Power supply connections - Noob question

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Well, everyone was a beginner once.  I am connecting a Seasonic 500w PS
to my Gigabyte DS3 board.

This mobo has a 4 pin ATX_12V connector, and a 24 pin ATX connector.

The Seasonic manual says to connect the 12V 4 pin AND the 24 pin
connectors to the motherboard.

The Gigabyte manual says 'If you use a 24 pin ATX power supply, please
remove the small cover on the power connector; otherwise, please do not
remove it'.

This leads me to believe that the 24 pin connection is an optional
alternative to the 4 pin connection.  Is the Seasonic manual misleading
here?  I'd like to minimize cable clutter.  Must I use the 24 pin
connection, or not?

Thank you, homebuild vets!

Re: Power supply connections - Noob question

On 8 Dec 2006 08:25:31 -0800  'Jonathan Currie'
posted this onto alt.comp.hardware.homebuilt:

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They may exist but.....
I've not seen a mobo with a 24pin *and* 4pin, only 20pin or 24pin.
Some with 20pin have also got a 4pin connector adjacent or nearby
to beef it up to a 24pin.

If your PSU has a 4pin plug are you sure the other one is not a 20pin?

Re: Power supply connections - Noob question

You got me curious, so I double checked.  The mobo is a Gigabyte
GA-965P-DS3.   There is definitely both a 4 pin connector, labeled
'ATX_12V' and a 24 pin connector labeled 'ATX'.

Here is the exact manual quote:

ATX_12V/ATX (Power Connector)
With the use of the power connector, the power supply can supply enough
stable power to all
the components on the motherboard. Before connecting the power
connector, please make sure
that all components and devices are properly installed. Align the power
connector with its
proper location on the motherboard and connect tightly.
The ATX_12V power connector mainly supplies power to the CPU. If the
ATX_12V power
connector is not connected, the system will not start.
Please use a power supply that is able to handle the system voltage
requirements. It is
recommended that a power supply that can withstand high power
consumption be used (300W
or greater). If a power supply is used that does not provide the
required power, the result can
lead to an unstable system or a system that is unable to start.
If you use a 24-pin ATX power supply, please remove the small cover on
the power connector
on the motherboard before plugging in the power cord ; Otherwise,
please do not remove it.

hummingbird wrote:
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Re: Power supply connections - Noob question

On 8 Dec 2006 11:43:13 -0800  'Jonathan Currie'
posted this onto alt.comp.hardware.homebuilt:

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This looks like your MoBo:
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OK, you need to attach the 24 pin PSU to the 24pin MoBo connector
and carefully make sure it's pushed home into the clip.

Don't connect the 4pin PSU to the MoBo 4pin connector at this stage.
It's likely you will not need to attach it at all, but when you get
the system running, check the voltages are stable etc via the BIOS.

It may be a translation error in your seasonic manual.
By connecting only the 24pin to start with will do no harm at all.

It looks to me that your devices have the 4pin plugs/connectors also,
to accomodate older versions of the other, if you see what I mean. So
if your mobo had a 20pin and seperate 4pin connectors, your PSU would
still be usable and vice-versa. This is reasonably common during the
upgrade to 24pin standards.

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Re: Power supply connections - Noob question

On 8 Dec 2006 11:43:13 -0800, "Jonathan Currie"

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The reason for this caution is to prevent you from installing a 20-pin
ATX power supply connector in a misaligned manner into the 24-pin
motherboard ATX power connector.

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If you don't use the 24-pin power connector, you won't be able to turn
on the power supply, since the 5-volt standby power comes through that
connector, and the motherboard turns on the power supply through that

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Re: Power supply connections - Noob question

On 8 Dec 2006 08:25:31 -0800, "Jonathan Currie"

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I think the 2X2 connector and extra 4 on the 20 pin jobber ARE NOT
I think the extra 4 pins on the 20 pin connector is for supporting
PCI+ cards. The 2X2 ( 4 pin) is for CPU voltage.

Re: Power supply connections - Noob question

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Indeed. And it makes a lot more sense to ask than
to just try it and hope for the best if you arent sure.

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That is correct.

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Nope, Gigabyte is talking about supplys which have
a 20 pin ATX connector as well as the 4 pin connection.

They mean that if you power supply has a 20 pin connector,
leave the cover on the end of the 24 pin connector in place
and plug the 20 pin power connector into the uncovered
20 pins of the 24 pin connector on the motherboard.

If you have a 24 pin connector on the power supply,
as you do, remove the cover on the end of the 24 pin
connector on the motherboard and plug in all 24 pins.

You need to connect the 4 pin connection as well in both cases.

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Yes, the 4 pin connector just has ground and +12V, two of each.

You need the 24pin connector for the other rails, +5V
and 3.3V and the other signals like PWR_Good etc too.

