I think I would try something like this. The trick will be finding the 2 plastic (Or better yet metal pieces)

The heat from the LEDs is conducted out their bottom so make sure they are in good contact with the plate.

 

That may have been part of my problem. The LED's soldered together with the wires, actually lift them off the aluminum mounting plate I have. The Heatsink is glued to the other side of my aluminum plate.

I think it does that because the thermal paste doesn't really grip the LED's down. The paste never actually hardens. What do you think about using an Thermal compound adhesive glue to glue them to the plate/heatsink?

I bought a sheet of thin aluminum, that I can cut mounting plates out of, or any shape.

I see what you are talking about in your image. So you are saying eliminate the heatsink all together and allow the sea water to come in behind the LED mounting plate? Wouldn't that eventually corrode the aluminum?

What if I ran a strip of metal from the heatsink, through the ABS plastic wall to the exterior of the case so that it can make contact with the surrounding water?

****Also, I did hear back from MG Chemicals tech support on the clear epoxy resin cracking. He said leave it in the oven for 4 hours, instead of 2 and you will get additional cross-linking****

 

Additional hardening of the potting material might make things worse instead of better. It may still shatter with uneven heating, unless the crosslinking increases the tensile strength so much that it overcomes that.

 

Hi watneh, I was thinking the epoxy cracked because there was too much wiggle room because I set the LED in the thermal paste. I am hoping if I can just get more heat out of the enclosure it will be ok. stumped

 

Have you looked at how the commercial products are designed to handle the heat problem? Removing heat or sealing against water are both "easy", but both at the same time is a challenge.

Water is obviously a great heat sink reservoir, but relying on the water means your lights might burn up if someone turns them on when the boat is trailered or stored. Maybe this is not a big concern for you, but it's something to think about. Murphy's Law has not been repealed.

FWIW, they make boats and canoes out of aluminum.

 

That's the truth, anything that can happen...will.

So, I've read about the commercial makers some. I even read assembly and parts instructions in their patent: https://www.google.com/patents/US7396139

I'm not too concerned, when it's trailered. It can only run for 2 minutes out of the water before overheating. Ill make sure it doesn't get switched on when not submerged.

What do you think about thermal glue. (LEDs glued directly to the heatsink or aluminum plate) My aluminum plate, I wrapped with a vinyl finish to give it a nice look. My LED's in the above image are seated on top of the vinyl. I wonder if that keeps heat from sinking through the vinyl down to the aluminum plate/heatsink?

 

Did he even state if this is for saltwater or freshwater?
With the "proper" alloy/material/fastening selection corrosion of aluminum is not really an issue. key word being proper...

sorry this thread just makes me giggle... I just love it when DIY'ers "try" to "engineer" stuff..

 

Eventually the salt water will break down the aluminum, unless its anodized.

Sorry I don't have access to engineers or engineering equipment to do the job. I do the best I can.

I tend "giggle" when people watch HGTV and become a real estate expert overnight, and decide they don't need an agent.

However, I did stay at a Holiday Inn Express last night.

 

use 5000 series aluminum alloys.. aka "marine-grade" aluminum...

use thermal adhesive.. "sparingly".. too much is just as bad as none..

There are tons of low cost metal clad PCB's already setup for bridgelux type leds also..
Thermal management will be your biggest fight..
Of course not having a proper current source/driver is not helping either.. Vf's fluxuate as does your power supply.. Hitting a moving target is much harder..

Also with the proper optics you could possibly reduce the number of LEDs..thus lowering the about of wattage you need to dissipate..

 

What do you think about thermal glue. (LEDs glued directly to the heatsink or aluminum plate) My aluminum plate, I wrapped with a vinyl finish to give it a nice look. My LED's in the above image are seated on top of the vinyl. I wonder if that keeps heat from sinking through the vinyl down to the aluminum plate/heatsink?

