At very low currents, LED light output is proportionally very very low?... - Page 1
Forum section: electronics / beginners

author: ocset
author_rank: Super Contributor
timestamp: June 16, 2020, 10:45:07 am
content: HiWe want to drive 350mA (average) through a Cree XT-E white LED. We want as much light output as possible.What will be the difference in light output if we drive the LED with…....a) 350mA constant DC current (as in the red waveform attached)..b) 350mA average but it’s a 500Hz triangle wave (as in the green waveform attached.)The light_Flux vs Current graph at the top of page 27 of the LED datasheet (below) appears to suggest that the maximum “Light flux per Amp” comes at around 100mA.Do you know what this curve looks like from zero Amps to 100mA?Datasheet: Cree XT-E Lamp (350mA LED)https://www.cree.com/led-components/media/documents/XLampXTE.pdf\ / \ / \ / \ /…..____......____…..____......____…..____......____…..____......____…..____......____…..____......____…..____......____…..____......____…..____......____i have read that due to "Auger recombination", then when below a certain level of LED current, the light photons just cant all properly escape from the LED silicon, and so at low currents, there is proportionally very much less light output....so in other words, if you view page 27 of the Cree LED datasheet ......the unshown bit below 100mA is a region of very very low light flux output....thats why they dont show it.Would you agree......if the bit below 100mA was just a plain extrapolation...then they would have shown it...but because it is in fact a region of (proportionally) very poor light output...they dont show it...for marketing reasons. Would you agree?

author: nfmax
author_rank: Super Contributor
timestamp: June 16, 2020, 10:49:54 am
content: Why not measure it yourselves?

author: ocset
author_rank: Super Contributor
timestamp: June 16, 2020, 11:11:40 am
content: Thanks, we dont have the gonigle, and they are phenominally expensive

author: nfmax
author_rank: Super Contributor
timestamp: June 16, 2020, 11:31:36 am
content: But you only need a relative measurement, comparing two different current drive waveforms (DC & triangle wave), so why not use a cheap illuminance meter and a home-made 'integrating sphere'? You could just compare on-axis measurements to start with, then have a look to see if the angular distribution is varying with drive waveform at few off-axis angles (without the 'integrating sphere'). If is doesn't, you have your answer; if it does, you may have to think again. But it is better than guessing!

author: tunk
author_rank: Super Contributor
timestamp: June 16, 2020, 12:34:44 pm
content: A guesstimate is that it will be slightly more efficient to use a350mA constant current: To find out, you could calculate the averageRelative Luminous Flux from 0 to 700mA. Or graphically, draw a linethrough zero and 350mA/100%, and what you gain below 350mA is lessthan you lose above.

author: rhodges
author_rank: Frequent Contributor
timestamp: June 16, 2020, 02:18:48 pm
content: If you can rig up a fixture to measure the waste heat, you could choose the method producing less waste heat (at the same RMS input.)

author: bdunham7
author_rank: Super Contributor
timestamp: June 16, 2020, 02:58:28 pm
content: Quote from: treez on June 16, 2020, 10:45:07 amYou can easily judge this by the efficacy chart you referenced. At 350mA you have the nominal 100%, at 700mA you have roughly 180%, so a 10% loss of efficacy at the peak. So the overall loss won't be much, a few percent, or even no loss, since you will bebelow350mA a good amount of time.The curve probably just continues in a relatively straight line to zero.QuoteIt's been a while since I thought about Auger (someone I knew did his PhD thesis on some aspect of Auger electrons) but this is typically a high-intensity phenomenon, not low. A quick web search revealed that 'LED droop' athighcurrents has been found to be caused by the Auger recombination phenomenon, not low currents. I don't know why Cree doesn't characterize the low end, but I don't think there are any surprises lurking there. Try putting 1 mA through it, then 100 uA. I'll bet it produces visible light at even those currents.If the triangle wave is what you currently have, I think 'optimizing' to DC will yield very little improvement, although it will reduce flicker--which is what I would care about.

author: TimFox
author_rank: Super Contributor
timestamp: June 16, 2020, 07:07:54 pm
content: If your drive waveform is a triangle wave, which is piecewise linear, it would be straightforward to calculate this answer using Excel: with time in column A, enter the current values as a function of the waveform and time in column B, then look up the output values for each current value and enter in column C. You shouldn't need a large number of steps for the accuracy required. Compare the sum of column C over a cycle with a constant light output.

