19:16 -0600, AZ Nomad

KRW is right that LEDs are not a constant voltage drop but the difference is not very much. The ones I was using dropped 1.7v at 3ma and 1.9v at 15ma.
The real issue would be whether you were dimming them to save energy or to just have a dimmer light sometimes. You will still be saving the amount between LED and incandescent no matter what and I might even argue dimming a LED with a rheostat is better, lumen for lumen than dimming an incandescent with a triac dimmer since the light falls off in an incandescent a lot faster than the current. Once I get some real high intensity LEDs I could plot the light on a light meter against total watts in the circuit. (LED on a rheostat vs Incandescent on a dimmer). If you are looking for that orange "dimmed" color you won't be getting it in a LED.
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snipped-for-privacy@att.bizzz wrote in wrote:

a resistor acts as a current LIMITER,not a "regulator",and it stipulates a constant source voltage. Since line voltage typically varies,you need some sort of regulator circuit(usually an IC)to keep LED current from exceeding it's limit and to keep a constant brightness.
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snipped-for-privacy@att.bizzz wrote in wrote:

E = IxR Ohm's Law. it's simple math. if you provide less current to the LED's because the resistor is a higher value,the voltage dropped across the resistor INCREASES.(and the power it dissipates) the resistor converts the additional power to heat.

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On Dec 25, 10:04pm, " snipped-for-privacy@att.bizzzzzzzzzzzz"

Nothing linear about a diode, he should actually try ploting a curve and see how much the resistance of a diode changes as you change the current, even much more so on a series string of the things like a lamp cluster.. Also no big deal to convert to DC to run an LED they are after all diodes they will do it for you. Thats the really cheap way out . Slightly more expesive and only arguably of better design is using a match head size bridge rectifier, cost about as much as a match too.
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On Sat, 25 Dec 2010 20:14:45 -0800 (PST), JIMMIE

No, they are diodes, but it's not a good idea to use them as rectifiers. The reverse breakdown of LEDs is pretty poor. As you note, a bridge rectifier is cheap enough. The probelm with running them on DC is storing the energy over the cycle (filtering the DC). That isn't cheap or particularly efficient.
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On Dec 26, 12:27am, snipped-for-privacy@att.bizzz wrote:

The string itself is a rectifier. The current through them will be DC, no reason to further rectify it.. In the case of a string of xmas lights they have nearly the full line voltage across them. Works fine.
Jimmie
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