# How can I tell if I've damaged my LEDs?



## Mattaus (Jun 4, 2011)

Hi all,

I recently built the XM-L based Flood lights as detailed in this thread.

It's been a few weeks since I finished all 3, and I finally got the chance to test them out last night. I ran all 3 flood lights off a single 105Ah battery for 4.5hrs at full power which for the smaller one is at 2.4A while the larger flood lights were running at 3A.

At no point did any of them get too hot to touch. The smaller one was getting close, but it reached this point after maybe 2 hours and remained at about the same temperature for the rest of the test. In fact you had to leave your hand on the light for a good 30 seconds before you felt it was uncomfortable. The larger flood lights were warm but never alarmingly so. The light output never seemed to change and there was no noticeable flickering or anything abnormal.

However! What I need to know is how can I tell if the LEDs are still OK visually? The LEDs are thermally glued to a 3mm thick aluminium bar which is in turn bolted and thermally pasted to the insides of the housing (as can be seen in the thread link above). What I'm worried about is that the thermal transfer is not good enough - so the cases might be fine to touch but the LEDs could be cooking themselves...

This morning I took them apart to get a close look at the LEDs and I noticed that a few (maybe 3 of the 9) had developed a small brown spot on the LED lens (or whatever the bubble is called) that cannot be scrapped off. 

Is this heat damage??? If it is how can I improve thermal transfer to the case? Whack some aluminium foil inside the back of the reflector or something? Obviously I need to avoid shorting the whole lot out...

Any advice would be appreciated. These are the first lights I have ever built and the first time I've ever used high power LEDs as well, so I may be making mistakes I'm not even aware of.

Cheers.

- Matt


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## videoman (Jun 4, 2011)

Can you take a photo of the brown spot on the leds ? Leds usually do not show brown coloring when they are overdriven or about to go. How about a Q-Tip with rubbing alcohol applied gently to the led lenses where the brown spots are ( powered off of course). Could be a thin film of solder flux residue changing color to brown for whatever reason. Strange.


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## Mattaus (Jun 5, 2011)

Something I forgot to point out before but when I opened the housing for the first time today it wreaked a bit...not a smell I've smelt before. I should note that I never cleaned anything after I soldered it all together. Poor practice I know, but it is something I am making a note to do in the future.

This is the best I could do photo wise. Ironically I just sold my SLR because it never got enough use, and it had a nice 100mm macro lens with it. Typical.





The best way I can describe the dot is like a piece of something burnt that's stuck itself to the lens, but it definitely feels 'embedded' into the surface. I'll grab some rubbing alcohol tomorrow and try clean it with that. Also after another inspection it's only 2 of 9 LEDs that have the dot. I'd think that if the heat was affecting 1 enough to start burning it, that I would have seen something with the others surely?


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## MikeAusC (Jun 5, 2011)

It looks like a speck of flux on the lens - with the heat, it has now etched its way into the lens.

To maximise heatsinking, keep in mind that the BEST thermal compounds have one thirtieth of the conductivity of aluminium.

So you need to aim for metal-to-metal contact at the microscopic level - using the compound only to fill in valleys in the metal.

Apply pressure so that contact is mainly metal-to-metal, squeeze out all excess compound.

Polish all metal surfaces to be flat to allow maximum metal to metal contact.


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## videoman (Jun 5, 2011)

Best to leave it alone. As trying to get it out may cause the entire led lens to fall out. Before the led goes to "poof". it will go to a very bluish color perhaps with white smoke and then you will see that the gold bond wires melt off like a burnt fuse. That brown circular spot is definitively hot resin flux from the solder most likely caused by a too hot soldering iron or solder that has a high percentage of resin in it. I have had the same problem myself on several occasions.


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## Mattaus (Jun 5, 2011)

OK but where did it come from? It certainly wasn't there before my extended test last night and I see no reason for it to work its way onto the lens...odd. Regardless of this if the cases and the LEDs didn't get too hot last night it seems the thermal transfer is working OK. I won't bother re-seating the whole thing unless the need arises because it's a pain the bum lol.


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## MikeAusC (Jun 5, 2011)

Initially the flux would have been clear - then continued baking would have discoloured it and the lens material.

When are you going snowboarding - I'll be at Perisher next week working on the Snowy Mtns of Music festival.


