What happens at the end of LED life?

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wle
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What happens at the end of LED life?

They all say 100,000 hours or something ridiculous. (Assume the electronics live that long, probably a bigger assumption than LED.)

What they don’t say is, “the brightness starts diminishing from hour 1. 100,000 hours assumes you can stand it to be X% of the original.”

But how do they die?

What kills them?

Does the LED just get dimmer?

Does the yellow phosphor (assume a white LED) get dimmer (which would change the color balance)?
Does it get cloudy or opaque, or change colors?

If you overdrive or overheat them, what effect does that have?

Is it “you only get so many lumen-hours out of this LED, your choice between bright/short and dim/long”?

How often will the LED itself just conk out suddenly, like an incandescent bulb?

Just wondering. Topic for discussion I had not seen addressed.
100,000 hours is only 11 years Smile

wle

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Edited by: wle on 10/24/2016 - 09:23
The_Driver
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They get dimmer. In the datasheets one can usually find “L70” lifespan. This tells you how many hours on average it takes for this led to dim down to 70% of its inital brightness. At this point it still works though! Just a bit dimmer.

Heat kills LEDs! The higher the average temperature, the lower the lifespan.
Also what most people don’t know is that higher currents are actually better for LEDs than really low currents (at the same temperature). Very low currents drastically reduce the lifespan of an LED. This is something that is not stated in most datasheets.
German video on the topic:

wle
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wow, really.. i would not have thought that!

{low current is bad}

wle

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EasyB
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The_Driver wrote:

Also what most people don’t know is that higher currents are actually better for LEDs than really low currents (at the same temperature). Very low currents drastically reduce the lifespan of an LED. This is something that is not stated in most datasheets.

Interesting. Could you explain? I couldn’t find a figure in that presentation that explained it. And I don’t understand German.
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Only sort of on-topic and doesn’t answer any of your questions, but I’ve had several household LED “bulbs” quit on me, and its always the driver/electronics and not the emitters. So yeah, the 100k hr lifespan quoted for most LEDs is meaningless for the average consumer because the other parts in the circuit are often the weaker link.

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The_Driver wrote:
They get dimmer. In the datasheets one can usually find “L70” lifespan. This tells you how many hours on average it takes for this led to dim down to 70% of its inital brightness. At this point it still works though! Just a bit dimmer.

Heat kills LEDs! The higher the average temperature, the lower the lifespan.
Also what most people don’t know is that higher currents are actually better for LEDs than really low currents (at the same temperature). Very low currents drastically reduce the lifespan of an LED. This is something that is not stated in most datasheets.
German video on the topic:

It’s too hard for me to follow what the speaker is saying..
But i’m suprised too about low current being ‘bad’ for the LED.
But what is low?
Is it less than half of the maximum recommended current?
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Jerommel wrote:
The_Driver wrote:
They get dimmer. In the datasheets one can usually find “L70” lifespan. This tells you how many hours on average it takes for this led to dim down to 70% of its inital brightness. At this point it still works though! Just a bit dimmer.

Heat kills LEDs! The higher the average temperature, the lower the lifespan.
Also what most people don’t know is that higher currents are actually better for LEDs than really low currents (at the same temperature). Very low currents drastically reduce the lifespan of an LED. This is something that is not stated in most datasheets.
German video on the topic:

It’s too hard for me to follow what the speaker is saying..
But i’m suprised too about low current being ‘bad’ for the LED.
But what is low?
Is it less than half of the maximum recommended current?

Yes, that is very surprising. Is it referenced somewhere else?

It must be far less than half the maximum current, because Cree’s LM80 testing doesn’t show any indication that low current hurts the LED lifespan. It primarily shows that high temperature is by far the main culprit when it comes to degrading the LED. Current (high or moderate) appears to play very little part.

I’d like to see a study based on very low current. I’ve never heard of this. And based on solar lawn lamps (that use a dim LED), I don’t see it anecdotally. Also, many electronics use dim LED to display status or time, and they run continuously for years or decades.

Though perhaps the poster is referring to modern high-output LEDs. I’m a moonlight mode fan, so I’d be concerned if the speculation is true.

