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Thread: LED goes poof!

  1. #1
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    LED goes poof!

    I finally finished my dual XPG helmet light( 1 mA Lux buck driver with 3S1P battery) and was extremely happy with the end results. Last night I set the light outside for a test run to see how warm the case would become (45 degrees with 15mph breeze.) I checked it after 10 minutes and one of the leds was out! The same thing happened last week before I assembled everything! Me very unhappy!
    Questions: Is it common for leds to fail like this? These were on a star which I filed to fit the case but I didn't get crazy or remove much material. The leds were wired in series. I used artic alumina to attach the stars and also to fasten the regina. I used quite a bit around the base of the reflector -could this be an issue? When I took it apart, all the wires looked fine. Part of the reason I chose this as a first build is I thought this would be pretty idiot proof but look at me! I am frustrated. Thanks for any thoughts.

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    LEDs are VERY reliable but there's things that will destroy them.

    1) overheating
    2) voltage spikes

    So, how well are the LED/Stars mounted to the heatsink/body of the light? Thinner layer of AA is better than thick.

    Are the XPG's mounted on quality star bases? or are these cheap/junk knockoff bases? Reason is that some LEDs mounted to Star bases have poor thermal path/soldering/epoxy etc to the top of the base. Some have such thin insulation that the LED can short to the Star base (aluminium).

    Have you measured the output of the buck driver to ensure it is working properly/wired properly, i.e. it is actually delivering 1A to the LEDs?

    You need to do some detective work here, since there is something wrong and you'll keep destroying LEDs until you figure out what is wrong.

    cheers,
    george.

  3. #3
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    Quote Originally Posted by georges80
    Thinner layer of AA is better than thick. cheers,
    george.
    I found an old thread looking a cheap MCPCB LED mounts in this forum dating from 2007, maybe? If you can't get the heat out of the LED into the MEPCB, anything you do later is of little effect.

    Under the star you want only as much AA as needed to fill voids in scratches you can't even see. Some around the edge of the star will be better than air, but will only help borderline cases and not by much.

    Fact 1: Metal-metal contact has about 100 X the thermal conductivity of AA.
    -so you would like the best match between the star and the heat sink you can have. Some stars are punched out and quite dish shaped. Sanding/polishing and screwing them down tight all help metal-metal contact.

    Fact 2: AA has 100-1000 X the conductivity of air.
    -so you want enough AA to displace the air in miniscule scratches of a sanded "smooth" surface, but not enough to prevent the high spots contacting metal-metal. Less is more to the point where air gaps are likely. Sqeezing excess out is a good way to get close to the ideal.

    Fact 3: A thin layer of AA adds little total resistance to heat.
    -that is because thermal conductivity is measured per unit of thickness, so a very thin 1/1000 inch of AA adds little resistance over a metal metal interface. If thin enough aa percentage of the contact is metal-metal and not metal-AA-metal at all, so that helps, too.

    Fact 4: Any resistances to thermal flow are additive.
    -so you can have a warmish body but a VERY HOT LED if the heat can't get to that interface with the air well and once there it doesn't transfer well to air.

    Think of it like a pressurized reservoir of water instead of heat flowing out. A low pressure or temperature at the air interface (faucet) is either due to low resistance flow and good heat removal to the air (or suction on the faucet), or a poor flow and what is getting through isn't getting away as fast as it should (like a crimped pipe does to water pressure).

    The temperature of the star and heat sink should be very close, preferably within a degree or few tenths of a degree and indistinguishable.

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    Dr.Mushy, you say that you checked 10 mins later so maybe you weren't watching at the time it died but did you notice a change in the tint from the LED? as this is a good sign of overheating(they tend to go blue when too hot).

    LEDs are very tough and don't die easy.

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    Georges80 and BrianMc: Thanks for the insights. I didn't think I had too much aa on the star but something is obviously wrong, and I tend to think it is heat. When I made the case originally I wanted to screw the stars down. My drill press pooped out and I drilled the holes free hand and tapped them. They weren't square and rather then do it right I glued them. I am going to make another mount and use screws and grease. Also I am going to put the meter on the buck and see what it does. I'll let people know.
    Yetibetty: I think maybe the hue was slightly bluer before death! It wasn't super dark last night. The moon was quite bright.
    There are a couple of things that always blow me away on this forum. The first thing is the quality and free thinking ideas people seem to have. The second is how people share their ideas and knowlege (especially to dummys like me!) Thanks.

  6. #6
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    You want to get your meter inline with with leds to see what current you are getting from the buckpuck.

    Have you got any pictures of your setup? - might show some issues etc.

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    This is a sad state of affairs but I've never sent digital pictures and am not sure how! My wife and I just got our first digital camera about a month ago. I guess I'm a luddite.

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    Quote Originally Posted by Dr.Mushy
    This is a sad state of affairs but I've never sent digital pictures and am not sure how! My wife and I just got our first digital camera about a month ago. I guess I'm a luddite.


    OK so assuming you can work the camera. take some pics of the light. inside and out

    sign up on a photo hosting site like. www.photobucket.com its free
    then upload your pics to it. and either click on the link it provides for the pics you want to appear on hear and paste it in your post .

    of failing that just paste up a link to your picture album and we can visit to see your pics

    I am sure somone will spot potential problems if they aer there.

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    Or when you click "post reply", click the paperclip icon and attach your images.

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    This is my busy time of year so sorry about the slow response. I junked the glued on stars and started again. This time I screwed the stars onto the mount using thermal grease. I also used thermal grease on the mount/ case interface (I did this the first time also.) I need to look into another voltmeter as mine doesn't measure over 250mA thus I can't measure the bucks output. I have taken pictures of the build but still haven't been able to download them on my computer-Duh!
    A concern- When I glued the regina on the star, I used artic alumina. The first time I was pretty sloppy. This time I didn't use as much. Could that have caused excess heat? Also , I recall seeing a source for XPG leds on slightly smalled stars. The tubing I'm using has an ID of 3/4 inches and the stars I'm using are slightly larger. I can make them work but it's a PIA. I have looked back but haven't found them. Once again, thanks for your expertise!

  11. #11
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    You can get XP-Gs on boards as small as 10mm from Cutter: http://www.cutter.com.au/proddetail.php?prod=cut937 Click on XP-G on MCPCB at top of page.

    It's just about possible to fit the Regina to one with a bit of work.
    Last edited by yetibetty; 05-02-2011 at 03:02 PM.

  12. #12
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    some tips - you don't need much thermal paste, just a very thin layer with the tiniest bit squidged out the sides. At every step of the wiring, check for shorts; star to body (if you're using screws make sure the heads don't touch the solder pads), driver to body, terminal to terminal and look very carefully at the ends of the wires to make sure they can't twist and short against another one. I use heat shrink stuff (what's it called?) on every wire-solder point, just to make sure.

    Hopefully none of the above is the problem, they're just good habits to get into.

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    Quick update: When I bolted in the new LED mount in the case I used thermal grease and thought all was well. Then I shined a bright light from front to back and low and behold, a gap where I thought I thought it was tight (where the grease was supposed to be.) A small burr at the hole the screw went through. I suppose this is a byproduct of a tight fitting case and carelessness by me. Will put it together again and see what happens.

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