I have a motion control PCB that runs some hydraulic valves via pwm in one of my products. We discovered that a drop of water on the right place on the PCB will turn on a MOSFET causing uncommanded movement of the system.
The argument around here is potting vs conformal coating. The boards that were exposed to water via pressure washing operations DID have conformal coating on them but some how the water still turned on the transistors. I had the P channel depletion MOSFETS pulled up with a 10K resistor. We changed that to a 680 ohm and it made it much less likely to get turned on with water droplets. But it is not perfect. Needs physical protection too. I am of the opinion that a good thick layer of conformal coating is just as good as potting and much less work. Opinions?
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