Hi,
Stupid question (that I probably know the answer to already), but does
anyone think it's possible that an EVGA GeForce 7950 GT KO graphics
card that calls for a minimum of a 450w. power supply in the computer
you install it in would work in a Shuttle SN27P2 computer chassis that
only has a 400w. power supply unit built into it? Since the Shuttle
computer chassis is one of those "mini" systems (can only cram a
limited amount of drives into it, plus only having two expansion ports
available for add-on cards), would having 50w. less power hamper it
from working well with the EVGA GeForce 7950 GT KO? Thanks for any
insights!
It is simple arithmetic. You total up the power consumption of the
hardware you stuff in the computer, then compare the results of
the arithmetic, against the rating printed on the side of the
power supply. And in cases where the system is really tight,
you measure the actual consumption, to be sure the computer
will operate trouble free over its life.
The critical supply rail is probably the +12V rail, since it powers
many motherboard Vcore processor power supplies, and it also can
power significant portions of the video card.
Small computers are generally very tight for power. I'm really
surprised, in fact, that you've got 400W to work with. According
to this, there is a 350W inside there.
On this page:
http://www.pcstats.com/articleview.cfm?articleID=1968
"Inside the system is a 350W A-PFC power supply, which might
sound a little restricting from the get-go, but it has plenty
of power to go around. PCSTATS tested the Shuttle XPC SN27P2
with an Athlon64 X2 5000+ processor, a 150GB WD Raptor and an
nVidia GeForce 7800GTX videocard and did not encounter any power
related issues."
They don't say whether they tested with 3DMark for an extended period
of time, but I hope they did more than just left it sitting idle in
Windows, as their test.
In the chart at the bottom of this page, the 7950GT draws 61.1 watts.
That is roughly 12V @ 5A. This number may vary a tiny bit, because
of all the different clock rates that the different manufacturers
use. (They show the split between power coming from the PCI Express
slot connector, and the Aux power connector, but since it all
ends up coming from 12V1 or from the single 12V rail in the
system, only the total is important at this point. If the power
supply was triple or quad rail, or this was SLI, then the
distinction could be important. But this is a single card system.)
http://www.xbitlabs.com/articles/video/display/geforce7950gt_3.html
A 5000+ AM2 processor would draw 89 watts (depends on version used).
Vcore conversion could be 90% efficient, so the 89W is divided by 0.90,
to get the level of power needed at the 2x2 power connector. That is
98.9 watts, divided by 12V gives 8.24 amps at the 2x2 connector.
(You can substitute the number for the processor you've got, instead.)
http://www.amdcompare.com/us-en/desktop/details.aspx?opn=ADA5000IAA5CS
The total so far is 5 + 8.24 = 13.24 amps from +12V. Add another
2.5A from the 12V rail, for a CDROM drive, a hard drive, and
the system fans. Result is 15.75A from +12V if the supply has a
single rail +12V output. Now, modern systems tend to use dual rail
supplies, and the CPU is powered by 12V2 on those, leaving 12V1 to
run the rest. If that was the case, then 12V2 @ 8.24A and 12V1 @ 7.5A.
You can check the label on the supply yourself, or post what is
written on the label here, and I'll compare it to the estimates
above. The 3.3V and 5V consumption might be in the vicinity of
50W, but there is no easy way to actually calculate a number.
Things like the power used by the chipset chips, are closely
guarded secrets it seems. Especially when the chipset chips
from some makers, are power pigs. The 50W number I use, is
based on a couple systems here I've measured - which means
the number is not based on a lot of fact.
It doesn't appear, on the face of it, that there is a problem.
Total system power is ~230W, of the 400W total you've got to
work with. Either total power, or rail current could be a limit.
I won't know if rail current is a limit, until you post the
numbers printed on the label of the supply.
Maybe if you overclocked the hell out of it, you can make it
fall over.
Paul
.