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I'd like to drive a high powered WiFi adapter, and I'd like to maintain the voltage safety mechanism and continue to power it all from the micro-USB so that I don't have to make unsightly modifications to the USB headers or my adapters.

I can splice the USB connection to pump 5V into the Wifi adapter only, while leaving the Pi running comfortably with the F3 polyfuse in place. That's the alternative.

Furthermore the external power supply may be fused. This is entirely serviceable, however I would really like to maintain the modularity of the system.

So to entertain the possibility of keeping things clean and boosting the power that can be accepted by the Pi, safely, I am reading up about the power circuit of the Pi... The schematic shows rather nicely how D17 upon overvoltage will short across F3. Since I expect to surpass 700mA through F3, F3 has to go. But then nothing protects me from catastrophic short circuit under an overvoltage situation. The answer must be to solder a slightly higher amperage polyfuse (or any other kind of fuse) on top of F3!

Has anyone done this? After looking at the schematic I'm pretty confident that this is a safe mod, but it sure will be a pain getting it soldered on properly (I may end up jumpering it).

  • Seems fine to me, but I'm definitely not an expert. Just note that the TVS (D17) diode also has to handle more current (when something goes wrong) when replacing the fuse. – Gerben Apr 22 '14 at 18:19
  • Yeah, that's true. I'm about to do a poor man's fuse mod with a 150milliOhm resistor parallel to F3. D17 will have a pretty tough job once the voltage hits 6V, but at least stuff won't ignite instantly. – Steven Lu Apr 22 '14 at 23:58
  • I you get a short, 33A will flow through that resistor. I think you might as well remove the polyfuse all together. You could also add a cheap car-fuse in parallel. – Gerben Apr 23 '14 at 15:14
  • It would burn the resistor and open the circuit. Hmmm. I will try to get a real poly fuse to do this with. – Steven Lu Apr 23 '14 at 19:20
  • I am also going to be running a model A (256MB RAM, 1xUSB no Ethernet) with the Wifi, so fingers crossed, it will actually function fine owing to not having the extra gear that makes the B draw so much more power, and I'll be able to leave it unmodified. – Steven Lu Apr 24 '14 at 2:40
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Yes you can replace it with another polyfuse.

You need to get a 6volt SMD PTC Ressetable Fuse (Poly fuse) rated at 2A. Ebay, Farnell or Mouser.

The dimensions don't matter, as long as the length is OK. There are other makes that are not as flat as the ones used on the Pi but more square shaped. That is fine.

The pain is DESOLDERING it. Make sure not to overheat the PCB from your soldering iron because it will cause issues. If you want to go down that route then get QuickChip SMD removal kit, watch some youtube movies on how to do it, practise on some old junk, then try it on the Pi. Soldering a new one back is a breeze.

  • 1
    One assumes that - given such a replacement will be with a hand-soldering iron - the user will have to wait awhile for the newly soldered in polyfuse to reset from the heat of being soldered in‽ – SlySven Dec 25 '15 at 2:01
  • Merry Christmas. – Piotr Kula Dec 25 '15 at 2:02

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