David said:
I had a system fail with a 500W PSU, because the ps only provided 25
amps on the +12 volt
rail, and the video card needed a minimum of 27. Result was spontaneous
reboots, at
unpredictable intervals, with nothing in the logs to indicate why.
Regards, Dave Hodgins
I bet a check of the hardware monitor, the one that measures
3.3V, 5V, 12V, would have indicated 12V off enough, to tip you off.
The 12V would start to dip if close to the limit. And the 3.3V and
5V would be higher than normal, due to the crossloading effect.
So if you see that pattern (two rails high, one rail low), that
indicates you're approaching a limit on the supply. It's a way of
detecting a heavy load, without using an ammeter. If the
primary side is cranked as much as possible, the output can
only continue to drop with the additional load. If a supply
actually meets crossload, it's supposed to stay within 5%,
on the two rails that are on the high side.
It's because a typical supply only has one feedback loop,
and all outputs get turned up at the same time. The lightly
loaded outputs, then end up looking "too high". The heavily
loaded rail becomes "too low" when the feedback loop no
longer has any adjustment room left, and is cranked as far
as it'll go.
The actual switch off, could be thermal, as there may be
a thermistor bolted to one of the cooling plates, perhaps
the one with the rectifiers on it. And if it's starting
to overheat, the thermistor can be used to shut off the supply.
The supply can also have overcurrent, something intended to stop
the supply if there is a dead short. But usually, that will be
set 30% above the supply rating, and that form of protection
probably isn't what was turning off the supply. It was more
likely to be the thermal protection.
There are many variations on supply design, and the above
only applies to the "$50 type supplies". There were a few
supplies with independent regulation, and viewing hardware
monitor might only show the 12V low in that case. And the
latest supplies with 87% efficiencies, some of those use
two stage regulation. The 12V feeds a 3.3V/5V regulator
board, and the adjustment of the primary only affects
the 12V, and the 3.3V/5V (about 20A max) board would tend
to be independent of the rest. But if your supply is a
run of the mill type, you may be able to spot an issue
using nothing more than the hardware monitor information.
The motherboard hardware monitor isn't very accurate.
It's job is to spot a trend. A multimeter should be used
to confirm any suspicions you might have. Or, sniff for smoke
Paul
