PSU Requirements

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Does anybody know of a website where I can estimate
PSU requirements (currents on the different ATX voltages)
for a particular motherboard/CPU?
Most MB manufacturers strangely don't supply data.
Many thanks, David

Re: PSU Requirements

David Johnstone wrote:
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The only manufacturer that offers figures, is Intel. And
the data they offer in their technical manual, is of little
use. It almost makes you wonder why they bother.

There are some web sites with power calculators, but they
have two problems:

1) Don't break their results down into spreadsheet form.
    If their results are to be trusted, the results should
    be peer reviewed. Takaman was the only site which gave
    a breakdown, but the Takaman site is gone now.

2) Sometimes have ridiculous numbers for things. For example,
    on RAM, you can get real data from a Micron datasheet.
    There is no need to make up numbers.

The method I've been using, is to:

1) Compute the 12V requirement. That is because much of the
    significant loading, is now on one or more 12V rails.

2) Estimate the remaining power on the 3.3V and 5V rails at
    about 50W. That is because, it isn't practical to try and
    calculate it. DIMMs can be at the 5W or so level, Northbridge
    might be up to 20W in some cases. Many other chips are only
    1W a piece. So 50W is pretty generous. Hard drives can use
    5V @ 1A each, so if you had enough of those, you'd have a
    measurable drain on the 5V rail.

On the 12V rail, you can get video card measurements from
Xbitlabs. They've measured a good number of cards.

For processors, there is and . I assume the Vcore conversion circuit
is 90% efficient, then take (P/12V) * (1/0.90) to get the amps
from the 12V rail, as the ATX12V 2x2 connector feeds the

That should give you enough info to start with. Power numbers for
hard drives are available. And numbers of a sort are available
for optical drives. 12V @ 0.6A is enough for a hard drive.
12V @ 1.5A is enough for an optical drive (I've measured the
optical drive on this computer, at 1.0A, spinning at max speed.)

In terms of a max config, right now that would be something
like a 130W processor, and two 8800GTX cards at 145W each. That
would be (130/12)*(1/0.90)=12A for the processor. And about
12A for each video card. With a bit thrown in for HDD, ODD, and
fans, that would be 12V @ 40A. At the other extreme, you can
probably build a (weak) computer using less than 100W total.


Re: PSU Requirements

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Hi Paul,
thanks, that is all very helpful.
I had an old bookmark to Takaman and found him very useful
in 2005. I'll try doing all the adding up you suggest when I
have a bit more time. Basically I have some Arctic T1 cases
with Seasonic 350W PSU's, and I am hoping to put some
Intel Core 2 Duo E6330 systems together in them, probably
with cheap Asrock MB's, maybe 1GB RAM, 250GB HDD
and an optical drive. Not sure about graphics yet, but nothing
really fancy planned there. I guess as you say the critical 12V
line is the issue, though some of the Asrock boards don't
seem to have an ATX 2.0 connector, which maybe means
that the CPU power comes from +3.3V.
Many thanks,

Re: PSU Requirements

On 19 Feb 2007 13:41:54 -0800, "David Johnstone"

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All Asrock boards running Core2Duo have the 4 pin 12V
connector, that connector is the only determinant of whether
using 12V for CPU or 5V, never 3.3V (at least, not since
Pentium 1 days in a PC).  You won't easily find any boards
using 5V rail either as the current requirement for a modern
CPU is just too high.  The last era of system practical to
run from 5V rail (derived vCore from VRM subcircuit of
course) might've been the early Athlon XP Palominos, with
anything thereafter doing better with the 12V supply design.

Your described system isn't very demanding but you don't
mention the video card which is typically the 2nd or 3rd
largest power consumer.  Assuming low end or integrated
video you should find a current generation Seasonic 350W can
power it.

Re: PSU Requirements

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Thanks - I know the Core 2 Duo is a relatively low power CPU,
one of the reasons for choosing it. I had been looking at the
ASRock 4CoreDual-VSTA, PT880, from this site:
the claim is made there that it has no ATX 2.0 connector, but
looking at the Asrock website it seems this is wrong.
The specs for the Seasonic PSU in the Arctic T1 say:-
active PFC 3.3V: 28A, 5V: 30A, 12V:17A
-5V: 0.5A, -12V: 0.8A, 5Vs: 2A
I'll try and do some adding up but I think you're right, should
be ok. Allowing for losses the CPU itself couldn't account
for more than about 7A at 12V.

Re: PSU Requirements

On 19 Feb 2007 05:38:16 -0800, "David Johnstone"

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In addition to agreeing with most of what Paul wrote (though
I think it might be easier than he does, to have 50W of
3.3V+5V power usage on a reasonably endowed system), as
important might be an estimation of which PSU can actually
deliver their rated current per rail.

In other words, if you had a list of parts in mind, odds are
we could come closer to an actual current consumption than a
random PSU would come to being able to continuously supply
what's stated on it's label.

Another way to look at it is that you don't really want to
buy a PSU with too little reserve power so you can fudge a
little, aiming higher than the actual requirement.  If your
high aim leaves only 25% margin versus 35% margin, you're
still in the safe zone (assuming as mentioned above your
PSU's actual sustainable current is what is considered, not
a momentary peak output at an atypical 25C environment).

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