On Fri, 02 Sep 2005 20:30:27 GMT, "George Hester"
<hesterloli@hotmail.com> wrote:
>I asked about a PSU issue I was having a few weeks ago here. One of my SCSIs wasn't being seen consistently at boot. After a little shopping around I found a 550 W PSU. But as I have become more familar with PSUs and from what I've read here Watts isn't the entire story. What looks also to be just as important is AMPs. The current delivered at startup. PSUs seem to be all over the map here for a given Wattage.
Most important is accurate ratings, a rating of the PSU's
sustainable current rather than it's peak current.
>Luckily mine is 24 amps and I no longer have my issue. But take a look at this:
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).
>
>http://cgi.ebay.com/Enermax-Pentium-...QQcmdZViewItem
>
>You will see this is a 330 W PSU but it is really low in AMPs 12 amps. That sucks right?
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.
>So my question is should I look at both Watts and Amps with PSUs? In other words 330 W PSU with 12 v 24 amp may be better than a 550 W PSU with 12 v 12 amp?
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.
>And finally I asked about this once before but I am going to try to be more specifuic. What exactly is the performance benefit if the 12 v is dual? If my PSU gives each device that uses 12 v, 12 v what's the signicance of having dual 12 V?
It isolates each "rail" a little bit, causes lesser
fluctuations on one when the load changes on the other.
>Would that normally have a higher AMP rating?
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.
PSUs Watts and AMPs 
Linear Mode