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Sounds concerning, the ~5lm mode I use 99% of the time will slowly dim to 4lm? Big Smile

Just for a moment... Okay I may stay a little longer

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WojtekimbieR: to alleviate your concerns, the 4lm will not likely occur until 10 years or so.

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WalkIntoTheLight wrote:
It must be far less than half the maximum current, because Cree’s LM80 testing doesn’t show any indication that low current hurts the LED lifespan. It primarily shows that high temperature is by far the main culprit when it comes to degrading the LED. Current (high or moderate) appears to play very little part.

I’d like to see a study based on very low current. I’ve never heard of this. And based on solar lawn lamps (that use a dim LED), I don’t see it anecdotally. Also, many electronics use dim LED to display status or time, and they run continuously for years or decades.

Though perhaps the poster is referring to modern high-output LEDs. I’m a moonlight mode fan, so I’d be concerned if the speculation is true.

Maybemaybemaybe it has to do with 1 of 2 things.

1, it might collect moisture in the phosphors, and high currents (thus high heat) helps “boil off” the moisture. Kinda like occasionally running the engine of a car that’s kept in long-term storage, vs just letting it sit, collect condensation, and rust away internally.

2, repeated operation at lower currents might “pull” the innards of the crystal a certain way over time, and high heat helps anneal the innards and reduce the stress. Back to the car analogy, having an engine that just runs at idle forever, and is never “opened up” to shake loose all the crap that might’ve collected.

Other’n that, I’m stumped.

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Lightbringer wrote:
WalkIntoTheLight wrote:
It must be far less than half the maximum current, because Cree’s LM80 testing doesn’t show any indication that low current hurts the LED lifespan. It primarily shows that high temperature is by far the main culprit when it comes to degrading the LED. Current (high or moderate) appears to play very little part.

I’d like to see a study based on very low current. I’ve never heard of this. And based on solar lawn lamps (that use a dim LED), I don’t see it anecdotally. Also, many electronics use dim LED to display status or time, and they run continuously for years or decades.

Though perhaps the poster is referring to modern high-output LEDs. I’m a moonlight mode fan, so I’d be concerned if the speculation is true.

Maybemaybemaybe it has to do with 1 of 2 things.

1, it might collect moisture in the phosphors, and high currents (thus high heat) helps “boil off” the moisture. Kinda like occasionally running the engine of a car that’s kept in long-term storage, vs just letting it sit, collect condensation, and rust away internally.

2, repeated operation at lower currents might “pull” the innards of the crystal a certain way over time, and high heat helps anneal the innards and reduce the stress. Back to the car analogy, having an engine that just runs at idle forever, and is never “opened up” to shake loose all the crap that might’ve collected.

Other’n that, I’m stumped.

Well, since I’ve never heard of The_Driver’s speculation about low current causing LED aging, I’m going to file that info in the “likely not true” bucket. At least, until I see some study that proves otherwise. If it were true, you’d think it would show up in some manufacturer’s testing results, such as Cree’s LM80 testing.

So far, the only thing I’ve read that significantly reduces LED lifepspan is heat, Specifically, when the LED junction temperature gets above 100C. Of course, electronics can fail in lots of other ways too, and as others have pointed out, the driver is likely to fail before the LED noticeably dims.

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From 13.16 to 16.20 he talks about how low current is responsible for dimming and color changes over time. much more so than high current. He shows three graphs to illustrate this.

My knowledge of electronics is very limited, and his Austrian(?) accent doesn´t make it any easier to understand. But i think that is what he is saying.

The_Driver
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The guy is an expert working for Osram on these things. You would think that he knows his stuff when he holds a presentation on this topic. I just saw this video a few days ago and it was interesting, but seemed plausible.

In the video he states that this is something that manufacturers don’t tell you in their datasheets. That is a known problem. For example most manufacturers don’t state the luminance (cd/mm^2) of their LEDs even though it is crucial information (for throw). Osram is one of the few which do this.

If you look into the datasheets of some power LEDs you will see that they state rather high minimum currents. Maybe part of the reason is the reduced lifespan.

@walkintothenight: this is not my speculation! You’re dismissing it rather quickly…

WalkIntoTheLight
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Kepling wrote:
From 13.16 to 16.20 he talks about how low current is responsible for dimming and color changes over time. much more so than high current. He shows three graphs to illustrate this.

