On Thu, 26 Jul 2012 22:57:25 -0400, "Tom Del Rosso"
wrote:
When a mobo has bad caps the PSU shuts down, but what triggers it? What
does the PSU see from its point of view? Peak current spikes?
The purpose of the caps that are close to the CPU is to reduce the
ripple on the processor power line to tolerable levels. The processor
can go from zero to 40 amps at anywhere from zero (ground bounce) to
GHz rates. The capacitors have to smooth all that out. Basically,
they form an energy storage system to deliver power during the high
current spikes to the power hungry CPU.
When the ESR (equivalent series resistance) of the caps increases due
to electrolyte loss caused by overheating, less energy is available to
the CPU during high current peaks. The result is lots of ripple and
noise on the power supply line. Eventually, this gets the attention
of the "power good" line to the power supply, which shuts down to
protect the MB and CPU.
http://en.wikipedia.org/wiki/Power_Good_Signal
One irritating problem is the dominoes effect on multiple MB filter
capacitors failing. When the ripple voltage is low across the
capacitors, there is little ripple current through the capacitors.
Capacitors only conduct during the time the voltage changes across
them. When the voltage is pure DC, no current is drawn. They also
only get warm when there's ripple current through the capacitor.
When one capacitor (usually nearest the CPU) fails, the ripple current
trhough the others increases slightly. That means that the internal
heat generated in the other caps increases. When the next capacitor
fails, the ripple current and internal heating of the remaining
capacitors again increases, result in even more heating. Eventually,
the few remaining caps can't handle the self-heating, overheat, bulge,
and die. This is why you want to replace ALL the capacitors in a
parallel string when one of them fails. Measuring the ESR of such a
parallel string is impossible.
Note that better MB's use polymer capacitors instead of electrolytics.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558