> >What I strongly suspect is happening is that plugging into header=20
> >pins the GND pins
> >are not contacting first (there is nothing to guarantee the order of=20
> >connections) and
> >when +12V pin contacts is couples power through discharged=20
> >elelctrloytic caps on
> >daughter board to GND rail, then "backfeeds" to +3.3V rail through=20
> >internal body
> >diodes of chips on that rail, pushing up the +3.3V rail and popping=20
> >the processor on
> >the main board. Similar could be happening on RXD, TXD and +5V=20
> >rail... but nature
> >of 2 failures so far suggest +3.3V rail more likely.
>=20
> Just a thought - can you add a power switch (MOSFET or whatever) to=20
> the 12V line as it enters the daughter board?  The 12V rail would be=20
> dead until something tells it that its OK to power-up.
>=20
> Note that this switch is on the daughter board, since you mentioned=20
> that you would like to avoid making design changes to the main board.
>=20
> For example, you could make a simple one-transistor controller that=20
> monitors for about 3V between the daughter-board ground and 3.3V=20
> rail.  If either ground or 3v3 was missing, don't allow the MOSFET to tur=
n on.
=20
Thanks Dwayne (and Oli). I had somehow overlooked that option, initially be=
cause=20
the 12V rail handles a fair bit of current (up to 4 x DC motors and 2 x lin=
ear=20
actuators) and I didn't want any voltage drops. Designing it into the board=
 now using=20
a IRF7424 P-channel MOSFET (30V, 10A, 20mOhm, SO8 package, low enough=20
price), and using the single remaining spare I/O line on the micro control =
it. Will put=20
power up delay in code before turning 12V rail on, nice to be able to turn =
it off also=20
to reduce quiescent current. Need a couple of BJT's and a few resistors for=
 level=20
shifting and to make sure it doesn't turn on during reset (8051 based chip =
has weak=20
intenal pullups enabled by default). Have included RC network of 5ms time=20
constant, to limit inrush current of some electrolytic capacitors.

Technically speaking the 5V rail could also couple to the 3.3V rail if GND =
pins not=20
connected, but these rails are through regulators so current is limited, an=
d take=20
away a diode drop and the voltage difference between them is much less like=
ly to=20
cause the same kind of damage the 12V rail does.

--=20
Brent Brown, Electronic Design Solutions
16 English Street, St Andrews,
Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell: +64 27 433 4069
eMail:  brent.brown@clear.net.nz


--=20
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