John Doe said:
My 380 W Antec power supply is 2 x 16A at 12 V.
Apparently wattage ratings are almost meaningless.
You know, I was wondering about this and asked a tech at OCZ but haven't
heard back yet... I'm sure this is a really stupid question, but when a
PSU has double or even triple 12v rails (as some now do), can you add up
all the amps from those [12v] rails to see if it would meet the
requirements of a particular video card? Because I was under the
impression you have to have at least one 12v rail running the required
amps, all by itself.
IOW, let's say a specific card requires a minimum:
450w PSU running 12v rail @ 22A:
PSU #1 hypothetical: 600w running (3) 12v rails @ 15A each
no good -- not enough juice on any single 12v rail
PSU #2 420w running (1) 12v rail @ 22A
no good -- wattage too low
PSU #3 500w running (1) 12v rail @ 32A
bingo -- this'll work
Now that's my understanding. (Obviously the second example is a
no-brainer... if the wattage is too low, it's too low).
But I spent HOURS looking at specs at Newegg trying to find PSUs that
ran enough juice on the 12v rail for these high-end cards! I guess if I
wasn't trying to spend the least amount necessary to get a good
candidate, it mighta been easier. Just go directly to the PSUs well over
$100, but I was trying to stick close to $60-$100.
I also heard mention of *adjustable* rails, where you can actually dial
in the amps you want (or allocate the available amps) for each 12v rail.
I didn't see those when I was shopping though, so they were out of my
price range.
Kate
I was reading an article the other day, where it was suggested that
some supplies with multiple 12V rails, use a single circuit to supply
all the current, and use a current limiter for each individual output
rail. I don't know if that approach meets the spirit of IEC60950, to
limit an individual 12V rail to 20A or not. (IEC60950 says that any
output rail should not be able to supply more than 240W, which is
12V @ 20A. Some supplies have huge single outputs, like a PCP&C
unit with 12V @ 34A, and I don't see them complying with that
safety requirement. I presume the limitation has something to
do with fire safety, but haven't seen an explanation of the
requirement in any detail.)
In terms of picking power supplies, you realize that a
"requires the +12v rail running min 22A (amps)" is pure
baloney. This kind of advice from the manufacturer, is almost
as bad as saying "only works with a 600W supply". You do a
calculation of the loads in the system, to come up with a
minimum requirement for a power supply. The manufacturer may
be assuming a different processor is being used, than the one
you bought.
Box:
MSI K8N Neo4 Platiunum Ultra nForce4 [non-SLI] (PCB 1.0a)
AMD Athlon 64 +3000
1 GB Mushkin DDR400 PC 3200 Dual Ch. RAM (2 x 512)
2 SATA II Samsung 160GB drives (not in RAID)
1 SATA Plextor DVD
1 XFX 7950GT
XION 600w PSU (+12v running 28A)
Sound Blaster Live! Drive Platinum (CT4760, 1999)
1 floppy
The important bits, for calculating the required 12V, are
processor, video card, hard drives, optical drives, and fans.
The rest uses +3.3V and +5V, and usually once the +12V rating
has been satisfied, the 3.3V and 5V are big enough to do the
job. It is not possible to do a meaningful calculation of
+3.3V and +5V in any case, and there are differences in
motherboard design, in terms of where current is drawn
from. Add-in cards, or even peripheral chips on the
motherboard, only draw a watt or two, and can be ignored
for the most part.
On a dual rail supply, 12V2 powers the processor. For a
3000+, there are 89W ones (CG stepping), and 67W ones (like
revision E). I can assume the worst here, and budget for an
89W processor. Your actual processor might be one of the
67W ones.
http://www.amdcompare.com/us-en/desktop/details.aspx?opn=ADA3000DEP4AW
12V2_amps = (89W/12V) * (1/0.90) = 8.24A [assumed 90% Vcore efficiency]
The 12V1 rail powers the rest. The major consumer is the video card.
The 7950GT is somewhere between the 7900GT and 7900GTX. We can again
be pessimistic and select 84W, or guesstimate your card at about 75W
based on clock rate. 84W/12V = 7A.
http://www.xbitlabs.com/articles/video/display/geforce7900gtx_6.html
http://www.techpowerup.com/gpudb/
Allocate 0.5A for system cooling fans. Allocate 2 * 0.5A for the
two hard drives. CDROM/DVD boilerplate value max current is 12V @ 1.5A,
and that current level might only be present during the acceleration
phase of spinning up the media. So, 12V1 total current:
12V1_amps = 7A + 0.5A + 2*0.5A + 1.5A = 10A
Our total current, with some pessimistic assumptions, is 12V @ 18.24A
for the whole supply, with 12V @ 8.24A for 12V2 and 12V @ 10A
for 12V1. If we wanted some head room, so that the supply wasn't
right at the edge, then maybe add 3A on top of the calculated figures
or so.
A Newegg advert quotes:
Minimum of a 400 Watt power supply.
(Minimum recommended power supply with +12V current rating of 18 Amps.)
Minimum 500 Watt for SLI mode system.
(Minimum recommended power supply with +12V current rating of 28 Amps.)
and that implies that adding a second video card adds 10A to the load.
Or 120W for the second card, which is kinda silly. So the stated SLI
value is likely on the high side, while the single card value is not
too far off. Finding a measured value for the card, under some kind of
representative condition, allows a more refined calculation of the
requirements.
The trick then, is to question the power requirement statement of
the manufacturer, when it pushes your purchase into some kind of
"blue sky" country. If the stated number cannot be met for a
reasonable price, then it helps to do your own estimating, using
video card measurements from Xbitlabs.
Every spec printed on the label of the supply, means something.
Some supplies have several "combined power" numbers. For example,
some supplies will list 12V1@20A and 12V2@20A, but may say
"total 12V power 360W", which in fact means if 12V1 is drawing
its full 20A (240W), then there would only be 10A left on 12V2.
Some supplies have limitations imposed in the form of
several of those combined power limits, and you have to
think through the scenarios (what if my xx rail goes to max,
what does that leave for yy rail...).
If you plan on overclocking, either video card or processor,
that can push the requirements up. A "gentle" overclock
doesn't represent that much of a difference, but taking
something like a Pentium D 805 dual core to 4GHz, can mean
purchasing a high end supply, instead of a mid-range one.
For the video card, assume linear scaling if the core and
memory are lifted by the same degree of overclock. For
a processor, power is linear with frequency, and if
Vcore is bumped, the power estimate increases by
(V2*V2) / (V1*V1). V1 is the original Vcore value, and
V2 is the Vcore voltage needed for the overclock.
Excessive Vcore bumping can make a big difference,
and only as much Vcore as necessary to keep Prime95 stable,
should be used.
Paul