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## Mattaus (Jun 5, 2011)

We're heading down July 16 for a week. Very excited but a bit worried I might have bitten off more than I can chew with the whole camping bit lol...hopefully I can get my Maglite mod done in time

I've checked the lights again and the spot is not causing any distortion to the the light that I can see so it looks like it won't be an issue.


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## RepProdigious (Jun 5, 2011)

Looks indeed like something worked its way into the dome, not as if the die itself is coming apart..

But if i were you i'd do something about the heat-sinking for the middle led because to me it looks like the strip aluminium you mounted the leds on is only potted at the ends. It takes time for heat to work its way through a material and because you have one of the leds basically surrounded by the others heat will build up on that one way quicker than on the others so you're basically creating a unnecessary bottleneck right there.


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## Mattaus (Jun 5, 2011)

RepProdigious said:


> But if i were you i'd do something about the heat-sinking for the middle led because to me it looks like the strip aluminium you mounted the leds on is only potted at the ends. It takes time for heat to work its way through a material and because you have one of the leds basically surrounded by the others heat will build up on that one way quicker than on the others so you're basically creating a unnecessary bottleneck right there.



Good point. Like I said I'm new to _all_ of this so it never occurred to me. Provided I don't short anything, would maybe stuffing a whole load of aluminium foil (food wrap stuff) into the back of the aluminium bar and reflector help dissipate heat at all? Without decent physical contact it might not do too much however. I would like to avoid modifying the housing if possible but maybe some drilled vent holes in the base and a small PC fan for active cooling would work? I like my first idea better though...a combo of the foil and some air holes.


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## RepProdigious (Jun 5, 2011)

Aluminium foil wont do a whole lot except maybe short something out, it could even prevent the little air-flow you have in there form happening and thus making things worse  The problem with stuffing foil is that you have a lot of surface-contact with air in between which is bad. A single piece bar of aluminium will conduct heat way way better from end to end than 3 shorter pieces put together with the same overall dimensions even if you put heat-paste between the parts because the connecting surfaces will always create bottlenecks. Wrapping multiple layers foil where you could also use thicker material is just dumb when heat is concerned. because you create one os this weak-spots between each layer. You'd be better off if you'd take another one of those pieces you have the leds mounted on and bend it in a u shape in such a way that you could connect one side to the middle of your led strip thing and the other side on the housing of the light if its a good conductor. Make sure there's plenty surface contact (more surface is better because its a bottle-neck) and use some heat paste.

You could also see how big your problem is exactly, turn the light on high and see how long it takes before it gets too hot to touch....


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## HooNz (Jun 5, 2011)

How about Bug doo doo , fly , cockroach all those types of things , out here in rural i see similar marks on glass/lcd monitor/u name it .


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## VegasF6 (Jun 5, 2011)

Mattaus said:


> At no point did any of them get too hot to touch. The smaller one was getting close, but it reached this point after maybe 2 hours and remained at about the same temperature for the rest of the test. In fact you had to leave your hand on the light for a good 30 seconds before you felt it was uncomfortable. The larger flood lights were warm but never alarmingly so. The light output never seemed to change and there was no noticeable flickering or anything abnormal.
> - Matt



Where are you checking the temp at that feels warm with your hand? On the steel enameled outer body? Or touching the aluminum plate? There are so many areas of thermal resistance I wouldn't expect the heat to travel well. I don't guess you have any sort of measuring device. If it was me I would use my meter and thermocouple, route the thermocouple behind the glass window at the bottom and attach it with polymide tape to the center led as close as possible to the ceramic package. That won't give you actual die temperature, but it's as close as you can physically measure. 

I hate to be a negative nancy here but I am concerned you don't have anywhere near enough heat sinking material or surface area for nearly 30 watts of led! I mean not even close.
The outer body of that light has very little value as a heat sink, the aluminum sheet metal is carrying the load and you don't have much of it. You have a relatively small piece in the back with that screw in the middle of it, and then an even smaller piece that your leds are mounted too. It's not really clear how you attached to two of them together. Then you have almost zero air flow.

If I saw a simple fix I would point it out, but I am thinking you really need to go back to the drawing board and start over again. I think if I was going to do this I would buy a premade finned heatsink and mount it to the top exterior. I would use copper plate folded in an L shape attached to that heatsink, pass it through a slot down into the lights body and mount my leds to that.


