I thought I would pass this along to all...

Two nights ago I was experimenting with an old resurected
O'Scope (it is an old Bell & Howell Schools kit scope).  It
had a shorted focus voltage cap which I replaced.  It seems
to work fine now..  But that where the fun starts!

I had a serial PIC programmer breadboarded up and I thought I would
poke around a bit and see what the scope could see. While doing this,
I accidentally shorted the ground against the metal enclosure of
the old PC power supply I was using.  This created sparks because
of some kind of ground loop. Immediately the LED that the PIC
was flashing quit.  I could smell that infamous burnt electronics
smell.  I cycled the power and the PIC started working again, but
then faded and quit again.  I touched the PIC and it felt very hot!
The burned smell was coming from the PIC!  After powering down and
letting things cool I plugged it back in and everything was fine
again!  My PIC was spared (actually two because the programmer used
one too!).

Two questions:

Does the PIC have some kind of thermal shutdown that was triggered
by the ground loop?  Why did it get so hot even though the ground
loop was very brief.  The only thing I can think of is some kind of
VDD clamp that was triggered shunting VDD to VSS and heating up.
Letting it cool down allowed the shunt to reset.  If a PIC can survive
a disaster like this, I am impressed.  BTW, I attempted programming it
again and all is well!

Is it normal for switchers from old PC's to have a hot chassis?
I can't figure out why the  ground shorting against the case of
the power supply would do this.  This wouldn't be very good if
a ground loop like this existed inside the chassis of the PC from
which the supply was removed.  Perhaps it was a hot ground from the
switching supply disaggreeing with the ground of the scope....  the
scope has a transformer based supply and the primary ciruit is
totally isolated from the chassis.

Dan


*******************************
* Dan Larson                  *
* Software Engineer           *
* Micro Control Company       *
* email: dlarson@citilink.com *
*******************************