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

On Fri, 24 Dec 2010 11:58:12 -0600, snipped-for-privacy@att.bizzzzzzzzzzzz wrote:

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.
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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.
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On Fri, 24 Dec 2010 11:58:12 -0600, " snipped-for-privacy@att.bizzzzzzzzzzzz"

OK you have a string of LEDs dropping about 95% of the line voltage and a resistor dropping the rest and limiting current now. How can making that resistance more by adding a rheostat in series be more inefficient?
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On Sat, 25 Dec 2010 11:48:23 -0500, snipped-for-privacy@aol.com wrote:

Resistors heat == inefficient
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On Sat, 25 Dec 2010 19:56:44 -0600, " snipped-for-privacy@att.bizzzzzzzzzzzz"

But the resistor will always be there. You are just making a bigger resistor, the current will drop and the light will dim in a vary linear way. The voltage you drop across your resistor will be the same no matter how big it is. That is not like a rheostat on an incandescent where you are changing the voltage applied to the filament.
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On Sat, 25 Dec 2010 22:26:43 -0500, snipped-for-privacy@aol.com wrote:

The voltage across the resistor *does* change. Also, P=I^2R.
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On Sat, 25 Dec 2010 21:56:12 -0600, snipped-for-privacy@att.bizzz wrote:

The voltage drop across a diode is relatively constant, so the voltage across the resistor has to also be relatively constant, with only the current being changed by the change of resistance. This is not 100% accurate, but for this discussion I believe it is close enough.
Not agueing with krw - just agreeing (to a point) with gfretwell. I say "to a point" because it is not totally linear. Much more linear than some would have you believe in a DC circuit.
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On Sun, 26 Dec 2010 00:45:29 -0500, snipped-for-privacy@snyder.on.ca wrote:

It is *NOT* "relatively constant" when you multiply the change times the number of LEDs in a 120V string and compare that to the voltage across the balast resistor. If you string a few together and have a large balast resistor it matters less but you're simply wasting that much more power, losing gains you made by using LEDs in the forst place.

Of course not. You could *NEVER* admit that you're wrong.

Of course you would be WRONG, as usual.
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snipped-for-privacy@att.bizzz wrote:

How does the voltage change across a fixed circuit? I think everyone here is talking about an Xmas tree lighting situation (now), in a series arrangement.
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wrote:

The circuit is not fixed. The conversation is that you change the resistor value to change the brightness of the LEDs. There is very little change in the voltage across the LEDs, but the big change is the voltage drop across the series resistor as the value of it is changed to change the current that changes the brightness of the LEDs.
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He's talking about dimming LEDs, as in under-cabinet lighting.
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wrote:

those are not single LED's,they are LED arrays. Probably parallel strings of LEDs.
--
Jim Yanik
jyanik
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He's building his own.

*Series* strings. Parallel does nothing but cause more trouble. ;-)
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On Sun, 26 Dec 2010 12:41:26 -0600, " snipped-for-privacy@att.bizzzzzzzzzzzz"

OK the science comes fast around here I took a flashlight apart. It appears they use a 4.5v LED, 9 in parallel with NO resistor at all. The internal resistance of 3 AAA cells seems to be the limiter
With just the batteries in there the lights are pulling about 244ma (they are fairly new batteries) This is hurt your eyes bright. I put my 1k pot in there and even all the way off I am dropping .05v, current around 211ma. The slightest movement of the pot, only putting a couple ohms in there rapidly starts dropping the current. Somewhere around 900 ohms we are at 3ma, dropping 1.3v and the light is "indicator bright".
OK so back to the junk drawer for some smaller resistors With 10 ohms in there it is dropping 0.8v 77ma the light is noticeably less but still pretty bright. When I double that with 20 ohms the light dims quite a bit, current drops to 48ma and voltage 0.94v
I guess somebody has to crack open a 120v LED bulb or just do an experiment like this to see how they work but I know what I need to know about a low voltage setup like I want to make. I am thinking a 25 or 50 ohm pot will do the deed for me.
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On Sun, 26 Dec 2010 16:02:42 -0500, snipped-for-privacy@aol.com wrote:

I've seen that too; a very poor design. There is nothing to current-share across the LEDs. Counting on the internal resistance of a battery is really piss-poor.

I assume you mean, "all the way *ON*", as in "zero" ohms (the wiper resistance is about 1/4ohm).

1.3V/.003A = 433 ohms.

Ok.
You can already see that it's nonlinear and you're using a DC source.

I thought you wanted an AC powered light? If all you want is to dim a low-voltage DC string, PWM is the way to go. It's pretty easy. There are cheap chips to do this automagically.
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wrote:

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.
--
Jim Yanik
jyanik
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HF buys their stuff from where? ;-)

The problem that I have with it is counting on the internal resistance of the battery. As long as the same *type* of battery is used there isn't a real problem. If another type is used things can go very wrong.
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wrote:

Communist China. :-)

they come with "heavy duty" AAA carbon-zincs,but I also tried alkalines with them,and they were brighter with the carbon-zincs. they may be UNDERdriven. I still have two more coupons for free ones,and the local HF store is only a short walk away! Plus,I see the ads are still coming out with the coupons.
they're nice little flashlights for the "price"! B-)
--
Jim Yanik
jyanik
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wrote:

don't forget the meter resistance,too.
--
Jim Yanik
jyanik
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Oh, certainly. Good catch.
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