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PCI Express Auxiliary Graphics Power Connectors

Power Supply 101: A Reference Of Specifications
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Although the ATX12V 2.x specification includes a new 24-pin main power connector with more power for devices such as video cards, the design was intended to power a video card drawing up to 75 watts maximum through the PCIe x16 slot. That is adequate for most video cards, but high-end gaming or workstation cards usually need quite a bit more power. To accommodate graphics cards needing more than 75 watts, the PCI-SIG (Special Interest Group) introduced two standards for supplying additional power to a video card via additional graphics power connectors:

  • PCI Express x16 Graphics 150 W-ATX Specification—Published in October 2004, this standard defines a six-pin (2x3) auxiliary power connector capable of delivering an additional 75 W to a graphics card directly from the power supply, for a total of 150 W to the card.
  • PCI Express 225 W/300 W High Power Card Electromechanical Specification—Published in March 2008, this standard defines an eight-pin (2x4) auxiliary power connector capable of supplying an additional 150 W of power, for a total of either 225 watts (75+150) or 300 watts (75+150+75) of available power.

Cards requiring even more power can use multiple connectors.

Graphics Card Auxiliary Power Connector Configurations
Maximum Power DrawAuxiliary Power Connector Configuration
75 Watts
None
150 Watts
One six-pin connector
225 Watts
Two six-pin connectors*
300 Watts
One eight-pin connector + one six-pin connector
375 Watts
Two eight-pin connectors
450 Watts
Two eight-pin connectors + one six-pin connector
*May optionally use one eight-pin connector instead.


The PCI Express auxiliary power connectors are six-pin (2 × 3) or eight-pin (2 × 4) Molex Mini-Fit Jr. connector housings with female terminals that provide power directly to a video card. For reference, the connector is similar to Molex part number 39-01-2060 (six-pin) or 39-01-2080 (eight-pin), but with different keying to prevent interchanging them with the +12 V motherboard power connectors. A diagram of the six-pin connector is shown below, as is the pinout below that. Note the Sense signal at pin five, which allows a graphics card to detect whether a six-pin power connector has been attached. Without the proper power connections being detected, the card may shut down or operate in a reduced functionality mode. Also note that pin two is technically listed as “no connection” in the official specification, but most power supplies do seem to include +12 V there.

PCI Express six-pin (2x3) auxiliary 75 W power supply connector.PCI Express six-pin (2x3) auxiliary 75 W power supply connector.

PCI Express Six-Pin (2x3) Auxiliary 75 W Power Connector Pinout (Graphics Card Socket)
Color
Signal
Pin
Pin
Signal
Color
Black
GND41
+12 VYellow
Black
Sense5
2
N/C-
Black
GND6
3
+12 VYellow
N/C = No connection; however, many PSUs include a redundant +12V (yellow) wire at pin 2.


A diagram of the eight-pin connector is shown below, as is its pinout. Note the additional +12 V power at pin two and the two Sense signals at pins four and six, which allow a card to detect whether an eight-pin connector, a six-pin connector, or no connector is attached.

PCI Express eight-pin (2x4) auxiliary 150 W power supply connector.PCI Express eight-pin (2x4) auxiliary 150 W power supply connector.

PCI Express Eight-Pin (2x4) Auxiliary 150 W Power Connector Pinout (Graphics Card Socket)
Color
Signal
Pin
Pin
Signal
Color
Black
GND51
+12 VYellow
Black
Sense06
2
12 VYellow
Black GND7
3
12 VYellow
Black
GND8
4
Sense1Black


Because of both the physical design as well as the use of the sense signals, the six-pin power supply connector plug is backward compatible with the eight-pin graphics card socket. This means that if your graphics card has an eight-pin socket but your power supply has only six-pin connectors available, you can plug the six-pin connector into the eight-pin socket using an offset arrangement, as shown below. The connectors are keyed such that they should only plug in the correct way, but be careful because they can be forced together in an incorrect fashion, which can potentially damage the card.

Plugging a six-pin power supply connector into an eight-pin graphics card power socket.Plugging a six-pin power supply connector into an eight-pin graphics card power socket.

The sense signals are used so that the graphics card can detect what types of connector(s) are attached, and therefore how much total power is available. For example, if a graphics card needs a full 300 W and has both an eight-pin and six-pin connector on board, if you were to attach two six-pin power supply connectors, the card would “sense” that it had only 225 W available and, depending on the design, it could either shut down or operate in a reduced functionality mode.

Due to special keying on the eight-pin connector, it cannot be plugged into a six-pin socket. Because of this, many power supply manufacturers include eight-pin connectors made in a “6+2” arrangement, where the portion containing the two extra pins can be disconnected, leaving a six-pin connector that will, of course, work in a six-pin socket.

Caution: The eight-pin PCI Express Auxiliary Power Connector and the eight-pin EPS12V CPU Power Connector use similar Molex Mini-Fit Jr. connector housings. Although they are keyed differently, the keying can be overcome by sufficient force such that you can plug an EPS12V power connector into a graphics card, or a PCI Express power connector into a motherboard. Either of these scenarios results in +12 V being directly shorted to ground, potentially destroying the motherboard, graphics card, or power supply.

The six-pin connector uses two +12 V wires to carry up to 75 W, whereas the eight-pin connector uses three +12 V wires to carry up to 150 W. Although these figures are what the specifications allow, the wires and terminals of each connector are technically capable of handling much more power. Each pin in the PCI Express auxiliary power connectors is rated to handle up to 8 amps of current using standard terminals—more if using HCS or Plus HCS terminals. By counting the number of terminals, you can calculate the power-handling capability of the connector.

