Mordoff Posted November 26, 2010 Share Posted November 26, 2010 I finished my DIY LED setup for my 3g pico yesterday. It has 4 LEDs and the heat sink is about 4"x3" from an old computer. But when I ran the LEDs last night for about an hour the heat sink was very hot. It wasn't hot to where it was burning my skin but almost. Is this normal? Or did I do something wrong and are my LEDs going to burn up? Link to comment
d9hp Posted November 26, 2010 Share Posted November 26, 2010 It's normal. If you're going to run those LEDs at that current (whatever it is...it's making heat!) you'll most likely need a fan to help dissipate heat. The heatsink basically pulls heat from the LEDs...but if it's absorbing too much heat it will get so hot that it stops being able to work, and you risk your LEDs burning up. So you either need to get a fan and blow it across the heatsink (or pull air away) or run the LEDs at a lower current. Link to comment
ReggieF Posted November 26, 2010 Share Posted November 26, 2010 Firstly, congrats on the DIY build! I have 6 LEDs at 700 mA on a video card heatsink that I bought for $4 and it doesn't get hot enough basically. If the room is hot and it's been running for 3+ hours (temperature has maxed out after about 3) then it'll feel like a hot dinner plate, but it's not at all too hot to hold. There are tons of CPU heatsinks that will be able to handle 4 LEDs without a fan, so you might just want to get another one. What exact heatsink did you use? Link to comment
Reefgeek Posted November 26, 2010 Share Posted November 26, 2010 Sorry for asking an obvious question, but did you use thermal paste? Link to comment
Mordoff Posted November 26, 2010 Author Share Posted November 26, 2010 Sorry for asking an obvious question, but did you use thermal paste? I used jb weld. The heat sink is from an old sony cpu I had lying around. It's about 2-3" wide so I figured it should be good enough. It had a fan on top of it when I pulled it out of the cpu but I took it off so Im thinking I should put it back on. Plus i'm running them at 1000mA by accident cause didn't really know what I was doing when I ordered the driver, but it is dimmable. Link to comment
ReggieF Posted November 26, 2010 Share Posted November 26, 2010 I used jb weld. The heat sink is from an old sony cpu I had lying around. It's about 2-3" wide so I figured it should be good enough. It had a fan on top of it when I pulled it out of the cpu but I took it off so Im thinking I should put it back on. Plus i'm running them at 1000mA by accident cause didn't really know what I was doing when I ordered the driver, but it is dimmable. Did you use thermal paste in addition to the JB Weld? To me it sounds like that heatsink is not going to be enough for what you need. For only 4 LEDs I would go without a fan because the added noise and hassle is not necessary given that it's only 4. Where are you in Southern CA? Because this place is in North San Diego County: http://www.geeks.com/details.asp?invtid=RV...-GP&cat=FAN Link to comment
Mordoff Posted November 26, 2010 Author Share Posted November 26, 2010 Did you use thermal paste in addition to the JB Weld? To me it sounds like that heatsink is not going to be enough for what you need. For only 4 LEDs I would go without a fan because the added noise and hassle is not necessary given that it's only 4. Where are you in Southern CA? Because this place is in North San Diego County: http://www.geeks.com/details.asp?invtid=RV...-GP&cat=FAN Im in Covina, South LA County. Link to comment
Reefgeek Posted November 27, 2010 Share Posted November 27, 2010 The heat sink is fine, the problem is how you attached them. JB weld is not a good conductor of heat, so you are basically running those LEDs without a heatsink at all. I don't know how easy it will be to remove your LEDs, JB weld is serious stuff. You need to use a nice thermal epoxy like this one: http://www.rapidled.com/servlet/the-11/Arc...Adhesive/Detail Until then, stop using your LEDs or you'll burn them up. J.B. Weld has a thermal conductivity of .00590 vs Silver which has a thermal conducivity of 4.3, or copper at 4.0, or even Nickel at .91. Basically J.B. Weld is 728x LESS thermally conductive than Silver. Arctic Silver Adhesive has about 60-65% silver content. Link to comment
ReggieF Posted November 27, 2010 Share Posted November 27, 2010 The heat sink is fine, the problem is how you attached them. JB weld is not a good conductor of heat, so you are basically running those LEDs without a heatsink at all. How would the heatsink get hot if it's not transferring heat? The title of the thread is "Is it normal for the heat sink to get really hot?" Link to comment
cptbjorn Posted November 27, 2010 Share Posted November 27, 2010 lol his heatsink is hot and you are saying JB weld doesn't conduct? Link to comment
Reefgeek Posted November 27, 2010 Share Posted November 27, 2010 Why yes that would be an important point wouldn't it Duh. Link to comment
