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Sometime ago I designed an bespoke enclosure for an el' cheapo action camera (GoPro clone), which pinned out the USB's power lines outside the waterproof enclosure - something like this, with the diodes on a tiny PCB inside the enclosure:

schematic

simulate this circuit – Schematic created using CircuitLab

The camera, with its enclosure, would be placed in a bespoke cradle to be charged (outside voltage of course slightly more than 5V to compensate for D1's Vf). The idea is that the enclosure will stay shut all the time, as this is important to the application (there also other specifics, which is why we haven't employed standard waterproof USB connectors in the first place).

I have a request to expose the data pins as well in a similar fashion, because they want to download movies from the camera quickly (the existing WiFi-based transfer is slow). I'm not well-versed with why the USB standard is designed the way it is, but I do remember that all USB connectors make sure that

  1. the power rails mate first, then D+/D-
  2. the D+/D- contacts are recessed, so it would be hard to accidentally apply ESD to them.

If I redesign the whole thing to pin out D+/D- outside the waterproof shell, I would likely violate the first point, and certainly will violate the second.

Question

How dangerous is this? Can't I just make the small PCB inside the enclosure a bit more complex and add e.g. ESD protection diodes from each data line to the power rails? Would the USB communication be affected by such protection measures?

anrieff
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  • The data pins have pull-up and pull-down resistors built-in so they do not float, however there is a limit to what kind of static charge they can tolerate. Can you make a cover or dummy-plug to protect them? –  Jun 26 '20 at 23:21
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    For a hobbyist, not a big deal, for production product you would fail ESD testing at the lowest voltage. You definitely need ESD protection on those data pins. Some would argue the data pins are actually more sensitive due to lack of capacitance and load. – MadHatter Jun 27 '20 at 02:25
  • @Sparky256, no, unfortunately physical protection is not an option. – anrieff Jun 27 '20 at 04:51
  • @MadHatter it's not a hobby use, and while no actual standardized ESD testing will be performed on these, the fear is whether they'd survive real-world use with operators handling the cameras and moving them around many times a day – anrieff Jun 27 '20 at 04:51
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    "How dangerous is this?" - this is not an engineering-level question. The "standartized ESD testing" with defined testing levels is precisely what you need. – Ale..chenski Jun 27 '20 at 06:37
  • @Ale..chenski: agreed, that would be best, but I don't have ESD testing equipment, and don't know where to borrow. I was hoping that somebody might have tackled with a similar problem. – anrieff Jun 27 '20 at 10:58
  • anyone that designs with a USB interface tackles this problem, you simply put a single IC chip on the USB line that has all of the necessary ESD diodes on it... Although I'm starting to realize are you just essentially plugging into an existing USB port on a camera? If so that Port probably has ESD protection already and you don't need external protection or the diode and whatever else you're doing there. – MadHatter Jun 27 '20 at 14:05
  • This answer might point you to right direction, https://electronics.stackexchange.com/a/260316/117785 The keyword is "IEC 61000" – Ale..chenski Jun 27 '20 at 21:21
  • And this, https://electronics.stackexchange.com/q/368444/117785 – Ale..chenski Jun 27 '20 at 21:22
  • Plus extra information about test arrangements, https://electronics.stackexchange.com/q/274820/117785 – Ale..chenski Jun 27 '20 at 21:23

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