If you intend to not solder those LEDs to a circuit board then thermal adhesive is a MUST.. (arctic silver is a 2 part thats commonly sold at computer parts stores,etc... but there are tons of others)..

and yes..EVERYTHING has a "thermal resistance".. make it work properly first.. then make it look pretty.. The vinyl is not helping.. The LED's need to be directly bonded to the heatsink.. You want the lowest thermal resistance from led junction to ambient (air/water)

And a flat plate is a horrible "heat sink".. surface area is key.. adding "fins" can greatly increase surface area thus greatly reducing its thermal resistance.

 

That's great input Mcgyvr. I do intend to solder them directly to the aluminum plate, which has a corrosion resistant aluminum heatsink under the plate.

I don't know how I would test it first and then make it pretty. It requires epoxy encapsulation to submerge it for testing. The contact with water is essential for cooling the entire enclosure. I can't submerge the exposed LEDs, nor testing it with ambient air. They get hot. 72 watts of LEDs do generate a lot of heat.

The vinyl film is to give it a finished look. I can etch a recatnagulaar opening in the center of the vinyl, for the LEDs to make contact with the aluminum surface. I do have some of the artic two part adhesive. You're right about that, it's good stuff.

These are the materials in the order they are to be assembled. As aforementioned, it all has to go together for testing, in it's final stage. I do have several adjustable 12 volt power supplies that I am using, that use clean power to bench test everything.

 

More general info..
You do NOT power LED's with a constant voltage source like you can with other "resistive" devices like a regular light bulb.. A regular resistive load will "draw" as much current as it needs to operate.. You need to "feed" LED's or they just keep gobbling up the current until they puke and burn up..

Yes "if" you match the sum of the Vf's to the power supply voltage closely enough and have enough fixed resistance (like with long/small enough wires to the device) it will light up.. BUT its like walking on the edge of a cliff.. one wrong step and oops...

"constant current" power supplies (or drivers) "feed" the LEDs with exactly what they need and adjust the output voltage so the LEDs can't be overfed..or underfed.. Using them is how you would "properly" power LED's and help to ensure a longer/safer life for them. For just a few dollars you can get some simple LED drivers that would help..


why solder to the aluminum plate? I'd just attach them directly to the heatsink with the thermal adhesive. or just buy star metal clad PCBs or similar..
special soldering techniques/materials are needed to get a good bond to aluminum.. at the very least you need the proper flux to get through the oxide layer..

If you do use the plate make sure you use thermal adhesive or grease if mechanically bonding it to the heatsink.. Every little gap/crevice of air hurts performance..
You need to get the heat to that heatsink so it can spread that heat around the enclosure as best you can to try to pull it away from the LED..then hope it can be removed through the water/flow on the outside of the enclosure well enough.
A painted diecast box with fins incorporated right into the outside and directly bonding metal clad circuit boards right to the inside wall would be the way I would want to go

 

ok, so the final installation will be connected to a 12 vdc marine system, or using a buckpuck as a steady driver...I can test with the buckpuck off of my 12 volt source.

The reason I am setting the LEDs onto the thin aluminum plate is for finishing aesthetic reasons. The aluminum plate will have the vinyl finish and hold the epoxy resin. If I set them directly onto the heatsink, the epoxy potting resin, will drip down the crevices wasting with excess. I need it to catch the resin when poured.

Star heatsinks take up too much space. 24 LED's on heatsink stars will need a larger enclosure.

How did your diecast box turn out?

 

I didn't make a diecast box thats just how I "would" look to do it.
but..
You could use something like
http://www.ebay.com/itm/1pc-20x-1W-...776?pt=LH_DefaultDomain_0&hash=item3a82c59818
There are tons of them.. ebay search "aluminum pcb"

heck even easier just get a 50W or 100W multichip and use a boost driver.. drill it right into the heatsink (with grease) then attach your vinyl right around it. you could get a lens for it too and that would look 100% better than the singulated..glue,epoxy job...
https://dl.dropboxusercontent.com/u/9726802/Reef/IMG_20121215_164729.jpg
I'm not sure why you are using potting compound anyways.. Its probably cracking from outgassing so a longer cure would help but I don't think its needed if your enclosure is waterproof..

