Archived from groups: alt.comp.periphs.mainboard.asus (
More info?)
In article <N673d.6533$mb6.4923@newsread3.news.atl.earthlink.net>, "X
Offender" <nospamplease@earthlink.net> wrote:
> The Asus P5AD2 Deluxe motherboard uses a 24-pin ATX power connector. There
> are very few power supplies available that support this new ATX 12V Version
> 2.0 format. Is it safe to use a standard 20-pin power supply with this
> motherboard? The manual is somewhat vague in regard to this issue, and I'm
> looking for a definitive answer.
>
> Thanks.
The difference between connectors is in the ability to deliver current
to the load, the ampere capacity. The Minifit Jr connector is rated
for 6 amps per pin, and that figure is used to derive the following.
If you compare the 20 pin connector to the 24 they show in the
manual, the 24 has an extra +3.3, +5.0, +12, and GND pin. Pin 1
of the 24 pin connector, I think, is in the lower left hand corner.
Pin 1 of the 20 pin connector should plug in there. The top four
pins are the "extra" ones.
The extra pins would be worth 6 amps a piece. For +3.3V, it boosts
the limit from 18amps to 24amps. For +5.0V, it boosts the limit from
24amps to 30amps. For +12V, it boosts the limit from 6amps to
12amps (from the ATX connector, plus it can draw another 2x8=16amps
from the 2x2 connector). Those are the amounts of current that
can flow from the PSU to the motherboard, without damaging the ATX
power connector. Generally, the 2x2 connector is used only for the
processor, whereas the single +12V pin on the ATX 20 pin is used
to handle fan headers etc.
On the load side, the new motherboards aren't much different from
older motherboards. PCI Express is a bit different, and especially
the video card slot could place new demands on things.
I found an Intel slide set, and it lists the PCI-E video card slot
power limits. These limits are based on the PCI-E connector and
how much current can flow through the pins of the video card.
+12V 5.5A (max)
+3.3V 3.0A (max)
3.3Vaux 0.38A (max) [ likely regulated down from +5VSB ]
(Card may draw a total of 75W. Ordinary 1x PCI-E cards are allowed
up to 25W.)
If you had a video card that draws as much power as it can from
+12V via the edge connector of the video card, and you used the ATX
20 pin connector, the card can draw 5.5A and the single +12V pin
on the ATX 20 pin is rated for 6A. Asus usually powers the fan
headers from this +12V pin, so if three fans drew an amp or so,
the total draw would be 6.5A.
PSU --+---- single +12V pin ------+-----+-----+-----+
| MiniFit Jr 6A per | | | |
| pin limit Fan Fan Fan PCI-E Video
| 0.33A 0.33A 0.33A 5.5A
|
2x2 +---> Up to 16A for Vcore.
12V | 115W Prescott draws
conn. +---> (115W/12V)*(1/0.9)=10.6A
If you buy a video card, and it uses an aux connector on one corner
of the video card, that connector will provide the extra current the
card requires. But, the trouble is, the PCI Express video cards I've
looked at so far, don't have an auxiliary connector for power,
which means it is all coming through the edge card.
This article measures power consumption of current generation cards.
This is for an AGP card, but the PCI-e will just use a hot bridge
chip to get to PCI-e country. The 6800 is measured at 72W at full
load.
http://www.xbitlabs.com/articles/video/display/ati-vs-nv-power_10.html
The current measurement is here. 12V@4.62A. With the three fans on
the same +12V pin, the total current of 5.62A is just under the 6A
limit. You could help things a bit by running the fans off Molex
disk drive connectors, and just route the tacho signal to the mobo
fan headers.
http://www.xbitlabs.com/articles/video/display/ati-vs-nv-power_2.html
http://www.xbitlabs.com/misc/picture/?src=/images/video/ati-vs-nv-power/6800u_table-b.gif&1=1
Any 1x PCI-E cards that you plug into the other slots, will
be drawing next to no power (like 1W for a typical function), so
they don't even enter into the equation. It would take a complex
function, like say an embedded computer on a board, to draw
enough power to be a concern. Any PCI-e card consisting of a
single chip on board (Ethernet, cheap sound card, Firewire card,
USB card) will draw around 1W or less.
Here is an example of a PCI Express video card that has two
Molex drive connectors for power. A card like this means you
can use the ATX 20 pin without worrying. It means the designer
is running the switchers from the Molex connectors.
http://www.tweakers.net/nieuws/29357
But the other PCI Express video cards I've looked at, don't use
an Aux connector, so the 5.62A of 6A test case above remains as
the closest to heating up the single +12V pin on an ATX 20 pin
connector.
In the description above, notice how easy it is to draw more
than the 12V@15A recommendation for a basic system. At 10.6A
for high end processor, 4.62A video, 1A fans, 0.5A per drive,
18A is the starting number, and a couple more amps for margin
would help. Since the processor and disk drive currents are
flowing through other pins and cables from the PSU, there is
no danger from those loads.
It is difficult to do a similar analysis for the other supplies,
except to use an Intel motherboard power spec. This spec is
for the Intel D925XCV, and table 37 is on page 85.
ftp://download.intel.com/design/motherbd/cv/C6859701.pdf
Table 37. DC Loading Characteristics
DC Current at:
Mode DC Power +3.3V +5V +12V -12V +5VSB
Minimum loading 200.00W 3.30A 10.00A 9.00A 0.03A 0.80A
Maximum loading 300.00W 6.00A 14.00A 16.00A 0.10A 1.40A
The +3.3V and +5V currents are low, compared to the 18A
and 24A limits of the ATX 20 pin connector, so those
shouldn't be a problem. The Intel manual says part of that
+5V allocation, is to power imaginary PCI-e cards drawing
an average of 2A per slot.
So, under some circumstances, the +12V pin on the ATX 20 pin
connector is running close to its limit, and that is
where the "you can do it, but not recommended" comes from.
HTH,
Paul
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