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

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.

Reply to
Home Guy
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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.

Reply to
G. Morgan

I haven't really been following, but I was in the borg last night and they now have quite a few. I noticed, right on the box, of at least one "Dimmable". There is little reason why not, unless flicker related.

YMMV

Getting better. I was looking at bulbs priced in the teens. I seem to recall prices 3 times higher not long ago. But I'm no expert.

Wound up buying a CFL that looked just light a regular bulb (for $4), with glass all the way to the screw. Got one floor lamp that a regular CFL won't screw in.

Jeff

Not being able to dim them makes them even

Reply to
Jeff Thies

Jeff Thies wrote in news:if23lv$8he$ snipped-for-privacy@news.albasani.net:

LEDs are CONSTANT CURRENT-driven,not voltage driven. they have to have some sort of current regulator inside that keeps the current constant regardless of input voltate swings. To dim a LED,you would have to vary the input current,and the typical triac lamp dimmers don't do that,they vary input voltage.

Plus,there's a fairly narrow range of current that produces light from the LED chip,it's not linear.Most LED dimming schemes pulse modulate the LED,changing the duty cycle of the current,and the eye averages the output. you get better efficiency that way,but it's more complex electronically.

Reply to
Jim Yanik

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

Reply to
Nate Nagel

G. Morgan wrote in news: snipped-for-privacy@4ax.com:

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.

Reply to
Tegger

One of the things that makes LEDs "harder" to dim is that they dim linearly with current. Normally this would be a nice thing, however present dimmers are made for traditional bulbs. Filament bulbs dim very non linearly with voltage. LEDs typically will have some sort of series limiting resistance, but the idea is to have as little series resistance as possible, to cut down on wasted power in the resistor. Pulse width dimming, as in car tail lights, is the obvious answer. But, I think they should up the frequency of said tail lights and future house lights dimming circuits, to prevent the strobing effect you see when you turn your head rapidly. Some people don't even notice it, but some do.

Reply to
Art Todesco

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 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.

Reply to
G. Morgan

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.

Reply to
Ralph Mowery

Unlike CFLs, LEDs are *ALL* dimmable. You just have to have the right dimmer. Since all lightbulb types require their own dimming technique, it's a challenge to design one dimmer that would work for any type of lightbulb a consumer can buy. Some are trying though, Lutron for example. Check out these "C-L" dimmers they sell that are for both *dimmable* CFLs and LEDs:

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Note that they are not intended for incandescent or halogen bulbs whereas bulk of the dimmers in HD or Lowes today are the other way around.

I'm guessing if LED lighting is ever to prevail, there will be an abundance of dimmers using PWM (Pulse-width modulation) that will work with any LED lightbulbs, even the crude and unreliable ones you can buy today. It's just the need to work with any lightbulb you can throw at them that's holding the dimmer manufacturers back.

------------------------------------- /\_/\ ((@v@)) NIGHT ():::() OWL VV-VV

Reply to
DA

I hadn't thought about this, but aren't light dimmers just varying the fraction of a half wave (pulse width)? It would seem to me that it should work with LEDs (although not particularly well) but I don't know what circuitry is in an LED light.

I think the key is how the current source is constructed. Limiting the power lost there is important.

Can you shed any light on just what is in an LED lamp? I'm curious now. I figured a wee chopper with a bit of output filtering. Add a resistor or a constant current configured transistor. Just speculating.

Is that a household type dimmer like on the wall?

changing the duty cycle of the current,and the eye averages the output.

For the money they are, they can throw a little circuitry at it! Chopping up DC is no big deal these days. With the low current drawn and high voltage, I wouldn't think it would need a very large cap.

I have no practical knowledge of this, so I'll defer to those with more experience.

Jeff

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Reply to
Jeff Thies

The problem is the clock rate. LEDs respond virtually instantly and at

60 or 50hz the flicker would be horrible. You could either clock your chopper a lot faster or just vary the current. Since the currents are so low, a rheostat may not be out of the question. As soon as Hosfelt gets my box here I am going to start playing with some high intensity LEDs to see what I can do with them, My goal is dimmable under counter lights that run off a wall wart with a very low profile. (perhaps even recessed into the cabinet bottom)
Reply to
gfretwell

Yes, you are correct, except that the LEDs have to have a "threshold"[*] voltage to light at all. If the dimmer is triggered when the AC (sine wave) is too low the LEDs don't light. The range of adjustment will be very small. [*]as others have pointed out, this isn't an on/off thing but is highly nonlinear

Yes, this is a very expensive thing to do, compared to a *cheap* Triac dimmer.

You'd have to make a DC source, which won't be cheap because of the sine-wave input. The source will have to store energy for the cycle.

No, they adjust the (phase) angle of a triac firing. This controls the power put into a resistive load just fine but doesn't work for a highly non-linear load like a (long) string of diodes.

Right. But you don't have DC to chop. ;-) Making the DC isn't all that cheap (or efficient).

Reply to
krw

As can be seen with LED Christmas lights. It's fairly cheap to double that (full wave bridge or more LEDs), which would solve the problem.

How do you "just vary the current" efficiently, without chopping it? It doesn't have to be a lot faster but you're right, 60Hz doesn't cut it IMO.

Huh? Sure they are. You might just as well use incandescent lights.

Good luck with that.

Reply to
krw

Uh the problem is an led fires at a certain voltage. To dim the led you need to apply that voltage via a PWM. Chop the firing voltage up into pulses too fast for the eye to notice. The smaller the width of the pulses the dimmer the led.

Reply to
A. Baum

Hey Don,

What do you know about what circuitry is in the commercial LED lamps and dimming compatibility.

For those who don't know, Don Klipstein knows more about lighting than anyone I know of:

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We'll see if he is around and interested.

Jeff

Reply to
Jeff Thies

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.

Reply to
clare

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.

Reply to
krw

No, as has pointed out here by others, it doesn't "fire at a certain voltage". It's light output is highly nonlinear WRT voltage, but there is no "on" or "off", rather a continuum.

The output of an LED is a function of the average current through it. You can either use PWM to change the average current or vary the current directly. The light output is a pretty linear function of the current through the LED so either method works.

No, the less the *ratio* of "on" to "off" the dimmer the LED. The important variable is the *average* current.

Reply to
krw

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.

Reply to
clare

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