 

I did see some of those pcb materials on ebay. It would increase the size needed to accommodate more LED's. I agree on the cracking of resin, I considered putting an acrylic window. I am potting everything to waterproof it for life, no dead air in the enclosure, not possibility of condensation or corrosion.

take a look at this: http://loomisled.com/shop/24-led-flush-mount-boat-transom-lights-12-12-5-vdc-only-non-charging/

 

Ahh, a heatsink.... better yet. I would also loose the plate. Bond the leds to the heatsink with a small amount of thermal epoxy. Cut a hole in the vinyl to expose the leds and epoxy it to the heatsink, then pot the whole thing on top and around the edge on the bottom. Be careful not to have any thermal epoxy touch the leads as it is probably electrically conductive. Then cut a big hole in the bottom of the box so the water can get to the heatsink. This is the trick to keep them cool. Even the heatsink in air will probably get to hot after a short time. I'm not to sure how well the epoxy will stick to the aluminum but you can try an experiment to make sure it is ok.
What everyone is saying is that your supply must be current limited, not voltage limited. So you need enough voltage to turn on the leds but then you must limit the current so they don't overheat. What did you use for your test. What will you use in the boat as a power supply. There may be some destruction of the aluminum in the salt water depending on the circumstances. Google aluminum in saltwater for the problem and some cures.

 

I see what you are saying ronv. My heatsink isn't the exact size of the enclosure. I looked for a 70 mm x 70 mm heatsink for an exact fit and could not find one. Even one I could cut. See the picture, there is spacing on each side. That was why I was using an aluminum plate. The plate fills in the gaps and gives me a surface to pour the potting epoxy onto. The vinyl was for a finished look, but I can loose it all together. Functionality is far more important.

Power Supply is a 12 vdc 3 amp power supply with leads, connecting a buckpuck to regulate voltage and drive current for the LED's

The challenge for me now, is exposing my heatsink to the water with water entry, to waterproof the heatsink from corrosion.

 

Give this a read...
http://www.aluminiumdesign.net/design-support/aluminium-corrosion-resistance/#toc-galvanic-corrosion

See zinc here.

http://www.onlinemetals.com/basket.cfm?pidadd=15715&CFID=8838285&CFTOKEN=82279321

Can't you use the vinyl as trim?

 

Those are good reads. Aluminum in the water was especially good.

Have you used online metals before? So, I looked at the zinc age. Were you thinking to order a sheet and fab my own heat sink?

I appreciate you taking the time to do a mark up above. I could use the vinyl as a filler around the edges of the heat sink. Finding a material to adhere it to around the edges of the heat sink spacing would be next. It sounds like the suggestion of holes in the rear section of the enclosure would be best. Allowing the heat sink to transfer heat produced, transferring to the surrounding water is the way to go. I wanted to keep the case completely sealed, but it looks counter productive. Making an enclosure block of swelling heat. ha

 

If you can tell I'm losing sleep over this I am, lol

Ronv, I believe you hit it right on post #41.

I may be overthinking this. I keep trying to use a heat sink, come hell or high water. Heat sink, being more applicable in an environment with more ambient air available to move around for cooling. ie motherboard, CPU with cooling fan, TV Y-Main boards.

Water has a higher thermal conductivity than air, it can move heat faster than air, water has a higher heat capacity than air.

Back to square one. Ronv, thank you for taking the time to share your design rendering. It has me convinced to scrap the heat sink all together. With enough water recirculating there should be no need, theoretically. Back to looking for another enclosure, I found the below enclosure. More sleek in design and practical for this project. Mounting bosses for a surface plate.

What are your thoughts on the enclosure, and it's size. With .58 inches in height, I measure enough room for the LEDs, aluminum mounting surface, gasket, and potting epoxy. There is another enclosure a little taller (.96 in height), 3rd image. However I like the first enclosure size. I would cut out the little reservoir openings on the lower and upper walls, to allow water to pass through easily.

Do you think it would be sufficient cooling?