How to "pot" a BB700?

How to \"pot\" a BB700?

No not that type of pot /ubbthreads/images/graemlins/grin.gif. I posted in the thread about heatsinking a BB700 and one of the options that came up was to 'pot' the BB700. I assume by pot it means to cover up in some type of epoxy that is RTV (not sure about that part).

Can anyone clarify for me how to pot an converter board and what type of materials I will need?

Thanks,
Daniel

Re: How to

an effective way of sucking the heat out of the unit would be to "drown" the board in the rtv epoxy. in and of itself, the rtv stuff would do quite a bit to help the thermal transmittance, but if it is in contact with the light of the body, that would help also.

if you were to attempt to try to pot a sandwhich, you would put epoxy in betweent he two boards and sand/file it so it is round and even with the two boards.

so...yup...you were right /ubbthreads/images/graemlins/grin.gif

Re: How to

Thanks Rothrandir!

One small question, which products are RTV? I haven't seen any products with the marking on there. Could you give me an example of an actual product that I should use to pot the converter board. Or will any epoxy that solidifies at room temperature do?

Daniel

Re: How to

Potting stuff.. RTV means 'Room Temerature Vulcanizing' and is used for rubber (and silicone) products. They work either by using a reactor and catalyst or moisture from the air. The most commonly available ones are silicone caulks from the hardware store. These set by a reaction with the moisture in the air and a 1/8" layer will set up in a couple hours at 70F/50% RH. Some silicone RTVs use an acetic acid reactor, they smell like vinigar, DON'T use them to pot electrical things, they corrode copper wires and circuit board traces rather quickly. Probably the safest is Silicone II bathtub caulk. Either can be used to make reusable molds for short poting runs, just make a wax master of the shape you want and coat it with a couple of layers of RTV, let harden 24 hours after the last layer and peel it off the master (or melt the wax out) and you can use it for casting epoxy or urethane parts.

The other common potting material is epoxy resin. There are dozens of brands available at hardware and hobby stores. Try to use the ones that say something like '30 min pot or open time'. They fill the mold well and let the bubbles come to the top were you can burst them. If you need an opaque epoxy use JB Weld. If you need to get heat out use Arctic Alumina Epoxy or if you can find someone who has some use Linde A polish and regular epoxy.

You can make 'one off' molds with wax, or production molds out of UHMW polyethylene. I have a couple poly molds that fit Wayne Y's BadBoy/MadMax series. Used a cheap spade drill just a tad bigger than the BB/MM and CAREFULLY ground it to the right size and drilled a few holes through a UHMW block cut to the thickness of a regular drop-in. The BB/MMs fit thghtly into the holes (coated lightly with cooking spray) and I fill the space through a hole drilled in the side with a syringe and 16ga needle. After the potting hardens you can push the BB/MM out.

Re: How to

Just wanted to offer my two cents worth... potting is a wonderful thing, and a dangerous thing! Properly done, it can protect a circuit from moisture, chemicals, and impacts, and some versions can provide a decent thermal path to the outside world or to a proper heatsink. Improperly done, it can destroy fragile components and trap moisture.

I'd suggest reading some of the technical literature. Emerson & Cuming make a wide variety of potting materials ( http://www.emersoncuming.com/ ). I think that Loc-tite and G.E. Silicones do also.

I've used a hard potting that contains aluminum oxide particles that are good at conducting heat. The downside is that any hard potting presents a risk to fragile components (usually as a result of different coefficients of thermal expansion). This is mitigated by using a soft potting or RTV over the fragile components.

The good potting materials are kinda pricey for amateur use. For home projects, I make do with stuff like clear RTV or Plasti-Dip.

Steve K.

Re: How to

Whiskypapa3,

I think I will take the epoxy route. You mentioned epoxy resin. So do you mean only use the resin part of the mixture and forget about the hardener?

And also, is this a good product to use?
http://www.lepage.ca/ProductCatalog/detail.asp?catid=21&subid=54&plid=435
It's the only one I can remember being at Home Hardware.

Daniel

Re: How to

dw_1984,

I burned my finger on the IC of a BB750 after a few seconds of testing! IMO, the best solution is to cut/file a small piece of aluminum/copper (from an old penny maybe) with a step, so it will rest against the IC and diode, and come to the same height as the inductor. I would then recommend bonding the "sink" to them with Arctic Alumina Adhesive. It is a 2-part epoxy with about a 5-minute "pot life" or working time before it sets-up. I would then "pot" everything. (The stuff's non electrically conductive, so no worries.) After that, I "potted" mine into a metal cavety, which contacted the top of the home-made sink.

Larry

Re: How to

dw_1984

You have to use both parts. Technicaly "resin" is the hard plastic you get after the two parts have polymerized.

If it gets as hot as tvodrd says you may have to check with dat2zip about how to pot it (or not).