On Sun, 28 Oct 2007 23:21:50 -0700, "JB"
<highlinex@yahoo.com> wrote:


>Thank you for your input Kony. I am sure you can do a power budget and
>allocate it per rail but that is not what I need.
>
>I am savvy on the engineering considerations but my gripe is with the
>ribbon- clerk type disclosure of power requirements.
>
>For example, the particular card whose requirement I mentioned as being 22A
>on the 12 V rail is an EVGA 8600GT. My gut feel is that the card does not
>need anywhere near that much current. In fact, IFhe link below gives the
>power dissipation for that card as being 30W, not the 264W implied by the
>requirement on the box. So, IF the numbers at the site are correct, the
>card itself needs 2.5 A.
>
>http://www.atomicmpc.com.au/forums.asp?s=2&c=7&t=9354

I agree a 22A rating is arbitrarily high for that card, it
is much closer to 30W than even 60W.

>
>Obviously EVGVA's intent is to tell the customer his total system needs that
>much capability on the 12V rail...a prognistication only a true seer should
>make. But that is not what they say so it's worse than no information at
>all AFAIC.
>
>BTW, that card is just one example, it's not one I intend to buy. I know
>other companies put similar requirements on the customer and think it's just
>sloppy work across the board. I don't think a user should have to buy a
>clamp-on ammeter and verify the ratings.
>
>When I get my components selected I will select a PS and then see what you
>folks think.


The problem seems to be that once PSU start rating current
from (supposed) split rails, it makes the current rating
seem higher than it really is. Suppose a PSU really had
split rails, suppose also it had 22A split 11A/11A. I doubt
any are labeled like this as they would tend to imply one
rail has more capability at such a low current and indeed
one might support more but getting down around this level
the question arises as to how they split the rail and how
much current remains per rail including powering other
devices.

It means there is a secondary reason why the current might
need be overspec'd to cover all scenarios when they use the
ill-conceived total 12V capacity rating as a guide, but
unfortunately some customers' eyes are bound to glaze over
if we attempted to tell them the real specifics they needed
to know instead of a simple number, and this doesn't even
consider yet that many PSU are overrated, not true ~ 22A (or
whatever the reating) capacity regardless of the shared rail
factor, are instead rated for peak instead of sustained (for
MTBF life rating) current and often at an unrealistically
low temperature like 25C instead of the real operating
state.