# what happens to LED's during thermal runaway



## ledlurker (Jan 16, 2002)

*what happens to LED\'s during thermal runaway*

Just want to know in general. Specifically what would happen to a pro-series 3 6V Luxeon in the dorcy cool blue style flashlight.

BTW found a Dorcy workhorse 6V with batteriesat the dollar store today for $3. I had a faceted reflector and looks like the reflecto with the cool blue. I will let ya'll know if if works out


----------



## Badbeams3 (Jan 16, 2002)

*Re: what happens to LED\'s during thermal runaway*

Ok, since no one is answering this I`ll take a shot at it...be easy on me fellow members...the LS in general I`m told...looses 10-15% of it`s brightness within the first 1-2 second of turn on according to John at Ledcorp...the cooler you can keep it the less loss...or at least the slower the drop as it heats up...Now as I understand it...for any given heatsink...there is a ma/volt point that it will no longer keep the temp of the emitter under control...as the led heats up it has more resistance...and creates even more heat for the already loosing the battle heat sink. The colder the chip (emitter) stays, the brighter at any given voltage or millamp it is. In thermal runaway...the point has been crossed...the led begins to use more power...but does not put out much addition light realative to the power it`s starting to suck. It gets hotter, the restance goes up, so it gets even hotter, the restance goes up more, and it gets even hotter...as this chain reation of heat happens the poor LS begins to dims, suffers permenant damage and finally sticks it feet in the air...dead. 

Bottom line...cold good...heat bad.

Ok fellow members correct me.


----------



## lambda (Jan 16, 2002)

*Re: what happens to LED\'s during thermal runaway*

you could probably kill a lux with 250ma, if it had no heatsink. But I've got a lux running 500ma for last 10 days and it's not even warm (mounted to large heatsink). Very bright, and I doubt there will be any probs. Yes, the led is rated at xyz, but like scotty on star trek, overdrive that sucker, you might be suprised what it will take. Just rember, in any event, there is lesson.


----------



## Badbeams3 (Jan 16, 2002)

*Re: what happens to LED\'s during thermal runaway*

Now here is a led heatsink! www.optotech.com Click on products and solutions...click on high intensity lighting...scroll down


----------



## papasan (Jan 16, 2002)

*Re: what happens to LED\'s during thermal runaway*

actually i thought it was the other way around...i thought thermal run-away was because as the LED heats up it's resistance goes down and therefore sucks more juice and therefore gets hotter and therefore the resistence goes down and therefore it sucks mroe juice...etc etc etc until it melts (probably the small bonding wire melts off?) and either short circuits (fuses together) or opens (seperates)...either way you got one taosted LED...


----------



## lambda (Jan 16, 2002)

*Re: what happens to LED\'s during thermal runaway*

Shark looks interesting, but who sells them? Nobody. Want to see my artist rendition of the 300 million candel power LED ?


----------



## Badbeams3 (Jan 16, 2002)

*Re: what happens to LED\'s during thermal runaway*

Hmm, that does make more sense...ok I`ll buy that...forget what I said...BTW cute kid.


----------



## Badbeams3 (Jan 16, 2002)

*Re: what happens to LED\'s during thermal runaway*

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by lambda:
*Shark looks interesting, but who sells them? Nobody. Want to see my artist rendition of the 300 million candel power LED ?*<HR></BLOCKQUOTE>

It wouldn`t be 300 million 20 degree Nachia`s stuck around a used carnival ferris wheel would it?


----------



## **DONOTDELETE** (Jan 16, 2002)

*Re: what happens to LED\'s during thermal runaway*

Actually, thermal runaway is a complex phenomenon, because resistance _increases_ as temperature does.

(Remember that supercooling leads to virtually no resistance, hence, superheating leads to higher resistance.)

Thermal runaway is simply the inability of a material to dissipate its own heat. Heat is directly proportional to power, which is resistance times current squared.

At small currents, temperature is not a factor at all.

At moderate currents, as current increases ever so slightly, temperature increases slightly, increasing resistance by a similar amount, which in turn reduces the current again. Here, the system operates in a steady state, and power is constant.

But beyond a critical value of current, the square of the current starts to dominate the increased resistance. The additional resistance no longer can lower the current, so the temperature can no longer go down.

The material continues to absorb power it can no longer dissipate, so its temperature starts to increase rapidly. It's now just a matter of time before the material destroys itself.


----------



## ledlurker (Jan 17, 2002)

*Re: what happens to LED\'s during thermal runaway*

Ok, I will just have to live with the maglite until I can find and alternative.

BTW, the 4# Dorcy Boss jeweled reflector fit the Dorcy Cool Blue perfectly. Now I have a much larger beam that is nice and even spot. If I figure out a way to heat sink it I will put the pro-series bulb in.


----------