My knowledge of electronics is very limited, and his Austrian(?) accent doesn´t make it any easier to understand. But i think that is what he is saying.

From Cree’s testing, high current doesn’t appear to be much of a factor in dimming. They have no info on low current. Their testing shows that heat is the main culprit with dimming over time.

I just don’t see any evidence of low current affecting LEDs. If this was the case, wouldn’t we notice it in electronics that have “always on” LED indicators?

The_Driver wrote:
@walkintothenight: this is not my speculation! You’re dismissing it rather quickly…

I’m dismissing it (at least for now) for 3 reasons:

1. I don’t understand German, so I can not evaluate what he is saying.

2. It’s a rather surprising and counter-intuitive claim. This requires good evidence to dispute it.

3. I have never seen such a claim anywhere else, nor can I find any reference to it using Google.

Nevertheless, it’s an interesting claim, so I would like to see some independent evidence to back it up.

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WalkIntoTheLight wrote:

I just don’t see any evidence of low current affecting LEDs. If this was the case, wouldn’t we notice it in electronics that have “always on” LED indicators?

No, because how low “low” is depends on the specific led type. A Cree XM-L has a different range compared to an indicator light.
Larger LEDs are usually more expensive compared to smaller ones. For an indicator LED nobody would use an LED that is underdriven. It makes no sense.
Similarily in flashlights nobody will make a light with an XM-L powered at 10mA in the highest mode. It just doesn’t make any sense.

I think this “problem” is not really a problem. Basically nobody will ever notice.

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Was there a figure in the presentation that represents how much more low current degrades the LED? At this point we (the non German speakers here) have no idea even the order of magnitude of whatever this effect is.

I agree it’s probably not a problem, especially for us flashlight users; thousands of hours is a long time.

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Well all I can say about lifespan is that my 10 year old fenix P2D that I EDC for 10 years, every single day. Was rated at 140 lumens new and still proudces 140 lumens today.

The low vs high current thing is interesting and on some level that I can’t quite put my finger on, makes sense. Just not sure why it does lol.

Maybe this is part of why PWM is used so extensively with LED’s? Thus allowing higher currents with lower outputs?

One possibility that comes to mind is that a higher current would like the LED up faster and “more completely” then a lower current.

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Texas_Ace wrote:
The low vs high current thing is interesting and on some level that I can’t quite put my finger on, makes sense. Just not sure why it does lol.

Ever hear of an “Italian tuneup”? Big Smile

Back in the day, at least in my nabe, when your car starts coking up with carbon deposits, etc., from stop’n‘go driving, an IT would be taking it on an open highway and running it wfo for as long as possible to burn off all those deposits.

Kind of instinctive, and it usually worked!

Texas_Ace wrote:
Maybe this is part of why PWM is used so extensively with LED’s? Thus allowing higher currents with lower outputs?

Mainly to prevent color-shift from high/low currents. Dunno if designers even knew about this phenomenon. I’ve played around with LEDs since dinosaurs walked the earth, and this is the first time I ever heard of it!

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WalkIntoTheLight
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The_Driver][quote=WalkIntoTheLight wrote:
Similarily in flashlights nobody will make a light with an XM-L powered at 10mA in the highest mode. It just doesn’t make any sense.

But it makes plenty of sense for the lowest mode. My Zebralights typically use less than 5mA in moonlight, and I’ve run some of them in moonlight mode for many months (cumulative). Often I just use them as a nightlight, and sometimes don’t even bother to shut them off (since they’ll last a couple of weeks on a charged cell).

Granted, I’ve probably only used about 10,000 hours in moonlight mode, at most. But I still see no sign of dimming (on the high modes). If low mode was really more damaging than high mode, I should probably start to notice it.

That’s only anecdotal, so I’m not saying it’s proof. But until I see proof that moonlight mode really is damaging, it’s difficult to accept it as anything more than the speculation of one person. And since I don’t understand German, I’m not even sure if that’s what he said.

