DIY LEDs - The write-up

[lynxvs]

The best thermal interface is a mechanical one free of thermal grease, epoxy etc. If the two surfaces are smooth a mechanical fastener is the best way to go. These seem expensive and you still need some sort of mechanical fastener anyway.

[ReefEnabler]

You really think aluminum heatsinks are smooth enough to not require a heat transfer medium? first I've heard that.

[lynxvs]

Normally you would machine smooth both metal surfaces.... I know that is beyond most DIY projects. If you have to use thermal grease you use as little as possible just enough to fill in gaps and have a tight mechanical fit.

[lynxvs]

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 – 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 – 70)/ 2.38 = 33.61 ° C/W
Thermal resistance between Case and Ambient air = 33.61 – 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]

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.

[ejmitch]

I like the thermal pad idea. It would probably be less messy in the end too!

[lynxvs]

Originally posted by Soundwave
OK. I tried optics just to satisfy my curiosity. I used the carclo wide ripple optics from LEDsupply and they made the fixture far far far far far far far brighter. I couldn't stand to look at it. However, I placed it over my tank a full 12 inches from the water surface only to be disappointed. It looked like someone had two huge spotlights over the tank. Not cool. I don't want to raise the fixture any higher so I took off every single lens that I just spent two hours installing.

If anyone plans on using optics, you may need to raise the fixture at least two feet off the water surface. This almost defeats the purpose.

Thought I would let everyone know.

If you want a decent spread, go without them.

Great on paper, not so much in real life.

Soundwave do you have before and after pics?

[stugray]

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.harbour...-SHEET-RC.html

I can get it from work, but have no idea what it costs.

Stu

[DeathWish302]

Originally posted by lynxvs
The best thermal interface is a mechanical one free of thermal grease, epoxy etc. If the two surfaces are smooth a mechanical fastener is the best way to go. These seem expensive and you still need some sort of mechanical fastener anyway.

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]

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.

[lynxvs]

Originally posted by DeathWish302
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.

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]

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/showth...8#post14697288

[Hookup]

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]

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.

[james3370]

i'll take a piece of orange monti

[Soundwave]

You are a little far away.

[zachtos]

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.

[ChknWing]

James I just noticed you are in Nashville. I am from Franklin and was just down there visiting family. Now I am back in KS with this crazy weather.

[ReefEnabler]

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).

Have you looked into the Luxeon Rebel Star Tri-Emitters?? Basically 3 Rebels premounted on a star Pure awesome.

[DeathWish302]

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.

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.

My point in my comments was you should use as little thermal grease as possible.

Agreed.

In the real world the two surfaces are not perfectly flat and you need thermal grease to fill in those gaps.

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....

[fernandokng]

One thing that I haven't been able to find a lot of information on in this thread is regarding dimming (actual wiring, programing).

Can someone shed some light in this area?

Also, Digital Aquatics RKE's LED controller can be used to control Aquailluminations LED. If I have the RKE, how can we use this for our own DIY LED purposes?

[Soundwave]

I spent all of last night doing this...



I don't like it yet but I plan on adding more corals in the near future.

I don't have any optics pictures. I was so mad, I just ripped them off.

I'll still be doing more experimenting this weekend.

[zachtos]

Originally posted by ReefEnabler
Have you looked into the Luxeon Rebel Star Tri-Emitters?? Basically 3 Rebels premounted on a star Pure awesome.

yes, the tri stars made by endor. There are very few optics to choose from though (and the few they have are over-priced). The problem w/ the PRE-soldered stars is that they don't come in the configuration or bin that I want. But I can get a blank star. The only good way to solder the rebel LEDs by mass is to use a hot-air rework station. All the other methods are too much work for doing a lot of these.

Yes, optics I consider a requirement to do this for our hobby, it increases the PAR so much that it's ridiculous to not do it. Speaking from experience here.

If you want to dim the lights, some of the buck pucks have 2 pins that you insert a potentiometer between. You can even get the harness pre-wired for a few dollars. Look up the spec. sheet for more detail. I am discouraging use of the buck pucks now as they are expensive, and you can achieve the same thing by using the above mentioned driver circuit. I found that at the candlepower forums (they ONLY talk about LEDs, they know their stuff). Also, the driver I mentioned above, you can dim an unlimited amount of blue and white with just two dimmer circuits feeding into those drivers instead of having several dozen potentiometers. This is the way the newer solaris works, shh.

**edited** selling items in this forum is not permitted.

[ReefEnabler]

ah I see. I guess if you're planning on optics maybe not the best choice, but I am using a few tri emitters just to cut down on the amount of different stars I have to mount and solder.

Might even get something done this weekend! Last night I started drilling some holes in my heatsinks. Even tapped my first hole but still have a few more. Definitely time consuming!


I should add that I am not seeking to use these LEDs for intense PAR. I am looking to get "fill lighting" at a 45 degree angle which will supplement my main lighting. Basically just trying to illuminate the shadowed areas from my other lighting (which is downward rays)... I think this will help the growth forms of acros and milles and get more colorful tissue from the frontal view.

Also like Soundwave, I will only be grouping LEDs in smaller areas, so I dont have room to have LEDS everywhere like optics require.

I am using Green blue and white LEDs.... probably the only one to use Greens so far

I am also aware that light passing through water at an angle loses intensity. Dont care

[zachtos]

I doubt you'll like the 'fill' lighting. I tried this and it looked like a 6500K spotlight mixed into my 14K lights. yuck. good moonlights though. The acros will be a different color (for the worse) under white only tri stars.

How are you soldering your tri stars? Are you mixing blue and white on one disc? I was going to do royal blue/blue/cool white on each star and buy a optic but it is too expensive to buy a hot air gun just for this project alone... eventually I want one though.

I think not using optics is the wrong direction either way. It's like mounting a MH w/o a reflector. Light is going at 140degrees! Why waste that light? you can half the amount of LEDs w/ optics for our usage.

If you can get strong enough LEDs, you can add shimmer to your T5 setup if you have one. I couldn't find a good way to mount mine or I would have done so. They do great for a refugium by the way, or a FW planted tank or even to add light for house plants.

Nope, I've used green in my array that I have for sale. 2 of them per 24 works well. It supplements some of the missing spectrum from the whites/blues.