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I'm a homebuild brain surgeon myself.

Re: Power supply connections - Noob question

The 24 pin connector is NOT optional.


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Re: Power supply connections - Noob question

Jonathan Currie wrote:
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This is an ATX12V connector. It powers the processor. The processor
cannot execute any instructions if this is not connected. The
fans may be spinning (because the main power connector is connected),
but the processor cannot work without this one. You will know you are
grabbing the right connector, when you check it and find two yellow wires
and two black wires.


This is the main power connector on the motherboard. The 12V on this
connector, powers the motherboard fan headers, and also delivers
some power to the PCI/AGP/PCI Express card slots. This connector
comes in two versions. The 20 pin version was used on older computers.
The 24 pin version is a superset of the 20 pin version. The 24 pin
main connector has four extra (redundant) pins. These redundant pins
leave more room for power to flow, but if the computer does not
use a lot of power, the 24 pin connector style is not essential.


Some power supplies come with a 24 pin main connector, and the four
(redundant) pins on the end of the connector, can be unsnapped. Thus,
the connector can be used for 20 pin or 24 pin motherboards, without
an ugly mess. The first picture here, shows the "rib" where the 4 pin
detachable section is attached, ready to slide off. The second picture
shows the wire colors associated with the detachable section. You cannot
confuse the detachable section, as the four wires are all
different colors.


Adapter cables, like this 24 pin to 20 pin, are not recommended.
Such a cable solves no electrical problems, and only adds voltage drop
to the cable assembly. It is better to plug the assemblies together
directly, unless there is a capacitor in the way that makes mating
the main connector impossible. In that situation, without a
detachable main connector, only then would an adapter cable
be a last resort.


If you have a 24 pin motherboard, you can plug a 20 pin power supply
or a 24 pin power supply. How do you know when a 24 pin power supply
is absolutely essential ? A good predictor, is if you are using
two video cards in SLI or Crossfire mode. Usually the power required
for such cards, makes the 24 pin connector and its extra power
pins, a useful thing.

If you have only one video card, you can use a 20 pin power supply
safely. The single 12V pin on a 20 pin connector, can safely carry
6 amps, and a power hungry single video card uses no more than
4.4 amps or so from that wire. The motherboard fan headers use
another 0.5 amps or so. The total is less than 6 amps allowed by
a 20 pin connector.

Each power supply normally has a "ratings label" stuck to the side
of the supply. This tells you how many amps each of the DC outputs
of the supply, can supply before the power supply is overloaded.
In addition to using the right cable type for the job, you also
need a power supply with the right maximum output for the job.
Usually I use a hardware inventory, to work out the power needed
for a project. While it is fun to guess at "how much is enough",
I work out the value, based on the 12V loading. The reason
I only calculate the amount of 12V needed, is because 12V provides
most of the computer power now, and it is not possible to work out
the (lower) demand of the 3.3V and 5V rails. The combined 3.3V and
5V loading will be less than 100W total, maybe somewhere between
50W and 100W, which many supplies can meet with ease. The really
big consumers are processor (65W to 130W) and video card
(20W to 145W). Those are just typical desktop system values,
and if someone searches hard enough, they can find exceptions
to those numbers.


Re: Power supply connections - Noob question

Wow, thanks all!  Good, helpful info from supportive and well informed
people.  Are you sure we're really on Usenet?  hehe....on with the

Re: Power supply connections - Noob question

Paul wrote:
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Wow! Paul, I have been searching for such a precise explanation and you
provided an excellent summary. This is a very confusing subject, and
your post clears much of it  up. I have two follow-up questions:

I'm planning to buy the ASUS P5B-E motherboard. The board manual
mentions these two primary power connectors:
1) 24 pin EATXPWR connector
2) 4 pin EATX12V connector
When I searched Newegg, I see that they list far more 20+4 pin power
supplies and only a very few 24 pin supplies. So what you're saying is
that a 20+4 pin supply is simply a way of having either a 24 pin main
power connector or a 20 pin connector, right? In other words, the term
20+4 pin refers to a connector that has that break-away feature where
you can either utilize those extra 4 pins on the main power connection
- or not, depending on whether the board has a 20 pin socket or a 24
pin socket, is that correct?

Second question: I'm faced with the choice of these 4 categories of
power supplies: 1) ATX; 2) ATX/BTX; 3) ATX12V; 4) ATX12V/ EPS12V. I'm
thinking that since I need that 12 volt plug to power the processor,
that I will need to stick with either ATX12V or  ATX12V/ EPS12V. But
what's the difference? In other words, what's the "EPS12V"?

Thanks again for your excellent summary.