I also have a Sunwayman D40A n/w that has a ultra-ultra moonlight mode “feature” (actually it’s a bug). If I shut it off from moonlight, it goes into a very very low moonlight mode, that stays on indefinitely (until I use it in high mode or I change the battery). It stays this way for months sometimes, if I’m not using the light. Again, probably at least 10,000 hours in that mode. Since it uses just microamps, surely I’d see some damage by now.

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Very low currents cause cancer.

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Ok, I will post some details from the video:
- Dr. Berthold Hahn is the Senior Director of Chip R&D at OSRAM OS
- here the pdf with the slides he shows during the presentation
- the important slides are 15 and 16
- they show the reduction in output of an LED depending on temperature and current
- B50 = L70
- B10 = L80
- the dottet lines are for the lower noted current and the solid lines for the higher current
- first slide shows 350mA and 100mA
- second slide shows 350mA and 10mA
- 10mA reduces the lifespan compared to 100mA and 350mA
- reason according to him: at low currents LED ages faster because of diffusion of interference currents in the “active zone” (he doesn’t go into great depths here…)
- so it tells you that an LED (probably 1mm^2 or 2mm^2) from Osram will lose 50% of it’s output in 9000hours when driven @10mA and 85°C
- at 100mA and the same conditions it takes maybe 30,000h (the graph doesn’t go that far)
- for flashlights it absolutely doesn’t matter! Why you ask? Because at low currents like 10mA the LED will never become hot. Low currents are only bad in combination with very high temperatures,.

The presentation was held in 2014 at a big lighting event from a university in Germany where experts from different companies held presentations on current lighting topics.

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I am only mildly interested in each and every detail so I just watched parts of the video to understand the problem.
Here is what I learned:

- LEDs age from impurities (defects) in the semiconductor that move into the active zone (where the light is ‘produced’). More defects – less light. The spectrum also shifts.
- The first mechanism that drives the motion of the impurities: High temperatures. The hotter the more motion.
- The second mechanism is low current density. The slide (in English) from 11:00 on shows a red area of low current density. A lot of the electrons go into the impurities without emitting light. They are feeding the impurities with energy and make them move more.

When you drive a 350 mA LED with 10 mA only it will fade faster that when driven at 100 mA (see 14:30 vs 15:40).
The low current ageing is so bad that it can be better to drive LEDs harder to get a better life span (13:30 green curve, different LED, under 1 vs 20 mA).
This is a bit unexpected and most suppliers do not tell you.

However with low current you will always have a much much lower LED temperature (around ambient), so we are talking about the upper right corner of diagrams 14:30 and 15:40. Maybe at 30°C the life span of the LED will not be dramatically low.
.

So good news for all modders: It is the temperature, not the current.

Bad news for firefly mode fans: Your light may not survive mankind.

WalkIntoTheLight
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The_Driver wrote:
Ok, I will post some details from the video: - Dr. Berthold Hahn is the Senior Director of Chip R&D at OSRAM OS - here the pdf with the slides he shows during the presentation - the important slides are 15 and 16 - they show the reduction in output of an LED depending on temperature and current - B50 = L70 - B10 = L80 - the dottet lines are for the lower noted current and the solid lines for the higher current - first slide shows 350mA and 100mA - second slide shows 350mA and 10mA - 10mA reduces the lifespan compared to 100mA and 350mA

Thank you for the translation.

I’m not sure I understand the slides. The first slide indicates that the 350mA 55C graph reaches about 94% after 10,000 hours. But the second slide it looks like this same graph reaches about 94% after only 1,000 hours, and hits 80% after 10,000 hours.

Are they showing the same thing?

Quote:
- reason according to him: at low currents LED ages faster because of diffusion of interference currents in the “active zone” (he doesn’t go into great depths here…) - so it tells you that an LED (probably 1mm^2 or 2mm^2) from Osram will lose 50% of it’s output in 9000hours when driven @10mA and 85°C - at 100mA and the same conditions it takes maybe 30,000h (the graph doesn’t go that far) - for flashlights it absolutely doesn’t matter! Why you ask? Because at low currents like 10mA the LED will never become hot. Low currents are only bad in combination with very high temperatures,.

Okay, that is a key point. Temperature is still doing the killing. A typical 10mA current in an LED will generally result in room-temperature (or close to it) at the junction point.

