On Fri, 02 Sep 2005 20:30:27 GMT, "George Hester"
<hesterloli@hotmail.com> wrote:
Most important is accurate ratings, a rating of the PSU's
sustainable current rather than it's peak current.
Enermax is way better than most generics, but unfortunately
they too overrate a little bit. Check out PCPower&Cooling's
website, they have a brochue that shows a list of why there
PSU is better than "some other", and that "some other" is an
Enermax (they don't come right out and mention that, but
it's easy to recognize from the picture and specs).
Well, it is rated as a 330W PSU, so that looks about right
_IF_ Enermax had a more conservative rating system. In
other words, if Delta or Sparkle had called it a 330W PSU,
it'd be beefier than that Enermax is. The Enermax in the
link is better than most generic 400W PSU, though.
First and foremost, you have to either buy one that's
accurately rated, or find out the true output power through
testing, OR guesstimate and buy more watts & amps than you
think you need, hoping it has your needs covered. Obviously
the latter option is the worst, but is the one most kids use
when they buy a generic 300W, find it insufficient, then buy
a cheap 500W, finding that it works so they conclude a
system like theirs "needs" 500W... when all it really needed
was something a little under what that pseudo-500W generic
can actually output, so long as we dont' consider how long
the generic will or won't work, or any other shortcomings
from many generics.
Typically a system loads one of the power rails the most-
used to be 5V, but now most often 12V rail. Items such as
CPU, video card, and hard drives (assuming you have nothing
exotic like Peltier cooling devices) use the majority of
that 12V power. An older system or one with a slower
Celeron or Pentium-M and only a couple hard drives, a meager
video card, could get by with 12A of "accurate" power from a
PSU. Getting by is not as good as some extra margin though,
so typically a target of around 18A 12V power is useful, or
a little higher for a very well endowed system. Since the
typical PSU has correspondingly higher 5V amperage as 12V
amperage rating rises, that shouldn't be an issue on such
systems consuming most power from the 12V rail.
It isolates each "rail" a little bit, causes lesser
fluctuations on one when the load changes on the other.
No, not for the (same otherwise) build quality and capacity.
Usually when a PSU has dual rails, it is built like a
single-rail supply, only one 12v rail. Then just prior to
the wiring harness, there are a pair of low-ohm resistors
separating each 12V rail. That isolates them more, but you
have a lower limit of total power on either rail. If a PSU
has plenty of reserve power, it can be more useful, but if
the PSU is marginal the benefits may be outweighed by having
more current potential with the single rail design.
Although advertised as a "feature", anyone competent to open
and modify their own single-rail supply could do so and have
same result.
Ultimately the final test is whether the voltage is correct
with regards to amperage- a supply with insufficient
capacity will start dropping one rail or rasing the other
(as it compensates). Voltage at the connector plugged into
the load(s) should be within +-5% of the spec'd voltage, and
5% off is a little less than ideal, a psu well matched to a
system is generally a little more accurate than that.
George Hester wrote:
That 330W Enermax seems to be made for older style mobos that draw lots
of amps from the +5.0V rail because the CPU voltage regulator runs from
it. C'T magazine once measured an Athlon XP2400+ system like this as
drawing 18A @ +5.0V, 15A @ +3.3V, and 2A @ +12V. In comparison, a mobo
that used +12V for CPU power (i.e., has a second 4-pin power connector)
drew 3.2A @ +5.0V, 9A @ +12V, and 10A @ +3.3V. So I wouldn't use that
particular model 330W Enermax for the latter type of system.
In case of a severe short, possibly a smaller fire or puddle of molten
plastic? 
I think that dual rail +12V design's only purpose is to meet a safety
requirement to keep the maximum volts x amps (sort of like watts) draw
from the wall outlet below 240VA in case of a short in the computer.
It offers no performance benefit but is absolutely necessary for
keeping up with the latest fashion trends. Normally, one +12V rail is
for the mobo and maybe the graphics card (if it has its own power
connector), the other +12V rail for the disk drives.
