Is heat a "cumulative" thing?

Rosoku Chikara

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Is heat a "cumulative" thing, or does it get "reset" each time you use your LED light?


Let me see if I can explain my question more clearly. I am not talking about driving a light at higher than recommended voltage. I can easily see where higher voltage might easily degrade a circuit and/or LED emitter in such a way that it eventually fails. (Even though it might work for quite a while, before it eventually fails.)


What I am talking about is a light that is driven at recommended voltage, but tends to slowly get hotter and hotter. (Or perhaps, I should say "warm" since I am not talking about something that is truly scorching to the touch.)


Does such heat eventually damage (lower the life expectancy of the light) by cumulatively harming parts? (If so, does that mean you might want to try to keep your lights as cool as possible?)


Or, so long as you "get away with it" at the time (meaning, so long as the light doesn't actually die when it gets "very warm"), would you say the life expectancy of the light is "reset" back to normal again the next time you turn it on?


I hope my question is clear. Sorry if it is a dumb question.
 

mvyrmnd

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There's no such thing as a stupid question :)

But I have no idea about the answer...

If I had to guess, I'd say that if damage were to occur each time the LED got too hot, then yes, the damage would be cumulative - as the LED has no function to repair itself.
 

FoxyRick

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Definitely not a dumb question, quite the opposite!

Yes, to an extent the effects of heat are cumulative. There are a number of effects that heat has on semiconductors that diminish its life. I'm more familiar with silicon but I remember reading that LEDs are particularly effected by the nucleation and growth of 'crystal fractures' from tiny impurities present in the crystal.

This growth is accelerated by current flow and heat, so every time you use the LED, the disruption of the crystal structure will be increased a little by the growth of the dislocation. These can also be nucleated by capture of ionising radiation (always present and causes occasional memory glitches in computers for example) or even by the intense light itself I think.

Another effect is the migration of atoms/ions from all the boundaries in the semiconductor, including the conductive joints. Again, this is accelerated by heat.

I would guess even the phosphor breaks down slowly, both chemically and mechanically.

Most chemical reactions are accelerated by heat, and mechanical action (breakdown, cracking etc) is spurred on by heating and cooling cycles. Like freeze/thaw cracking a rock.

None of this requires the LED to be abused, it just happens with use and is inevitable.

Whether the reduction in life from this is significant compared to other failure modes though, I don't know. I've never had an LED flashlight fail other than by abuse, or quickly probably due to manufacturing defect.
 
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LEDburn

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Think of it like the dashboard on older cars: it wont crack the first time the sun hits it but after a certain amount of damage it will eventually crack.
For one car that gets left out all the time, it might be 10 years whereas another car of the same variety may last 20 years if housed indoors or under cover most of the time.

Maybe the manufacturer rated life expectancy is rated at a certain temperature and drive current.
Anything under those values may lengthen the life expectancy but I would say anything over would therefore shorten it.
 

subwoofer

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I would say that the question as in the thread title needs to be changed to:

"Is the damage caused by overheating an LED cumulative?"

Base on this, the answer is yes.

However, first we must consider that all LEDs deteriorate if used at all. The higher the output the faster the LED's performance will degrade.

Take a new LED light with stated lifespan of 100,000 hours. This is the time after which the output of the LED drops to 70% of its original intensity. However, this depends entirely on how hard the LED is driven. You could run the LED at 5% of its rated output for probably many times this stated 100,000 hours and see virtually no drop in output, however, run the LED at its maximum rated output and the lifespan will be much closer to this.

If however you do not heat sink the LED properly, it may be running hotter than the manufacturer specified this lifespan, and the actual life would then be possibly far less than this.

Overheat the LED and the normal gradual deterioration will accelerate significantly and this will be permanent, just as all the deterioration from normal use. Overheating however will shorten the life of an LED significantly.

If you equate this to a car tyre. The tyre has a certain amount of tread when new (the running time of the LED). In normal use, the tyre will gradually wear at a normal rate (LED's normal lifespan). Now do some doughnuts in the car park and you have fun burning away the tyre tread and making the rubber hot and sticky, which makes it wear away even faster (over drive, or over heat an LED and it is the same). The damage is cumulative, you have worn away your tyre's tread. It will not 'reset' and magically come back. Nor will the LED's usable lifespan.
 

dougie

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Wow, subwoofer has put it very succinctly and with a great analogy about car tyres and how they wear. However, LED's are but one component in a myriad of electronic components used in modern flashlights each of which has the potential to fail from either manufacturing defects or physical impurities used in the components. The best and only hope for a long trouble free life is to use the item within the manufactures specifications and having confidence in who made the product and how well!
 

