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Topic: How to isolate a circuit from water  (Read 487 times) previous topic - next topic

valerio_sperati

Hi all,
I'm developing a small PCB hosting:
  • an ATMega 328
  • a miny Lipo Battery 3.7 volt
  • a Lipo charger
  • 3 addressable LEDS



The pcb should be sporadically dipped in water, for about 1 second.
Questions:

  • Is there some water resistant resin I could use to cover and isolate the entire PCB?
    I think the PCB does not produce significant heat, so I guess this will be not a problem.
  • For Lipo recharge, I'm using a USB cable connected to a Lipo charger on the PCB. One end of the cable (that one I use for recharge) will not touch water but I need to isolate the other end (that one connecting on the Lipo charger).


Thanks for advices.


 

TomGeorge

Everything runs on smoke, let the smoke out, it stops running....

MarkT

Even with conformal coating, you are likely to get corrosion set in.  Water and electronics do not mix.
Sunlight + water will be even worse.

You need to pot the circuit in a box really, but a LiPo battery should never be potted, it becomes a
hidden fire risk.

Why do you want to do this?
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

ElCaron

Taking about conformal coating, is this conformal coating?
I have a can of that, and I cannot say I trust it a lot. Ok, it's a thin transparent film, so it is probably normal that I do not see a lot, but I also didn't have a lot of problems using a multimeter afterwards on the contacts. If it works at all, it does not give a lot of mechanical resistance.

I still use it a lot when 240V are involved, just as an additional layer of security. I do not reduce trace clearance because of that, though.

Noobian

#4
Sep 20, 2017, 06:29 pm Last Edit: Sep 20, 2017, 06:32 pm by Noobian
The pcb should be sporadically dipped in water, for about 1 second.
Questions:

  • Is there some water resistant resin....
When you dip something in water you should look for water-proof resin and not water-resistant.

how deep in water? purity of water? is it saline? is it acidic? is it hard water?


I think the PCB does not produce significant heat, so I guess this will be not a problem.
Your PCB does not produce much heat when it's open and ventilated. When it's enclosed it might start to build up heat. You should double check this first.

TomGeorge

Hi,
The can you linked to is good for corrosion protection, but not for overall waterproofing.

What is your project that needs to be dipped in water?
What is  the application?

Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

Paul_KD7HB

Further considerations. Your PC board probably has wires attached. Wires are usually flexible. Water will follow the surface of the wires into the conformal coating because wire movement has made the coating release form the wire insulation. If water gets inside the wire insulation it will move eventually the circuit board.

Design your project so wires never get wet.

Paul

ElCaron

The can you linked to is good for corrosion protection, but not for overall waterproofing.
I am not the OP, I did not intend to use this to make anything waterproof. However, this stuff is called conformal coating in the datasheet, and I wanted to check if that is what you meant. Because, as I said, I really wouldn't trust it for that, and actually not really for anything regarding safety ..

Grumpy_Mike

What conformal coating will protect against is condensing humidity. It is good at that.

ElCaron

What conformal coating will protect against is condensing humidity. It is good at that.
At least the one I linked seems to be super easy to mechanically damage, though. As I said, I had no problems holding a not very sharp DMM probe to a sprayed contact and get a reasonable reading.

Grumpy_Mike

Yes that is the point of conformal coatings. It is called probe through. It is meant for servicing. Once serviced you need a quick spray to it again, unless the coating is self healing.

The coating is ment for a board that is mounted in a box, not a free board that is handled.

ElCaron

Yes that is the point of conformal coatings. It is called probe through. It is meant for servicing. Once serviced you need a quick spray to it again, unless the coating is self healing.

The coating is ment for a board that is mounted in a box, not a free board that is handled.
Ok, that is good to know. I was using it for stuff in boxes, mainly when mains is involved, but I was never quite sure if it makes any sense at all.

valerio_sperati

Hi all, thank for your advices. I attach here a sketch of my project, so to better illustrate my needs:




This is a device to be used by a bubble artist, on his hand; I write here a list of additional info:

  • he will dip his hand in water with soap, approximately  up to the wrist; dipping time: about a couple of seconds. This is needed by the artist to soak the hand with water soap.
  • he will switch the leds on/off through two mini pushbuttons.
  • I'm still thinking how to arrange all stuff: probably I'll sew everything on a glove worn by the artist.
  • the artist will wear a fabric wristband on the wrist, in order to avoid water dripping on the arm. Probably I'll fix the pcb on this wristband: note that the wristband, even if not dipped directly into water, will be very very wet
  • The critical parts, as shown in the sketch, are the led strip, the pushbuttons and the wires connecting to the pcb: all these elements dip in water.



Now, according to you, how can I isolate all the circuit?
Do I need necessarily a waterproof box for the pcb?
How can I isolate leds, pushbuttons, and wires, which are necessarily out of the  (possible) waterprof box?
Thanks a lot!




[/list]

123Splat

As mentioned before, Google "potting compound"

hammy

Why not put the PCB further up his arm , so it doesn't get wet .

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