snipped-for-privacy@att.bizzz wrote in news: snipped-for-privacy@4ax.com:
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.
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.
snipped-for-privacy@att.bizzz wrote in news: snipped-for-privacy@4ax.com:
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)
" snipped-for-privacy@att.bizzzzzzzzzzzz" wrote in news: snipped-for-privacy@4ax.com:
those are not single LED's,they are LED arrays. Probably parallel strings of LEDs.
Tell that to "free-lunch" Clare. The datasheet he linked had a 2:1 efficiency reduction (4x current for only 2x lumens) rather than 30%, but that was by overdriving them above their average current spec.
Actually, phase control, not PWM. Similar to, but not to be confused with. With a nonlinear load, like an LED, it's quite different.
Completely clueless.
He's building his own.
*Series* strings. Parallel does nothing but cause more trouble. ;-)
Who said anything about phase control?
PWM is pulse width modulation, control of power by duty cycle.
PWM works just fine on non linear loads as its output isn't operated linearly. The output of PWM is either on or off. What controls average power is the percentage of the time that the device is on. LEDs do fabulously well with PWM actually.
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.
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.
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.
Sadly, I have yet to look inside any retail-available LED lights over 2 watts, nor most 2 watts and under. I don't remember too well what the packages that I looked at so far say.
Best I can say is, look at the fine print on the packages for compatibility with dimmers.
The Philips ones at Home Depot *may* be a good bet. However, I did little more than glance at their photometric and color specifications after getting "sticker shock".
So far, I am seeing only here-and-there applications where LED "bulbs" appear to me more appropriate than CFLs or incandescents, mostly for light output near or less than that of a 25 watt incandescent and with a lot of "on time" per day.
One more thing: Most white LED units claiming 100,000 hour life expectancy, especially cheaper and non-major brand ones, significantly fade in 4,000 to 50,000 hours. I generally recommend green or blue LEDs for nightlights. Any LED "security lighting" that must be at least basically white should be by or have LEDs made by *major* brands of "lighting grade" LEDs, such as Philips/Philips-Lumileds, Cree, Nichia, Osram/Sylvania, Citizen/Cecol, and the like. Also, the usual "bullet shape" low power LEDs are unlikely to get past
10,000 hours before significantly fading if they are white unless they are greatly underpowered. I know of one LED nightlight "bulb" that does make use of underpowering to achieve true long life from cheap white low power LEDs. That is the Feit 3-LED candelabra base one.One more thing: For outdoor or basement nightlighting or security lighting, I strongly recommend that any white lights (LED or otherwise) be of cooler color (more bluish, higher color temperature). This is because night vision is significant in such dimmer lighting environments, even if colors and sharp outlines are visible. A spectrum richer in mid-green to mid-blue favors greater illumination in this case.
Dummy, common incandescent (triac/SCR) dimmers use PHASE CONTROL, not PWM. There *is* a difference.
No, it is PHASE CONTROL. Dimming is done by controlling the firing angle of a triac or SCR. For an incandescent (resistive load) this works out to be similar to PWM, but it is a *lot* different when you're talking about a load as nonlinear as an LED. PWM is often used to dim LEDs, but from a DC supply.
PWM works, phase control doesn't work well.
It would help if you had slight clue.
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.
Brightness of LEDs is controlled by how much current flows through them
- same as with dimmable CFLs. All that is necessary is to make the current control circuitry in the LED "bulbs" compatible with and making use from the usual dimmers, similar story as with CFLs.
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.
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.
but it hasn't been done with CFLs yet, so don't hold your breath waiting for it.
nate
There is a major myth about peak rather than average light output of LEDs determines how bright they appear to humans, even when pulsed rapidly enough to appear continuously on.
I mention its origins and the truth in:
Shows efficiency decreasing with overdrive.
Effectiveness of combining overdriving and pulsing is for machine vision applications where strobing is suitable.
Mentions usefulness of pulsing, but not for increasing visual luminous efficacy.
Mentions short pulse overdrive - apparently for strobing, single-pulse or machine vision applications.
Machine vision is in the name of the link.
Mentions for machine vision.
Stated to be for machine vision lighting.
That is for a visibly strobing application - mentions 10 flashes per second.
For cameras, frame grabbers and machine vision.
It has been done for CFLs. I have seen dimmable CFLs at Target for a year or something like that already, maybe almost 2 years. They have existed for much longer than that.
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