I wired up a circuit with an LED strip, 3 N-channel MOSFETs, and a separate
12V power supply, with the gate pins on the MOSFETS connected to I/O pins
on the Beaglebone.
some mosfets have protective diodes on the gate inputs.
When I didn't connect the circuit to the Bone ground, I apparently fried 2
boards. Neither will boot up. One gives a flicker on the power light and
then goes off, and on the other the power light turns on, but the other
LEDs don't flash and I can't get into either board.
If I assume that the ground on the circuit didn't have a connection to
the BBB ground, and that the 12 volt supply was connected to the
circuit +12 and gnd, then you put floating lines on the BBB outputs,
and any leakage in the power supply to ground (or the BBB ground)
could put any voltage on the output pins of the BBB.
The gates were floating, which is why the circuit was unstable.
BTW: these had to be enhancement mode FETs for this to work.
Here is the thread on the Adafruit forum with a photo of the circuit:
*The Adafruit support said that setup should not have hurt the board. *
If connected properly.
I checked the voltage on the gate pin on each MOSFET that was connected to
the Bone and it was nominal (~200mA).
Voltage is not measured in ma, do you mean 200 mv?
The meter you used loaded the circuit, and would not show the true
voltage. It's not a matter of current into the chip, it's a matter of
voltage (period). The voltage can cause nasty things in the chip
transistors driving the pin, and the power supply of the chip itself
then does the damage.
Does anyone have any ideas on this? How could this have fried 2 boards?
A suggestion for you. Next time, do not make single connections to
pins on the BBB (or anything else unless you're quite certain of what
Best to take a ribbon connector that makes connections to everything
at once, run it to a breakout board, and then hard wire your
connections to that. A frequent problem in breadboards is wires
coming loose. This way the ground is automatically made to the chips
on the breadboard.
Another option would be to use a darlington transistor array, such as
the ULN2803 or the ULN2003 instead of the FETS. Again, the same
warning about grounding is in effect.
Since you have two non-functional BBB's I'd suggest taking a meter and
checking the voltages at the various regulators on the board. You
have a schematic, and it may be possible that you have a bad regulator
somehow. That's better than a fried CPU.