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Faulty mbed - overcurrent?

Topic last updated 23 Apr 2010, by Jan Mulders. 1 reply
Jan Mulders
23 Apr 2010

Hey all,

I recently ordered two mbeds from Farnell, and one has conked out after I tried to get it to drive an LCD using the TextLCD interface (http://mbed.org/projects/cookbook/wiki/TextLCD).

As soon as I connected my LCD up and hit the power, I noticed it was pulling an awful lot of current: 1A at 4.8v. I turned the power off and double checked my connections - everything's in order.

Disconnecting everything and running the mbed with only +5v and GND connected (same story on USB) results in it pulling an amp and not lighting up whatsoever. One of the large 3-pin surface mount components I presume is a regulator (LD33 C749, nearest pin 1) is heating up like crazy - but this isn't new, as this got pretty hot when the mbed was running normally.

Have I fried my mbed, and if so - how? Surely there's overcurrent protection on the +3.3v line, and all the data lines (pin 24 to 30) can handle getting 3.3v dumped into them, worst case - right?

It just seems a little infeasible for me to have cooked this thing just from pulling too much current from the 3.3v line.

Maybe someone can enlighten me?

Varka

Jan Mulders
23 Apr 2010

Some more observations:

When powered with 5v, the regulator gets really toasty as well as the processor on top (the LPC1768). Sounds to me like I've managed to fry the micro.

Any suggestions on why this has happened, what protections the mbed is meant to have to prevent this kind of thing happening, and anything I can to do to resolve this?