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Embedded PC with many USB devices seem to have been exposed to overload or short-circuit. (some ports only deliver ~1V)

Q: Would it crash the MB if regulated 5V is led from external PSU to the USB-connectors? Mainly thinking of when pc=off, still 5V on USB.

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I'm not quite understanding the configuration you're asking about. Are you saying your +5V USB lines have an external +5V source connected to them? –  Doc Jan 5 '12 at 18:27
    
Thanks, Currently no, but that's my intention; soldering a 5V-cable from regulated PSU to the USB-ports on the MB. (Lack of time so change of mb not an option). –  user65823 Jan 5 '12 at 18:35
    
Strictly speaking, that could work. But I think it's a bad idea. I'd be worried about how the circuit on your MB providing USB power would respond when the machine's powered down. It could be okay. I suppose you may be able to rig up a diode on there to help prevent the external source power from getting through to the machine - but diodes will introduce a Vdrop.. Still, that may work. I'd probably try it, but with hardware I didn't care for - still different implementations of similar gear could respond differently. Interesting question. –  Doc Jan 5 '12 at 18:40
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2 Answers

USB is a fickle animal today... most of the time on modern motherboards... the 5v line is used as a reference for the data lines... and simply cutting the trace going to the 5v pins on the header and adding 5v to it won't bring the hardware back to life. Sure, it'l supply power... but if the USB controller chip itself is only getting 1v instead of the necessary 5v or damaged to the point where the output voltage is only 1v... you're going to be out of luck.

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First I would use more than one test method to check the power at the USB. one of the small USB toys they have. Plus a voltmeter ,I would test the voltmeter on something if I wasnt sure about it.

I have read on the web where a user who damaged the power on a USB port, was able to use a good quality 4 port powered hub. In that situation the rest of the port was still functional, and he/she was able to use the port for something.

I would try that method of the powered hub , in hopes of knowing more, and to be sure that it would work if I supplied power there to begin with. The hub could be powered with batteries for any portable use still.

There are other devices which require no power from that port, they could be used to test also.

next I would analise how much I had to lose , and go for it. if the connection one way is actually busted, it isnt very likely to to head back into the computer any ammount that would mater. Proper polarity and isolation of the added power would be important, if your getting the power from the the same computer, even if converted with a DC-DC or a voltage regulation. Out of pure fear, and just general good electronics, the power going in there would be limited to ~.5A , using a mini fuse or tiny curcuit breaker, or via the max output of the regulation.

It still would be better to know what device this is, or anything else about it.

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