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Powering the Mega 2560 from External Power

e14 Contributor

I recently blew up the USB  i/f and the 3 Volt regulator on my 2560.  I am powering it from the output of a 9 V LDO regulator connected to the Vin pin on the board. I subsequently connected my laptop to the board via the USB connection to perform some basic I/O.  The board got very hot and now the USB I/F is fried.  I'm also only getting 2.2V out of the 3 V regulator on board.

 

Is connecting the USB to the board while powered from an external supply a no no or is there a documented procedure on doing this?

  • Rick

    Normally there is no issues with connecting to both power sources as there is a fet that does the switching .....

    Some laptops have switch mode power supplies and may be floating at a high voltage.

     

    You say you are powering it from a LDO 9v source, but how is this wired. Is the negative connected to earth, or is the whole thing floating.

    If you have a multimeter you could measure the voltage between the USB metal case and the metal of the USB socket on your laptop.

     

     

    It may be possible to identify what is dead, the least of which is the 3v3 regulator.

     

    It might be time to replace the Mega, but making sure the next one doesn't suffer is a good idea.

     

    Mark

  • in reply to mcb1

    Hi Mark & thanks for the quick reply.  The LDO ground & the USB case ground are both verified to be at my controller box ground (earth) so nothing is floating.  Can't see any issues with USB case to laptop case either.  I've already received another 2560 from Mouser and after checkout of all my connections I gave this another try.  So far so good.  I'm now thinking that the usb plug in while the system was hot was done on the board side connection and not the laptop side.  A little wiggling of the usb cable is necessary to slide it into the 2560 due to physical contraints of by box.  I might have inadvertently shorted something in the process.  Anyway, the cable from the 2560 side is now permanently in place so all hot connects are done plugging the usb cable into the laptop.  And away I go.

    Thanks again so much.

     

    Rick

  • in reply to e14 Contributor

    Rick

     

    The checks do seem to verify the voltages are okay.

    In theory it should not matter which end of the USB cord is attached first, as the voltage pins are slightly longer than the data pins.

     

    3v3 regulator chips are dirt cheap, and sometimes non smd ones are found in mice, so you could try removing the one you have and replacing it.

    Of course the issue may be somewhere else, and its dragging the 3v3 line down ....

     

    Cheers

    Mark