Mini 12 volt ATX PSUs

Thanks donncha,

I thought the oil idea might work! Cooling AND salt protection for the price of one. The only hitch I see is the oil dissolving something. Like the "rubber" inserts at the ends of some capacitors, for example. On the other hand, silicone oil should be OK. You can get the stuff by the gallon at some cosmetics supply places, I've used it for ultra high pressure transmission (75,000 psi) where petroleum oils turn into solids. I don't have any thermal data for it though. A high heat content would be nice. But then, just about any liquid should have a higher heat content than air.

I'm building an aluminum boat and I've been thinking of extending a "fat" frame at a convenient location so I can bolt a second aluminum box containing my motherboard, etc., to it with some thermal compound. Inside the computer box I could machine some stubby fins oriented vertically, and on the motherboard I could use conventional heat sinks also oriented vertically. Hopefully the back of the box where it attaches to the frame would be "cold" relative to the oil, thus causing it to convectively fall downward. The "hot" components would make the oil rise, and if I design it all so there is an easy path for this circulation to follow I think I could have a nice passive cooling system, (almost) direct to the hull. Those people with non-thermally conductive hulls (wood, glass) could do something similar by making a heat exchanger for the back of the computer box and placing the entire box and heat exchanger in the bilge below the water line. Two lines rising to a sea inlet should result in convective circulation of the water within this heat exchanger, and again you have an entirely passive system. The whole heat exchanger could be made from copper and thermally coupled to the aluminum box (or the box made from copper) via an electrically non-conductive material like is used for mounting transistors on heat sinks. Copper would have the advantage of resisting attack by all those biologicals . . . but you would sure need to make sure you weren't setting up a nasty galvanic corrosion problem!

BillyDoc

BillyDoc

 

Corrosion Protection

For the non-extreme methods, here's a pointer to a product that apparently works well in Marine and other hostile environments like chemical plants and irrigation systems:
http://www.actonrust.com/electrical.htm

I'm thinking of trying this on some samples, and in a fanless 15 watt Mini-ITX for onboard use.

Although the ABS plastic NEMA4 type boxes are great for marine environment, I'm thinking about one of those aluminum gasketed mini-briefcase boxes with snap fasteners. The thin aluminum should do a good job of inside air to outside air or inside air to heatsink/aluminum hull heat transfer.

With only 15 to 25 watts total, this isn't a big problem. If someone wants to run a 150 watt 3 Ghz multi-core system and a big hard drive in a box, that's another story...

 

With flooded oil-cooling of electronics, you may also need to take into account the different dielectric constants involved; air being 1 and transformer oil typically around 2.2 Especially on high-speed FSB equipment. I guess the only way to know for sure is to test the technique ...

You'd also need to figure out a reliable means of sealing the case, especially with the need for multiple leads to pass through it. You'd also need to fabricate some kind of expansion facility (as on fluid-filled compasses), and a drain plug of course - just in case you ever need to gain access to the innards.

Many oils will harden pvc over a period of time, but this shouldn't present a problem if flexibility isn't required.

'best
Colin

 

Yikes! I didn't think of the dielectric constant at all, and you are so right. These circuit boards with multi gigahertz signals blasting all over the place look like pure magic to me under the best of circumstances. And I can't imagine designing one without the use of a very sophisticated design program . . . that would take into consideration the dielectric constant of the surround to simply be able to send those signals from one point to another without being swamped with reflections and who knows what else.

Thinking about it I can't see how signal propagation would NOT be strongly effected, unless the solder mask layer has a similar effect and this has already been factored in. This is a nasty problem. I guess I'll just have to do some experiments.

Thanks for pointing this out SAE140, this is a very important point to consider.

BillyDoc

 

also have a look at
http://www.tomshardware.com/2006/01/09/strip_out_the_fans/

They found that the cpu had to be sealed as the difference in the dielectric constant affected it.

"We have the following explanation for this phenomenon: On the motherboard in the area of the CPU base, the oil is responsible for increasing the capacitive resistance between the individual wiring. In short, the oil acts as a dielectric material. Since very high frequencies occur on the motherboard, the capacitive resistance goes down. Accordingly, this then influences (or tampers with) the digital signals, particularly in the area of the CPU base. After all, 939 pins are located there in a very tight space."

As I thought only the cpu is affected be the change in the dielectric constant.

You would still want to be careful.

I have heard of successful trials done without this step in sealing the cpu. But it was with older pc's, perhaps the less amount of pins or lack of all together solve this proplem. If you are using a mini-itx motherboard or something with a built in cpu this might not be as much of a problem.

Also watch out for the corrisive effect of oil on plastics and rubber.

 

Hi Donncha,

Interesting link! One thing though, I've tried to seal against oil with RTV silicone before, and eventually the oil penetrates it. It can take a few months, but . . . Epoxy works well, though. And another type of material which might work and is somewhat flexible are some of the "casting" urethanes. I haven't tried these (or looked at the manufacturer's site) for oil resistance, though.

BillyDoc

 

There is an oil used in X-ray equipment that is great. It has consistancy of vegetable oil but it does not react with any materials. It has highest isolation properties. It is not cheap. Last time I bought it 15 years ago it was $200 for 5 gallons, Exxon made it. I dont remember name - but it exist and it works really good.

 

Aluminum & copper dont mix

If you have an aluminum boat, you can not have any copper on board. Not even pennys. Eventually copper will ooze green stuff, copper sulfate and it will eat your boat alive. Aluminum is not a bad heat conductor. Also there is no need for salt water cooling. A small tank of fresh water made of metal, perhaps aluminum will dissipate heat from inside of computer box to outside that is all that is needed, unless someone wants to fit Cray in a ship.