I’d still like to see some further testing of this, especially at room-temperature, but this is an interesting start.

Mitko
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The thing that kills leds most is the heat- all the way heat: ofc that is valid if you dont overdrive them great time.
Overcurrent and voltage spikes also kills them rapidly

Mousture is quite cunny too, it atacks solder joints

Usualy, leds dergade slowly and after some point they start blinking, either cob chips and single die emitters
Some of them die all in an instant yet most of them do the final disco dance Cool

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I watched this video

And I am native german, so I did understand well why low currents are far worse than heat to kill LEDs

The current protects the very thin active light emitting zone from electrically defective spots to migrate into it from the surrounding material, this active zone is a lot thinner than the rest so migration is a huge issue
Of course heat also increases the migration of electrically defective spots

On our CC moonlight flashlight the LED can age over 10 times faster than on max. rated current
You see the plot at 16:00 of the video
And that plot shows the LED running on 3% of its normal current
Our moonlights and firefliey modes on CC lights get below 0.1%

The odd thing is if we get good cooling an overdriven LED ages slower than at rated current when they got the same junction temperature

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Interesting.

If this holds up, it occurs to me that people who almost always use low modes might be better off carrying a light with a less powerful LED – think XPE, not XPL – for routine use of low modes, and a second light with a more powerful LED if they want higher output from time to time.

That way, the same low current for the low mode would represent a larger percentage of the less powerful LED’s optimum current, hopefully reducing the defect migration effect.

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Still have to watch the complete video, but as a conclusion I’d say our often used FET+1 design is a good idea to realize moon modes since the LED is driven with at least 350 mA (with a very short PWM duty cycle).

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Yeah but in flashlights you use an LED which can live >50000h
If the life cycle gets below 5000hours on CC Moon mode its not a big deal as its not your living room light that burns on abeverage 4 hours a day

Gor people that use flashlights as joule thiefs for their bedroom lighning every day its not good news if those lights have CC driver

Same counts for dimmable lights in your household, if you always dim em to a very low level they age pretty fast, but the effect is not dramatically above 10% of current, but below its really noticeable aging of it

I was having my floor lightning every night on the lowerst dimming at approx. 2% on so my cats have light in the night, ot does not take much power, but now I know it wears down the lights very badly

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The closest thing I have in terms of long running LEDs is a pair of LED Lenser blue 5mm pig tail LEDs wired in series. I am pretty sure they are Nichia re-branded and marketed by Lenser. I can’t for the life of me remember what kind of current and voltage I am driving them with. But they have been running constantly and continually for 8-9 years in a solid state headphone amp that never turns off (a modded Larocco PPA for those interested in a google search). They have dimmed slightly over that time. I am pretty sure I am over-driving them to some degree.

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As the Osram guy explains in his video, an overdriven LED ages less if the temperature is the same as on lower currents because you cool em better

Two things let age LEDs
1. low current
2. heat

It sounds odd but current protects the active zone from aging by migration of faulty spots into it from the surrounding p-n substrate
At 16:15 you see the aging diagram from an 350mA LED driven at 10mA

and the impact is far worse than one driven at 100mA at 150dC junction temperature

Also what testers like Djozz see that at hard overdriven LEDs the radiaten decreases again does not come from heat
In fact its an effect calles Auger recombination
He explain it that there are 3 electrons in the active zone and instead of all 3 producing light two of em use their energy to kick the 3. out of the active

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Okay, this is all interesting, but has this study been confirmed by an independent lab?

Since I don’t understand German, I’m not sure if they tested moonlight mode currents at low junction point temperatures, and compared it to high currents at high temperatures. That is what you will get with flashlights. It’s always been my understanding that heat is what ages an LED.

If moonlight mode only ages an LED rapidly when it’s also hot, who cares? I’m not sure if that’s what they were testing, but it isn’t what occurs in real life.

And, in my own anecdotal experience (which I know doesn’t make for any scientific evidence), I have used some flashlights on moonlight mode for thousands of hours (XML2 LEDs). They’re on almost 24/7, and have run that way for over 2 years. If moonlight mode was so damaging to them, you’d think I’d have noticed a significant dimming by now.

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