DIY LEDs - The write-up
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ReefEnabler
Premium Member
You really think aluminum heatsinks are smooth enough to not require a heat transfer medium? first I've heard that.
I've posted this before I think but it might be helpful. I use just a flat plate of aluminum as my heat sink. I did some calculations below to justify heat sink size. I attach a PCB directly to the plate using screws.
Max Junction Temp = 150° C
Power of Single LED = 3.4 Forward Voltage X 700 mA = 2.38 W
Ambient Temp = 70° C ( A SWAG)
Thermal Resistance between Junction â€"œ Case (From Data Sheet) = 10° C/W
Thermal Resistance between Junction and PCB (From Rebel application note) = 7° C/W
Total Thermal Resistance = 10 + 7 = 17° C/W
Total Thermal Resistance between Junction and ambient air = (150 â€"œ 70)/ 2.38 = 33.61 ° C/W
Thermal resistance between Case and Ambient air = 33.61 â€"œ 17 = 16.61 ° C/W
The amount of heat dissipation that can be achieved with a flat plate of aluminum is indicated below.
Using a 3mm plate looks about 20 cm^2 per LED converting to inches is equal to 3.1 in^2 * 50 LEDs = 155 in^2 The plate I am using is 24 X 7.25 = 174 in^2 not sure if you can count both sides of plate as surface area… I also have two cooling fans to help…
Max Junction Temp = 150° C
Power of Single LED = 3.4 Forward Voltage X 700 mA = 2.38 W
Ambient Temp = 70° C ( A SWAG)
Thermal Resistance between Junction â€"œ Case (From Data Sheet) = 10° C/W
Thermal Resistance between Junction and PCB (From Rebel application note) = 7° C/W
Total Thermal Resistance = 10 + 7 = 17° C/W
Total Thermal Resistance between Junction and ambient air = (150 â€"œ 70)/ 2.38 = 33.61 ° C/W
Thermal resistance between Case and Ambient air = 33.61 â€"œ 17 = 16.61 ° C/W
The amount of heat dissipation that can be achieved with a flat plate of aluminum is indicated below.
Using a 3mm plate looks about 20 cm^2 per LED converting to inches is equal to 3.1 in^2 * 50 LEDs = 155 in^2 The plate I am using is 24 X 7.25 = 174 in^2 not sure if you can count both sides of plate as surface area… I also have two cooling fans to help…
ReefEnabler
Premium Member
Soundwave, do you know how hard it would be to remove a luxeon from the heatsink with the AA Epoxy??
I read elsewhere that its possible to mix AA Epoxy with regular Arctic Silver compound which will keep enough strength for mounting the LEDs but will make it possible to pry off a star without damaging it.
I read elsewhere that its possible to mix AA Epoxy with regular Arctic Silver compound which will keep enough strength for mounting the LEDs but will make it possible to pry off a star without damaging it.
stugray
Premium Member
lynxvs,
"The best thermal interface is a mechanical one free of thermal grease, epoxy etc."
In my experience, this is not true & you still need something to "complete the thermal circuit".
When we bolt a flight box onto a spacecraft, the two surfaces are required to be VERY flat and we still use something between.
Long ago, we used plain old silicone RTV. It was one of the best.
Now that we know silicone is an awful contaminant, we cant use it anymore.
So we now use something known as "Q-pad":
http://electrical.appliances.harbourdirect.com/Q-PAD-3-4quot-X-4quot-SHEET-RC.html
I can get it from work, but have no idea what it costs.
Stu
"The best thermal interface is a mechanical one free of thermal grease, epoxy etc."
In my experience, this is not true & you still need something to "complete the thermal circuit".
When we bolt a flight box onto a spacecraft, the two surfaces are required to be VERY flat and we still use something between.
Long ago, we used plain old silicone RTV. It was one of the best.
Now that we know silicone is an awful contaminant, we cant use it anymore.
So we now use something known as "Q-pad":
http://electrical.appliances.harbourdirect.com/Q-PAD-3-4quot-X-4quot-SHEET-RC.html
I can get it from work, but have no idea what it costs.
Stu
DeathWish302
Clown Hoarder
This statement defies Classical Heat Transfer Theory... There MUST be a medium for effective heat transfer from the aluminum CREE star to the heatsink. This is a fundamental 'law' when designing any system that requires heat transfer from one surface to a heatsink. The air gap created from asperity/porosity between the surfaces will dramatically decline heat transfer. Aluminum is one of the best 'conductors' (copper is another but heavy and $$$) and can run circles around convective heat transfer. With a thermal gap you are relying on convection and therefore are shooting yourself in the foot w/o some transfer medium. I personally prefer thermal epoxies over grease.
kcress
New member
stugray; I usually go with grease because all those pads are stunningly expensive. Every time I need to use them it grates on me. You bend over backwards to design the cost out of some design, then find out you need to pay as much for the thermal pad as you paid for the IC you're putting on top of it. I think it borders on being a scam, but maybe the stuff it expensive to make. I just don't see why.
Berquest is a huge supplier of it and I always get these product announcements for great new thermal pads. I hunt them down and am horrified by the cost every time.
Grease works just fine if you have reasonably smooth surfaces. Most of the time, and I'm guilty of it too, people think, "Grease is, cheap and more must be better", and we make a mess.
