On 14/09/2011 23:08, Dwayne Reid wrote:
> At 06:16 PM 9/13/2011, Brent Brown wrote:
>
>> What I strongly suspect is happening is that plugging into header
>> pins the GND pins
>> are not contacting first (there is nothing to guarantee the order of
>> connections) and
>> when +12V pin contacts is couples power through discharged
>> elelctrloytic caps on
>> daughter board to GND rail, then "backfeeds" to +3.3V rail through
>> internal body
>> diodes of chips on that rail, pushing up the +3.3V rail and popping
>> the processor on
>> the main board. Similar could be happening on RXD, TXD and +5V
>> rail... but nature
>> of 2 failures so far suggest +3.3V rail more likely.
> Just a thought - can you add a power switch (MOSFET or whatever) to
> the 12V line as it enters the daughter board?  The 12V rail would be
> dead until something tells it that its OK to power-up.
>
> Note that this switch is on the daughter board, since you mentioned
> that you would like to avoid making design changes to the main board.
>
> For example, you could make a simple one-transistor controller that
> monitors for about 3V between the daughter-board ground and 3.3V
> rail.  If either ground or 3v3 was missing, don't allow the MOSFET to tur=
n on.
>
> dwayne
>

I was thinking along those lines too - you could also add an RC filter=20
on the base to bring the rail up slowly.

--=20
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