On Thu, 25 Oct 2007 03:07:13 -0700, "JB"
<highlinex@yahoo.com> wrote:

>It seems like both industries operate in a vacuum.
>
>I looked at a video card whose requirement was "a 350 W PS with 22 A
>available on the +12V rail".

.... but you didn't mention the card nor the rest of the
system. Since a video card manufacturer can't know other
system details, they just have to generalize because some
psu aren't honestly rated and they can't know the 12V
current the rest of the system uses.

>
>Most PS ratings that I checked are rated for a maximum of around 18A on the
>+12V rail regardless of the advertised power rating.

You haven't checked many online then? Newer generations of
higher wattage PSU typically have more than 18A rating,
though sometimes it is for current on multiple 12V rails
which is ok, since one rail on modern systems powers the CPU
which is part of the consumption the video card manufacturer
factors in when spec'ing 22A.



>
>Some have dual or triple +12V rails whose combined ratings may be 36A or
>higher.
>
>So how do we make a determination as to what the video card really needs and
>how to provide it?

Start with details. Video card, CPU, # of hard drives, PSU
make/model/rating.


>
>Some would, I suppose, put the rails in parallel and hope for the best. But
>if the PS is not designed for that mode, the result could be instability
>with unpredictable effects.

Not necessarily, depends on the specfics. Generally if the
rails are truely in parallel it is more versatile, not less.


>
>If the actual load requirement for the video card were disclosed, we could
>make some guestimates as to all other loads and choose a power supply that
>would probably work. IMO the video card requirement is ridiculous and shows
>a lack of concern for the customer.


I agree the video card manufacturers should plainly state
current:rail consumption, it would make things a lot easier
for those who know the other variables in their system. It
is a shame the engineers and marketing departments can't
(dont') provide these specs. You can have some idea of
certain models of psu that are shown acceptible with your
card by seeking fellow users of same card with equivalent
systems, but cold hard numbers are always the more accurate
way to go.


On the other hand, it is seldom a good idea to buy a PSU
with barely enough capacity rather than one with quite a bit
of margin, not just for stability reasons but to achieve
longer life. Granted, sometimes even that is not enough,
certain PSU cost corners like poor fans or capacitors might
reduce life regardless of having a capacity margin, but
generally if one has the budget for a power hungry video
card it just makes sense to put a bit more of the budget
into a higher quality and capacity PSU as well.

There are some online comparisons of video card power
consumption, if you're lucky a Google search might find one
that has tested your card or at least same GPU, memory,
frequencies... even if a different brand of card it should
be very close to same power consumption as they're generally
built on the same reference design or at least close enough
to that in consumption even when certain manufacturers like
MSI or Asus decide to change the design a bit.