Gatotsu77 Posted November 27, 2010 Share Posted November 27, 2010 If the heatsink is getting hot, and JB Weld is a poor thermal conductor, would that not suggest that the LED's are getting much hotter than the heatsink? If there is a very thin layer, I don't imagine it would act as too severe an insulator, but probably not pulling heat from them as efficiently as a thermal adhesive. How old was the Sony the heatsink came from? Newer heatsinks are designed to pull quite a bit of heat out of the cpu, but older designs might not cut the mustard. How tall is it? Solid aluminum I'm assuming? Link to comment
cptbjorn Posted November 27, 2010 Share Posted November 27, 2010 meh I knew it was a bad idea to even bring up JB weld... I'm assuming the thermal conductivity quoted above for JB weld is in W/cmK, and the conductivity of arctic silver adhesive is .075 which if you look at just that number is 12 times better, assuming those values are correct. But if you think about what this actually means, the heat is only being conducted through a vanishingly small distance of the paste, generally measured in thousandths of an inch. In practice you would need to set up a delicate test rig to measure any difference between the two. What matters is that both materials are many orders of magnitude better thermal conductors than the still air gaps that would be present if nothing were used. The measurable value that actually determines how "good" a mounting technique is is the completed thermal resistance between the LED star and the heatsink, which for 20mm star LEDs will range from about .5°C/W for perfectly screw mounted, up to maybe 2.5°C/W for a poorly applied pad on a rough heatsink. I would be shocked if there was even a .1°C/W difference between JB weld and arctic silver, which at 3 watts is only 1/3 of a degree difference in junction temp. The differences in this value among using screws+grease, epoxy and pads are much larger than any difference between different epoxies. Link to comment
Gatotsu77 Posted November 28, 2010 Share Posted November 28, 2010 That's why I was more interested in the heatsink itself... happen to have any photos of it? Link to comment
Trick Posted November 28, 2010 Share Posted November 28, 2010 sorry to butt in but my heatsink gets really hot too. How hot is too hot? Link to comment
Trick Posted November 28, 2010 Share Posted November 28, 2010 i can hold onto my heatsink for maybe 10 seconds and then i realize that it too hot and i let go. I might need to put a fan on it. how hot is it supposed to get? i would say it's as hot as an ecoxotic panorama module. Link to comment
cptbjorn Posted November 28, 2010 Share Posted November 28, 2010 hand on heatsink for ~10 seconds means you roughly in the 45-50C range. depending on mounting and LED type that puts the junction temp at ~70-90C which is fine. lower is always better though, are you using a fan? Link to comment
Trick Posted November 28, 2010 Share Posted November 28, 2010 no fan right now. but i might add one. i made a diy heatsink. its made from extruded aluminum that i jb welded together. Link to comment
Trick Posted November 28, 2010 Share Posted November 28, 2010 ok so i got a meat thermometer and i let it sit on my heatsink for 5 minutes and it currently reads 114 degrees farenheit. Link to comment
Hans Dorn Posted November 28, 2010 Share Posted November 28, 2010 114F is plenty cool. I'd also like to point out that the critical thermal junction is between the LED chip and the star board. A tiny surface area has to conduct a lot of heat in this position. You don't need to worry about the thermal contact between the star board and your heat sink, as long as you don't use PU foam to stick them there. Link to comment
Trick Posted November 28, 2010 Share Posted November 28, 2010 ok its been some more time now and the temperature reading is now 123 degrees farenheit!!! Link to comment
disaster999 Posted November 29, 2010 Share Posted November 29, 2010 I am running 12 LED driven to 0.64amps (according to my multimeter) with an extruded heatsink. LEDs are mounted with thermal compound and I dont have a fan over them. During the summer months the heatsink gets to around 45-50c. Right now when the weather is cool, the heatsink is around 40-45c. I am not running fans and im able to grow SPS. I do plan on adding a fan later on if i decide to drive them to 1amp. Link to comment
Trick Posted November 29, 2010 Share Posted November 29, 2010 mine are running at 50C right now Link to comment
disaster999 Posted November 29, 2010 Share Posted November 29, 2010 50c shouldnt really harm the LEDs, but I've heard some suggesting running a fan to bring it close to 35-40c. the LEDs will be brighter. But 50c should be fine. Ive been running my LED for a few months at those temps and never had a problem. Link to comment
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