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if I am plugging in the power supply the red power led lights very dim.

I found several guides to test the the board with a multimeter but these were for the other revisions, and it seems like B+ is a little bit different.

I had a device on my GPIO pins which drew way to much current, so actually there is a reason why something could be broken. But the Pi worked fine for several weeks afterwards. Now I would like to know if the power supply is broken or the board. And may it help to power the board via GPIO?

  • you should get a different board and a different power supply, then use different combinations and try to figure out what's broken. – lenik Jan 7 '15 at 10:11
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    Carefully measure the voltage between pin 2 (5V) and pin 6 (ground) on the expansion header. – joan Jan 7 '15 at 10:33
  • This exact same thing happened to my B+ 1.2 while I was sleeping! (I must have rolled on top of it, or something). Now the 3.3v rail is screwed (〰1.8v) and nothing seems to be able to revive the stupid little thing. Shiite! – alex gray Nov 30 '15 at 12:59
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I measured the 5V test points, and it turned out that the 5V are okay, but the 3V3 gets just ~1.8V. After some testing around I noticed, that the CPU is very hot.

Most probable szenario: I accidentally connected the 5V GPIO pin with a normal GPIO pin, and this fried the processor.

Solution: get a new pi, don't fry this one, tape connector cables when other people are around to prevent this.

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And may it help to power the board via GPIO?

I would only do that as a matter of last resort if it appears its not usable otherwise. I'm not an electrician, but my understanding is that the B+ has two forms of power protection, a brown out circuit to detect under voltage, and a polyfuse to deal with over current. If you blew the polyfuse, it would reset in time (at most a day or so, I think).

In other words, if you've damaged something, it will probably is not part of the power regulation so by-passing that won't make any difference.

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Also, Don't connect power LEDs directly to the GPIO pins of the Pi, but use for example a FET inbetween. Sometihng like this:

https://electronics.stackexchange.com/questions/40007/remote-control-led-has-3-volt-power-source-output-i-need-it-to-switch-on-a-12-vo

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