Fireclaw18

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I would say that the question as in the thread title needs to be changed to:

"Is the damage caused by overheating an LED cumulative?"

Base on this, the answer is yes.

However, first we must consider that all LEDs deteriorate if used at all. The higher the output the faster the LED's performance will degrade.

Take a new LED light with stated lifespan of 100,000 hours. This is the time after which the output of the LED drops to 70% of its original intensity. However, this depends entirely on how hard the LED is driven. You could run the LED at 5% of its rated output for probably many times this stated 100,000 hours and see virtually no drop in output, however, run the LED at its maximum rated output and the lifespan will be much closer to this.

If however you do not heat sink the LED properly, it may be running hotter than the manufacturer specified this lifespan, and the actual life would then be possibly far less than this.

Overheat the LED and the normal gradual deterioration will accelerate significantly and this will be permanent, just as all the deterioration from normal use. Overheating however will shorten the life of an LED significantly.

If you equate this to a car tyre. The tyre has a certain amount of tread when new (the running time of the LED). In normal use, the tyre will gradually wear at a normal rate (LED's normal lifespan). Now do some doughnuts in the car park and you have fun burning away the tyre tread and making the rubber hot and sticky, which makes it wear away even faster (over drive, or over heat an LED and it is the same). The damage is cumulative, you have worn away your tyre's tread. It will not 'reset' and magically come back. Nor will the LED's usable lifespan.

Sounds about right.

Personally, I think heat degrading the LED is probably a non-issue for most flashlight owners. Even if your LED is only getting 20% of its rated 50,000 lifespam, it's still probably far outlasting the rest of the components of the light. Chances are very high, that the light will cease to function for some other reason or be replaced as outdated long before heat causes the LED performance to degrade noticeably.
 

AnAppleSnail

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Cree's published study indicates that the temperature around the LED dome has the greatest impact on dimming over time. But it still takes tens of thousands of hours to get to 70% at rated current, and thousands at max current.
 

Rosoku Chikara

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Thanks for the many thoughtful and kind replies.

I'd say that if damage were to occur each time the LED got too hot, then yes, the damage would be cumulative...

I guess my question becomes: How much heat is "normal wear and tear" (presumably factored into the rated 50,000 hour lifespan of the LED) and how much heat is "damage"?

Whether the reduction in life from this is significant compared to other failure modes though, I don't know. I've never had an LED flashlight fail other than by abuse, or quickly probably due to manufacturing defect.

But, as a more practical issue: Is the reduction in life from "non-lethal" heat significant compared to other failure modes? (I say "non-lethal" heat here, because as I understand it, high enough temperatures result in immediate and total failure.)

I would say that the question as in the thread title needs to be changed to:

"Is the damage caused by overheating an LED cumulative?"

Base on this, the answer is yes.

However, first we must consider that all LEDs deteriorate if used at all. The higher the output the faster the LED's performance will degrade.

The reason this question even occurred to me was that I was testing the run times of several lights and I was leaving them on (in order to see just how long their batteries would last) for much longer periods than I would normally use them. And, I began to notice that most of the lights started getting quite warm after long periods on their highest output setting. I couldn't help but wonder whether or not I was "damaging" my lights. (I thought it would be ironic if, in my attempt to verify exactly how many hours I could "count on" a light in an emergency, I was actually dramatically increasing the chances that the light would end up failing in an actual emergency.)

Personally, I think heat degrading the LED is probably a non-issue for most flashlight owners. Even if your LED is only getting 20% of its rated 50,000 lifespan, it's still probably far outlasting the rest of the components of the light. Chances are very high, that the light will cease to function for some other reason or be replaced as outdated long before heat causes the LED performance to degrade noticeably.

I am thinking that Fireclaw18s response that "heat degrading the LED is probably a non-issue" is true, at least for me.

At 20% of a 50,000 hour lifespan, I could still get over 9 years service out of a flashlight, even if I used it 3 hours a day, everyday. (Which is far more than I currently use any of my flashlights.)

I am more concerned about increasing the probability of future "sudden death" (in an emergency when I really need the light). I am less concerned about any slow dimming of the LED (I am willing to use a dim light, so long as it works).

Anyway, thanks again for all the replies.
 
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AnAppleSnail

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The LED is quickly damaged when parts of it get over 150C. In a reasonably well built light made of metal, the flashlight will be within 50C of that temperature. Your fingers would probably stick to an assembled light that was cooking its LED. Mostly bare LEDs with no heatsink get killed by heat.
 
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