In article , "Đavīd"
wrote:
Paul wrote...
I haven't a clue why they did that. It doesn't make much sense,
because as a rule, the only load on the main ATX power connector
is the fan headers, for the single +12V signal. The 2x2 connector
supplies the heavy processor Vcore current via two of the four
pins (the other two pins are ground returns).
Adding a second +12V pin via the 24 pin version of the connector
just doesn't add up. Neither do the other extra pins (3,5,gnd
listed
in the manual) appear to be necessary, as the existing +3.3V
and +5V pins were already enough for the job. This is, after all,
a P4P800 board, with a LGA775 processor socket, so power
requirements
should be very similar to the P4P800. Hope I'm not missing
something
obvious there...
Maybe the marketing department wanted all the P5xxxx boards to
be the same :-)
Thanks Paul for your wealth of information. I have a brand new Antec
480 watt True Power I was hoping to use with my new build. It has the
ATX12V connector. Are you saying the 24 pin connector used for PCI
Express is different from the ATX12V connector? If it is different,
what should I do? Get an adapter or another power supply with the
correct PCI Express connector?
Davīd
Greenville, NC
The previous generation of power supplies (which is all I've got for
my computers) has 20 pin ATX power for the main power connector, and
a 2x2 power connector that feeds the processor exclusively. The
previous generation of motherboards used PCI bus for add-in cards,
and AGP slot for video card. If the AGP card needed extra power, that
power came through a cable plugged directly into the video card.
On the PCI Express motherboards, the motherboards use a mix of
normal PCI slots (for backward compatibility, so you can plug in
a SoundBlaster or an old modem card) and they have PCI Express
slots. For PCI Express, the slot has some extra power pins,
intended for handling larger current requirements. Currently, the
only card that really needs that kind of power, is a PCI Express
video card. I think the power coming through the slot can be
up to ~75 watts or so.
So, the intention was, to make PCI Express video cards draw their
power through the motherboard slot, so no more auxiliary cables
would need to be plugged into the cards. But, depending on the
mood of the video card designers, you might still see a place
for aux power connector.
The situation with +12V is this. If you examine the ATX 20 pin
power connector, it has one +12V pin on it. Each pin in that
connector can handle a current flow of 6 amps. If you look at
the new 24 pin power connector, it has two +12V pins on it.
That means a total of a little less than 12 amps could flow if
needed.
Some of the high end PCI Express cards are drawing the majority
of their power from +12V. If a card like that has no aux power
cable, all the power comes through the PCI Express slot.
This article has some power measurements for video cards.
http://www.xbitlabs.com/articles/vid...powercons.html
http://www.xbitlabs.com/articles/vid...-nv-power.html
http://www.xbitlabs.com/articles/vid...00gt-oc_3.html
Some of the hottest video cards are drawing 4.5 amps from +12V.
The fans powered through the motherboard header draw 1 amp or so
from +12V. The total could be 5.5 amps. If you only use the
ATX 20 pin power connector, to power a 24 pin motherboard, then
only one pin carries that 5.5 amp current. The pin itself is
rated for 6 amps. All is still within spec, but the practice
of drawing close to the rated current is not exactly a good
one. In a situation where you are using an expensive video card
like that, it may be a wise idea to either find an adapter
that converts from 20 pin to 24 pin (which will take one +12V
wire and connect it to two connector pins), or get a proper
power supply with the 24 pin power connector on it (which will
have two wires and two pins for +12V). It all depends on how you
feel about drawing 5.5 amps through a 6 amp pin.
So it is not a big deal. If the pin carrying the current starts
to fail, it will get hot, and the plastic will start to
melt. Probably by the time it gets that bad, the video card
will be crashing out in 3D games. I haven't read or heard of
anyone having a problem with using a 20 pin connector on a
24 pin motherboard, and if anything, it is more likely that
an old power supply simply wouldn't be up to the task of driving
the heavier load of a P5xxx board.
On the P5P800 motherboard, which is a P4P800 AGP based board,
only with a LGA775 socket for the new processors, there is little
reason to be using the 24 pin power connector. That is because
the board is using an AGP video slot, and any heavy power
requirement will be met by plugging a disk drive connector into
the AGP card.
Summary: Low risk using an existing 20 pin ATX power in a
P5xxx 24 pin power connector. Depending on the way it
is designed, an adapter could be used, and reduces the
risk further. (I wouldn't bother buying an adapter
myself.) If the current ATX power supply seems
unstable (due to not meeting the motherboard total
requirements), then upgrade. If you do a lot of
overclocking, consider a better supply.
HTH,
Paul