[LAD] [Somewhat OT] Strange failure mode of a PC

[Ralf Mardorf]

On Sun, 2010-07-18 at 14:37 +0200, Jörn Nettingsmeier wrote: 
> On 07/17/2010 10:46 PM, fons at kokkinizita.net wrote:
> 
> > What I don't understand is how the contacts got so dirty.
> > If a resistance of a few kOhm is enough to make it look
> > as a closed contact then it can't be handling large currents,
> > so there should not be any arcing. And the construction of
> > the thing is such that it is virtually closed, no dust or
> > whatever could ever creep in.
> 
> it can't be totally sealed, as that would require an expensive gasket. 
> my guess is that the way servers are designed, air is drawn in through 
> the front (the "cold aisle" in datacenter lore), and expelled through 
> the rear (the "hot aisle"). after some years of use, it's not so 
> unlikely for gunk to accumulate pretty much anywhere in the path of the 
> airflow.
> and the conductivity of pure industry-strength gunk should never be 
> underestimated.
> 
> btw, if the bios does not allow the boards to be configured as "always 
> on after power loss" (common on older hardware), a capacitor in place of 
> the momentary power switch works nicely, provided its charging current 
> has dropped to zero before the bios thinks it's a 
> more-than-four-seconds-turn-me-off event.

Just for fun I used the German Google to search for the switch issue. It
seems to be something that happens very often :D. Wow, my PC case is
that old, that I had to manipulate the LED jacks, because they don't fit
to modern mobos, but the switches never were broken.

What's a good value for a capacitor?

Btw. I pushed the momentary switch while running a GNOME session on Suse
11.2. As expected the ShutDown-Restart-Suspend-Hibernate menu appeared.
I closed the menu and pushed the momentary switch again, but the the
computer turned off, without doing a shutdown.