author: David Hess
author_rank: Super Contributor
timestamp: June 17, 2020, 12:01:37 am
content: Quote from: treez on June 16, 2020, 10:45:07 amThe highest efficiency occurs at a specific current which depends on the LED technology so at low output levels, it is advantageous to operate with a higher current at a lower duty cycle.

author: foxtrot824
author_rank: Newbie
timestamp: August 24, 2020, 12:35:01 pm
content: Hi there,In my experience many LEDs in the 1-3w nominal drive range have some CCT shift when they are very under driven. I have done lots of testing in a 12" sphere on individual emitters but usually I am testing at 100mA and greater. I can see if I have any data on anything Cree in the single or double digit drive current. Is lumens per watt the ultimate goal?Cheers

author: Kleinstein
author_rank: Super Contributor
timestamp: August 24, 2020, 02:46:34 pm
content: With the white LEDs the efficiency is usually highest at some point a little lower (e.g. 1/10 to 1/2) the nominal power. They specify for nominal power for a level where is efficiency has not dropped too much and the cooling still works. They get more money for a 3 W 100 lm/W LED than for 1 W with 120 lm/W, so they sell it as 3 W and not as 1 W.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 24, 2020, 02:55:51 pm
content: One of the first sentences of the datasheet states:Optimized for directional, high-lumen applications,And that may be the reason they have not bothered to characterize linearity at low current.My own suspisions are confirmed by Kleinsteins explanation.I've bought some 10W LED panels on aluminimum PCB from Ali (Yeah, I know) and when you go above 3 to 5 watts the heating goes up "significantly" more than the light output. But I have not done measurements of this yet.Part of the loss of efficiency at high current is due to simple ohmic losses.Just punch in some numbers of a 1A dc current through a resistor, compared with a 10A current at 10% duty cycle.I did not see anything about temperature effects when skimming through the datasheet.LED's don't like heat.But why is this important to you?Human eye's are notoriously bad at estimating light conditions.A classical example is the "chessboard illusion" where some object is placed on the chessboard, and it throws a shadow over the chessboard. It's done in such a way that a "light" square in the shadow is exactly the same brightness as a "dark" square on the direct lighted part.You won't believe it until you do a pixel for pixel comparison.https://duckduckgo.com/?q=chessboard+optical+illusion&t=hk&iax=images&ia=imagesAnd doing some comparative measurements in a closed (shoe?) box with fixed orientation and distance between LED and some light sensor is fairly easy to do, and much more accurate than trying to use your own eyes.

author: ArthurDent
author_rank: Super Contributor
timestamp: August 25, 2020, 01:29:42 am
content: If you've ever had someone set a camera flash off while you were looking at it and temporarily blinded you, you realize that the eyes are pretty slow in reacting to light so if you had a 50%/50% duty cycle driving an LED you would be using 1/2 the power but the eyes wouldn't notice the difference if you use a high enough frequency and they will "integrate" the light so it looks like a solid beam.You could try different frequencies and duty cycles and see what works best for you.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 25, 2020, 02:01:07 am
content: Main thing is different recombination rates in the phosphor, leading to shitty CRI. The spectrum goes lopsided. But these are illumination LEDs, there's not much point running them very low.Tim

author: Circlotron
author_rank: Super Contributor
timestamp: August 25, 2020, 02:10:26 am
content: Quote from: ArthurDent on August 25, 2020, 01:29:42 amIf I understand what you are saying, given a high enough chopping frequency, if you varied the pulse width your eyes would still only perceive the peak brightness and not see any variation in brightness with pulse width. Is that what you mean?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 25, 2020, 03:20:38 am
content: Not peak but average. Eyes integrate light exposure over a modest time scale (10s ms).(The one way you can kinda-sorta game this, is a repeat frequency much slower than POV (persistence of vision): you will perceive blackness inbetween, while a pulse of modest length (10s ms) looks like a flash of full brightness. The blackness allows power saving, for purposes like indication -- obviously, only where a flashing source is tolerable/desirable. A faster frequency (above POV) would merely integrate out to a constant intensity, and you've done nothing but PWM it, for which you're probably better off using a constant current of the right value in the first place, to save on circuit complexity and I^2 R losses.)Put another way: a photoflash looks about the same whether it's 10us or 10ms long, if it's the same energy (so, peak current inversely proportional, say) in each case.Tim