Usually you need about a fifth of what you actually use. If applied correctly none should even ooze out from under the device.
Sometimes the pads stick on both sides, and get torn, then you have real leveling issues with trying to replace a part. Grease just wipes off, or a little alcohol does the trick.
Berquest is a huge supplier of it and I always get these product announcements for great new thermal pads. I hunt them down and am horrified by the cost every time.
Grease works just fine if you have reasonably smooth surfaces. Most of the time, and I'm guilty of it too, people think, "Grease is, cheap and more must be better", and we make a mess.
Usually you need about a fifth of what you actually use. If applied correctly none should even ooze out from under the device.
Sometimes the pads stick on both sides, and get torn, then you have real leveling issues with trying to replace a part. Grease just wipes off, or a little alcohol does the trick.
Can you please explain heat transfer theory to me and why this statement defies Classical Heat Transfer Theory? If two metal surfaces are perfectly flat (not saying this is possible) and mechanically fastened to each other there is no better way for heat transfer to take place. Anything else you put in between the two surfaces reduces the heat transfer including thermal grease.
In the real world the two surfaces are not perfectly flat and you need thermal grease to fill in those gaps.
My point in my comments was you should use as little thermal grease as possible.
ReefEnabler
Premium Member
we need more peeps to vote in the April ThOTM poll..... the lighting thread is really good, but not better than this thread!
http://reefcentral.com/forums/showthread.php?s=&postid=14697288#post14697288
http://reefcentral.com/forums/showthread.php?s=&postid=14697288#post14697288
Hookup
New member
Great thread.. Just read all 21 pages and obviously some brilliant people contributing... Had to vote, just had to.
Soundwave, do you have any coral color/growth coloring photos... Technical stuff is wonderful, but at the end of the day it's all about the healthy corals, amazing colors and good growth... (for me..)
Soundwave, do you have any coral color/growth coloring photos... Technical stuff is wonderful, but at the end of the day it's all about the healthy corals, amazing colors and good growth... (for me..)
Soundwave
New member
I just got done spending 6 hours redoing all my rock work. Unfortunately, I broke my orange monti cap into about 4 pieces. I guess I can sell the frags cheap, locally. A good size piece that looks whole is staying in there. After this little remodel, I have about 50 some odd pounds or more of rock left over. Amazed me. I kept out all the Tufa rock that I think was part of my problem with nutrients. That stuff traps everything.
I'll post pics when the tank clears up and I get final positions on things.
I'll post pics when the tank clears up and I get final positions on things.
zachtos
Active member
LED driver Cheaper LED driver. This is a $3-4 driver that will run many more LEDs then the over-priced buck pucks. They do not have the dimming feature, but you can make one pulse width modulator (cheap) dim an entire series of these. Just a suggestion for doing the project much cheaper for DIYers.
The Luxeon Rebels are still in the lead for intensity, but are not cheaper and are not easy to solder (as we discussed at NR). I'm waiting for stronger and cheaper bins to arrive. (200lumen bins exist but are hard to find).
I think I told you about how much of a difference an optic made in my old Luxeon K2 design didn't I? It increased my intensity by 2.5 times with no checkerboard pattern. I think you will find it WORTH the investment to buy and mount optics on each lens. I think I used 20 degree optics, you can crunch some numbers to see how much coverage the cone at x" will give you. The LEDs are throwing light in 140 some degrees and we want them around 20-45 degrees, going DOWN not out. You will be amazed at your new PAR readings. Shoot me a PM if you want any more advice.
The Luxeon Rebels are still in the lead for intensity, but are not cheaper and are not easy to solder (as we discussed at NR). I'm waiting for stronger and cheaper bins to arrive. (200lumen bins exist but are hard to find).
I think I told you about how much of a difference an optic made in my old Luxeon K2 design didn't I? It increased my intensity by 2.5 times with no checkerboard pattern. I think you will find it WORTH the investment to buy and mount optics on each lens. I think I used 20 degree optics, you can crunch some numbers to see how much coverage the cone at x" will give you. The LEDs are throwing light in 140 some degrees and we want them around 20-45 degrees, going DOWN not out. You will be amazed at your new PAR readings. Shoot me a PM if you want any more advice.
ReefEnabler
Premium Member
Have you looked into the Luxeon Rebel Star Tri-Emitters?? Basically 3 Rebels premounted on a star
Pure awesome.
DeathWish302
Clown Hoarder
In a 'real' world situation NO material is void of asperity/porosity (bumps and cavities in material), so why even mention it? Regardless of surface finish, there will be an air gap which equates to convective heat transfer. Even if this gap is filled with thermal grease, this interface still transfers heat by convection. This is the ideal case due to the higher thermal conductance of the grease/epoxy.
Agreed.
Agreed.
I agree conduction (transfer through a non-fluid medium) is the best method for heat transfer. Transfer to a heat sink with a void between the two materials IS not ideal. Thermal compounds between surfaces truly act as a medium for convection, so there is no ideal case except having a solid block. This is obviously not possible in most all cases.
I'll agree to disagree about the specifics of a 'mechanical' interface and the value of this argument to the thread.
I'm done ranting, sorry....
:rollface:
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