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## Mattaus (Jun 5, 2011)

> How about Bug doo doo , fly , cockroach all those types of things , out here in rural i see similar marks on glass/lcd monitor/u name it .


 

They're sealed up pretty tight. I'd be very surprised if anything got in there to be honest.


RepProdigious and VegasF6 are probably both right - I need to sort my heatsinking out better. I have a temperature gun that measured 110F but I can’t remember if I removed the glass or not when I took that reading. Besides when I do the conversion that’s 43C which in my mind is not hot at all, so it must be wrong.
 

I should point out the larger flat bar that makes contact with the sides of the housing is wedged in there so tight it can’t be pulled out without a great deal of difficulty. It’s also bolted on at the back. So the contact at those points is as good as it’s going to get with out welding them in. The large amount of thermal paste you can see in the build pictures was simply added as a safety net – though I now know that was a bit of a waste.


Taking the whole lot apart is going to be a massive pain, especially seen as my budget is in a painful state because of these lights already lol. I’ll have to investigate further but at this stage I’ll probably take the smaller bar the LEDs are mounted too off, and make a larger bar that connects to the top and bottom of the housing. So it’ll be like a big cross sort of deal. I’ll drill and tap some mounts so that the 2 bars are tight together as well.


:sigh: 


At least I built them for running outside in the snow which should help. Though they'll then be useless in our Australian summer.


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## LedGod (Jun 6, 2011)

What you have looks very much like EOS, loosely defined as electrical overstress, worst case you will blow bond wires, more moderate case is discolouration in areas for the die. Typically occurs due to electrical stress event, power supply issue, current surge etc. It is not necessarily catastrophic but will have an impact on the led performance either life or day to day performance

Cheers
LedGOD


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## RepProdigious (Jun 7, 2011)

Another thing you could do is place the light on its back (glass side up) drill a hole in the case and fill the whole back with epoxy with heat conductive yet electrical insulated filler (dont know the name of the powder stuff in english). Makes servicing a big no-no but it adds mass (which is good for relative short uses) and also provides a wider heat conductivity path to the outside world. And it isn't all that expensive and relative easy to do.


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## netprince (Jun 7, 2011)

Have you considered an active cooling solution, such as a fan? I'm following in your footsteps and building something similar, I've got a large heatsink and I'm planning to attach a cooling fan which will have in and out channels. 

Also, would be cool to use a cpu water cooling system which would allow your enclosure to remain mostly sealed, only a hole for a fluid hose would need to be drilled.

Good luck on your trip!


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## deadrx7conv (Jun 8, 2011)

Fans, water cooling.... just make a simple project too complex. 

Reduce the power to something more reasonable for the thermal area available. 
For the size of the aluminum bar, along with the size of the housing(insulated with paint), you're probably limited to <20w. You pushing 25w-30w?

If the light isn't easily disassembled, remove the reflector and add some forward heat sink fins(chopped up CPU heatsinks). Don't forget to strip any insulating paint from the the housing's light mounts. This way, more heat can possibly travel to the light stand. The reflector, if metal, can be thermal epoxied to your aluminum LED mount bar. 

I also wonder if you can drill a hole right through the aluminum bar stock(+ light housing) and use some copper bolts(as heat pipes) bolted to a finned additional heatsink mounted to the back of the light. 

The overheat shortened its life and might have reduced its output. As long as the LEDs produce acceptable light, keep using it. 

I also wonder if your aluminum(what type of aluminum) bar is a good enough to use a heat transfer medium. And, what thermal glue did you use? Generic thermal epoxy might be insulating. 

Your aluminum bar needs to be finned. The housing needs to be finned. A copper bar would be another option if you build more. Old CPU computer heatsinks would be a better mount than that bar stock.


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## RepProdigious (Jun 8, 2011)

Drilling a couple holes in the back and placing a heatsink on the back of the strip would not be hard at all as long as there's room for it..... Give a northbridge sink and one of those tiny 3 or 4 cm chipset fans a try, you could just wire it up with the switch. Should work like a dream.

Imho watercooling is overcomplicating things for a light.


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## HooNz (Jun 11, 2011)

Mattaus , Wot was the stuff?


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