On the importance of a heatsink - The Planted Tank Forum
 
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post #1 of 6 (permalink) Old 02-15-2012, 10:52 PM Thread Starter
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On the importance of a heatsink

I previously have posted that I thought we were using excessive heatsinks on some of these LED builds. After some testing, I can confirm that I was wrong.

I have built 2 LED rigs, and I can confirm that heatsinks matter quite a lot.
The first rig I built is constructed of XR-E Cree LEDs. I used simple L-channel aluminum. It worked and I had no failure, but to be truthful I believe that the LEDs lost some considerable lumens during usage. This might also be the result of the corrosion caused by having these inside of a closed hood.

I built another rig using Bridgelux LEDs and the same L-stock. I figured that they would work just as well. I was wrong. I did an initial test of a few hours, and my heat was staying around 50C. I assumed that this would be acceptable, since everything I found had the Bridgelux LEDs rated up to 150C(at the diode). I was wrong. After leaving them on for 8 hours, I found all of my LEDs were burned out and operating at a very dim glow.

Unless anyone can think of anything else that could have happened, I am completely stumped.
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post #2 of 6 (permalink) Old 02-16-2012, 08:41 AM
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Sorry to hear this.

I'm not real educated on LEDs or even electronics in general, but for a brief time-period I was following a couple flash-light forums, and one of the main concerns they had was adequately heat-sinking the various LEDs.

I think the brightness of the LED was somewhat dependent on the voltage, but the brighter LEDs also put out more heat, and tended to loose efficiency (and lifespan) the hotter they ran.

so, yeah, not a lot of help in this post...
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post #3 of 6 (permalink) Old 02-16-2012, 07:12 PM
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pucksr now u see why i keep stating how important heat sinks are... :P

Now imagine if u were using 10W XM-L's :P
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post #4 of 6 (permalink) Old 02-16-2012, 10:14 PM Thread Starter
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Well, I will admit I was wrong. However, I don't know if the exact same lesson can be learned. A few things came out of this...

1. The Bridgelux LEDs are NOT to be run at 700mA.
2. The Bridgelux LEDs from GroupBuy are 90C(at least from what I can tell) and not 150C like many of the other Bridgelux LEDs on the market.

Heatsinks are very important no matter what...but previously I had been commenting on the temperature you could push an LED to without serious problems. I still don't have any conclusive evidence on the Cree's which run at about 40-45C on the heatsink. I had a huge splash/humidity issue...it caused A LOT of problems

I think I have it licked though. I am going to buy the new black heatsinks from RapidLED, they have a slot for a shield. I will also be closing off the shield a little to try and reduce the issue further. I am going to do a little testing to see how well it works. I am also planning on adding a little thermistor setup to monitor the temp. It should work nicely.
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post #5 of 6 (permalink) Old 02-16-2012, 10:51 PM
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Just remember that the critical temperature is the junction temperature, not the heatsink temperature. It isn't at all easy to measure the junction temperature, so we have to make do with heatsink temperature. When the heatsink is at 50C, you can be sure the junctions are higher, but you have to do some calculating to guess how much higher.

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post #6 of 6 (permalink) Old 02-17-2012, 02:57 PM Thread Starter
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Calculations are my strong point

That is a good point. There actually is a pretty simple method for measuring the junction temperature, but it is somewhat destructive. Bridgelux has a methodology on one of their white papers.

You can calculate it, but many of the numbers you use will be speculative. However, I would assume that a safety margin of 30 degrees C between a heatsink point near the diode and the junction. I think you can actually assume something a bit lower than this, but 30 degrees would cover almost all cases(unless you did something silly like not put any thermal grease/adhesive between the sink and the diode)

However, I was assuming that max temp was 150C for the Bridgelux. If anything the max temp of the bridgelux is LOWER than the max temp for Cree
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