Re: Power supply connections - Noob question

bobsmithhome@gmail.com wrote:
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Yes :-)

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Well, the terminology is not consistently applied. Which can make
things harder to verify. I like to examine pictures of stuff, to
look for hints as to what I'm getting. The "ATX" in your list,
could be, say, something without the 2x2 ATX12V connector. The
ATX/BTX doesn't mean too much to me, as BTX is a motherboard form
factor, and doesn't necessarily imply a power difference. Which
leaves ATX12V and EPX12V.


There is a summary of some types here:


An actual EPS12V spec is here, and you may notice that the power
supply you are interested in, doesn't exactly match what is in
this spec. The EPS12V here, has a 2x4 processor connector, with
two 12V rails on it. That means according to this spec, the
EPS12V would power a two socket (server style) motherboard.


That is one problem with power supplies. On the one hand, they
use the names of the standards, as part of their product
description. As if that leaves them off the hook, when it comes
to documenting their product. But on the other hand, they may
have done something in the wiring, which is non-standard, or
at the very least, leaves a user wanting to verify what they've
done. At least I don't find all power supply issues, to be
simple ones.

As for ATX, there has been a progression of connectors on the
ATX form factor. You can see the latest specs here, but there
are also older specifications for the 20 pin supplies.

(Main spec page)

(24 pin main connector, 2x2 ATX12V, latest spec)

An older spec for ATX - you can still buy some of these.
(20 pin main connector, 2x2 ATX12V for the processor.)

Even older, with a 1x6 Aux power connector:
(20 pin main connector, 2x2 ATX12V, 1x6 Aux)

Asus, in their documentation, calls the 2x2 connector here EATX.
I think I'd call it ATX12V myself.


There are some Asus and Gigabyte boards, where they use a 2x4
connector for processor power. Using all the pins on the 2x4
connector, is for processors over 130W. AFAIK, the hottest Intel
processor at stock speeds is 130W. Power can go over the 130W level,
if you overclock. At which time, the 2x4 connector comes in handy.
Fortunately, the older hotter processors are dying out, leaving
fewer opportunities to need the full 2x4 connector. The safest
powering choice, is to buy a 2x2 and plug it into the 2x4
motherboard, because otherwise, you may need documentation for
both the motherboard and the power supply, to determine if they
are compatible.

The 2x4 power connector comes in at least two styles (because
I'll ignore GES for now). The one on the right is for dual
socket server boards. The one on the left is for desktop boards,
such as some Gigabyte and Asus desktops. But verifying what
you've got on the power supply, can be a challenge. In theory,
the one of the right, could be detected by wire color. The
"12V1" and "12V2" should have a yellow wire, with a second color
stripe mixed with it. But some supplies may use solid yellow wire
for all four wires. You cannot rely on wire color in a case like
this. So it is a lot easier to just avoid using stuff like this,
than to find someone to verify it for you.

12V 12V 12V 12V                 12V1 12V1 12V2 12V2
GND GND GND GND                 GND  GND  GND  GND

Some of the supplies with 2x4 connectors, allow the connector
to be split in half. The 2x2 portions are both available,
as a potential cable to plug into a 2x2 ATX12V hole. While
that solves the wiring question, you still need to know which
rail inside the power supply that the current is coming from,
in order to avoid overloading the wrong rail. (You wouldn't
want the entire computer, running off one rail of the supply.)

A situation where more than 130W can be drawn by a processor,
might be a Pentium D 805, overclocked to 4GHz. Such a processor
uses over 200 watts, which is a bit absurd, but there are people
who like to do stuff like that (i.e. Tomshardware). Running a
processor at the 200W level, causes the Vcore regulator to get
hot, so hot in fact that it can melt adjacent plastic if it is
present. Running gear that hot, means it won't last very long.
(And an expensive motherboard with eight phase power, might be
a better candidate, than a cheaper motherboard with only
three phase power, but that spoils the "something for nothing"
philosophy of overclocking.)

When it comes to the simple power supplies with 2x2 connectors,
it is pretty easy for me to offer advice. If you are dealing
with >500W supplies, with two, three, or four output rails,
2x4 connectors, strange standards names, then you really need
to get a wiring diagram from the manufacturer. I do read
occasional accounts where someone buys an expensive supply,
and they report their system isn't stable - and in the back
of my mind, I'm always thinking about all the crazy things
that can happen with the exotic, expensive stuff.

I have yet to find a power supply manufacturer that offers
enough documentation.


Re: Power supply connections - Noob question

What a great explanation of power supply wiring.  I'm piecing together
my first homebuilt and am in the selection stage, and just came to the
power supply.  Your post could not have been more timely and easy to
As soon as we started programming, we found to our surprise that it
wasn't as easy to get programs right as we had thought. Debugging had
to be discovered. I can remember the exact instant when I realized
that a large part of my life from then on was going to be spent in
finding mistakes in my own programs.
  Maurice Wilkes discovers debugging, 1949

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