Creating small permanent waterproof electronic modules.

Project date: 2008

Occasionally you may have to make a small electronic circuit for the boat, as I did with this LED switching and current limiting circuit. This circuit, used on the stereo system, allows a LED to indicate one of several conditions; such as which zones are powered. Since there is some common equipment in the zones, "steering diodes" were used to ensure only the correct LED is on. As well, its standard practice to current-limit LEDs, and I added those resistors to the circuit.

But this project is not about the circuit itself, rather how to best mount those components in a boat, and maintain a degree of waterproofness. I am going to borrow a technique from the aerospace/electronics industry, and use "potting compound" to fill a box containing the circuitry. Potting compound is nothing more than an epoxy, with special electrical isolation and other properties.

Potting compound can also be used to seal the backsides of connectors, so I am sure you have seen this stuff without knowing it. One caveat up front though; the compound has some wicking properties, and if you are not careful, you can destroy a connector from the potting compound leaking into the contacts. Since it is a form of epoxy, you'll have a hard time removing it. So if you are wanting to seal a connector, look for connectors that are specifically designed for potting.



The first step in creating your own module is to wire the circuit. Here I simply used some thin pieces of plastic, poked a hole in them, then inserted the resistors and diodes required for the circuit. I then added the needed wires, and did a preliminary check out of everything. Most electronic components can be potted, but I would be careful about mechanical items such as potentiometers, switches, and connectors as mentioned above.

Since epoxy has sharp edges, to protect the wiring from chafing, I added a couple of rubber gromments, and I will fill the box up to that level. Other acceptable forms of wiring are terminal posts or studs that poke through the potting compound after the box is filled.

The potting compound will ooze out of any hole, so avoid drilling into the box sides or bottom if you can. Its far better to have any connections exit the top of the box.

The potting compound has a very long curing time - about 24 hours, so you'll need to find a place where your project won't be disturbed. As well, you'll drip some potting compound, so make sure to use a piece of plastic or otherwise protect the surface of your project bench.

There is an odd procedure to mixing and stirring the potting compound, so read the instructions carefully. You typically mix the compound, then come back in 15 minutes and mix it again to remove any air bubbles that has formed.

When you are ready, carefully pour the potting compound into your project box. Stop when you are about 3/4ths full, and wait for the epoxy to level out. It is somewhat on the thick side, and you do not want to overfill the box, or you may cover the gromments - which will defeat their purpose of chafing protection. The potting compound's working - or "pot life" is at least an hour, so you have plenty of time to add epoxy in small amounts until your box is fulled to the correct level.

After 24 hours, the potting compound is rock-hard, and you have a custom-made waterproof elecronics module you made yourself. Occasionally you may get some bubbling at the surface as I did here, but it is fully cured as well, and is of no consequence, other than cosmetically.

There are several formulations of potting compound, and the one I used is 832B, purchased from Mouser Electronics. This formulation has a high degree of both electrical and electrostatic isolation. This is traditional 2 part epoxy, with a resin and hardner. Together, they make about 1/2 liter of material, but that is quite a bit if you are potting connectors and small projects. The potting box, also from Mouser, is a Bud model PB1574, and measures 2"x 1.5"x 2". Other sizes are also available.




Mouser Electronics
Bud Industries Potting Boxes
MG Chemicals 832B Potting Compound

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