Dan Larson wrote:
>
> 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
>

I don't know how or what went on, but i've had a similar experience.
Two p16C84's were a little mistreated here, and got really hot. But
after reprogramming, they worked fine again! (I'm not complaining, just
surprised...)

Hans