What does that ATX_12V plug do on a Pentium IV?

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What does that ATX_12V plug do on a Pentium IV?

I was going to swap a power supply from a P III to this P IV until I
saw this extra plug did not exist on the old P.S.  I was able to
google some photos of it and a description, but it seems there is some
controversy on the web as to the purpose.  One said it supplies 12
volts to the CPU (why dont that just come from the regular power
supply 20pin plug)?  Another said it's only needed for extra powerful
CPU's, and yet another said it's to assist high end video cards.
Worse yet, one site said it's not needed, while another said the
computer will not boot without it.  So much for useless

What does it do?



Re: What does that ATX_12V plug do on a Pentium IV?

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    I would definately use it. I just went to one of my older Intel 915
chipset manuals and it said "Make sure to connect the 4-pin ATX12v connector
or the system will not boot". As to what it does, I would expect it supplies
the additional voltage needed by the P4 processors. On boards for the Core 2
Duos there is usually a 4 or 8 pin aux. connector so it is really nothing


Re: What does that ATX_12V plug do on a Pentium IV?

On Mon, 17 Sep 2007 03:56:37 -0500, george41407@noemail.com

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It supplies 12V power to the processor through the
motherboard's step down VRM supply subcircuit.

On "most" boards, without this plug the system won't run at
all.  On some older boards or something poorly designed,
this VRM subcircuit might have a common rail to the 12V lead
on the main ATX connector and the system might work, but
incurring a sometimes-significant voltage drop across the
board from the ATX connector to the VRM input.  All boards
with the 12V socket should be used with a PSU having that
mating 12V plug.

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There is a secondary problem in that older systems which
didn't use 12V for CPU VRM circuit, used 5V instead...
without a separate connector.  That means older PSU were
optimized for more 5V current and less 12V current (in most
cases), such that the old PSU may not have enough 12V
capacity or even if it could run the system with an adapter
to get the 12V power to the ATX_12V plug, it might result in
the 12V rail drooping too low because these older PSU
predominately regulated based on the load on the 5V rail
which is significantly lower in a system using 12V for CPU
power opposed to 5V.

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Because more modern processors use enough current that the
single lead (wire) and (metal) contact on the main ATX
connector was becoming insufficient.  Further it would
require putting the main ATX connector much closer to the
CPU VRM subcircuit to shorten trace length as the trace
resistance has a proportional voltage drop to the input

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Depends on the era and specific board.  Generally, you need
a PSU with the connector and ample 12V current rating if the
board has the ATX_12V socket.

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A system might run but be closer to a problem state, if not
using the 12V connector, if/when it could run at all.
Generally most systems won't run without it connected so
it's a non-issue wondering about consequence.

Also consider that the PIII system's PSU is now several
years old, a significant portion of it's viable lifespan is
probably gone already and it  wouldn't be so suitable for
redeployment, for another tour of duty in a newer system
instead of buying an optimal PSU for the newer system, one
having the ATX_12V connector and an optimzed higher current
rating for the 12V rail.  

While some basic, minimally endowed ATX_12V systems will run
from roughly 8A or more 12V current, that is when the PSU
has a higher 12V current rating so that the rail is not
loaded to max capacity.  Generally a better ballpark for a
PSU powering an ATX_12V system is for the PSU too have at
least 18A @ 12V rating, or if a split rail design, at least
15A per rail.  These numbers are a bit overgeneralized, when
cutting it close the details of a specific PSU and the whole
system it's powering would have to be considered...
certainly some P4 based systems won't run stabily if at all
from 18A of 12V, particularly when a power hungry gaming
video card is installed.

Re: What does that ATX_12V plug do on a Pentium IV?

george41407@noemail.com wrote:
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You can find out what the 12V2 connector is for, and a whole host of other
info in :

Cheerz - Brownz
Beta TR34 (Ring ding ding ding pop pop)

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