PCI Express Graphics Power Connector Maximum Power-Handling Capabilities
ConnectorNo. +12V
Pins
Using Std.
Terminals (W)
Using HCS
Terminals (W)
Using Plus HCS
Terminals (W)
Six-pin2
192264
288
Eight-pin
3
288
396
432
Only two +12 V pins are used in the six-pin connector, even though most power supplies include three.
Standard terminals are rated eight amps.
HCS terminals are rated 11 amps.
Plus HCS terminals are rated 12 amps.
All ratings assume Mini-Fit Jr. connectors using 18-gauge wire under standard temperature conditions.


Even though the specification allows for a delivery capability of 75 W (six-pin connector) or 150 W (eight-pin connector), the total power-handling capacity of these connectors is at least 192 and 288 W, respectively, using standard terminals, and even more using the HCS or Plus HCS terminals.

These two auxiliary power connectors are sometimes called PCI Express Graphics (PEG), Scalable Link Interface (SLI), or CrossFire power connectors because they are used by high-end PCI Express boards with SLI or CrossFire capabilities. SLI and CrossFire are Nvidia and AMD’s methods of using two video cards in unison, with each one drawing half of the screen for up to twice the performance. Each card can draw hundreds of watts, with many of the high-end cards using two or three auxiliary power connectors. This means that most power supplies that are rated as SLI- or CrossFire-ready include at least two or more of the six/eight-pin PCI Express graphics power connectors. Using two video cards drawing 300 watts each means that even if you have a 750-watt power supply, you will have only 150 watts of power left to run the motherboard, processor, and all the disk drives. With high-powered processors drawing 130 watts or more, this may not be enough. For this reason, systems using two or more high-end video cards require the highest-output supplies available, and some of the current ones are capable of putting out up to 1000 watts (1 kilowatt) or more.

Note: Nvidia has trademarked the term SLI as meaning scalable link interface, but its primary competitor, AMD, uses similar dual-graphics card technology called CrossFire to achieve comparable performance improvements.

If your existing power supply doesn’t feature PCI Express auxiliary power connectors, you can use Y-adapters to convert multiple peripheral power connectors (normally used for drives) into a single six-pin or eight-pin PCI Express auxiliary power connector. Note, however, that these adapters will not help if the power supply is not capable of supplying the total power actually required.

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  • 5 Hide
    joytech22 , December 14, 2011 4:19 AM
    Quote:
    On the other hand, if you plug into a 240 V outlet and have the switch set for 120 V, you can cause damage.


    Did that when unboxing a computer, must have flipped the small red switch on the supply and boom, at the Windows XP loading bar the PSU exploded. lol.
  • 8 Hide
    cmcghee358 , December 14, 2011 5:05 AM
    Did I miss them covering efficiency and the whole 80 PLUS thing?

    I can't imagine as detailed as it is, omitting something like that...
  • 9 Hide
    cangelini , December 14, 2011 5:59 AM
    cmcghee358Did I miss them covering efficiency and the whole 80 PLUS thing?I can't imagine as detailed as it is, omitting something like that...


    There's still one last part to go!
  • 3 Hide
    cmcghee358 , December 14, 2011 8:55 AM
    But the last part isn't for PSUs. It's just the last part in the series of PC components.
  • 0 Hide
    nikorr , December 14, 2011 9:44 AM
    Thanx ...
  • 0 Hide
    neiroatopelcc , December 14, 2011 10:54 AM
    I wonder how much the power_good signal prevents? is it just the powering of the cpu ?
    I recall once using two power supplies to power a sli board and accidently use a molex from the second supply to power a sli power connector on the motherboard - resulting in fans powering up if you powered the second psu even when the first wasn't on (and if you didn't, the geforces would screech due to lack of power)..... maybe that was just the creative yet rubbish asrock board design, but it certainly didn't need a power_good to power up the fans.

    ps. "Note: If you find that a system consistently fails to boot up properly the first time you turn on the switch, but that it subsequently boots up if you press the reset or Ctrl+Alt+Delete warm boot command, you likely have a problem with the Power_Good timing. You should install a new, higher-quality power supply and see whether that solves the problem."
    Could this explain why I only have 4-6GB memory at post, but 10GB after a quick power off and back on (didn't bother with a reset switch when designing case). Note that 10GB is still 2 short. It used to initialize 10GB - then power off and back on would provide the full amount. Running less than 6GB memory doesn't cause the error.
    Someone said I'd have to reseat the cpu, but maybe it's just that rubbish coolermaster power supply?
  • -2 Hide
    chesteracorgi , December 14, 2011 12:35 PM
    Very informative and interesting. The part about single 12V vs. multiple 12V rails is important reading for system builders who opt for "safer" multiple 12V PSUs. With the current state of design of PSUs anyone planning a sli or Xfire rig is well advised to opt for the single 12V design rather than risk an imbalanced PSU that overvolts a component.
  • 1 Hide
    JohnnyLucky , December 14, 2011 1:06 PM
    Great article. It's not just for beginners.
  • 1 Hide
    Reynod , December 14, 2011 1:30 PM
    Compatibility Issues was a useful section.

    Overall very well written.

    Cheers,
  • 0 Hide
    kd0frg , December 14, 2011 2:39 PM
    awesome information! nice work!
  • 0 Hide
    elbert , December 14, 2011 3:28 PM
    cmcghee358But the last part isn't for PSUs. It's just the last part in the series of PC components.
    Quote:
    covering efficiency and the whole 80 PLUS

    If you picked one of these books up you would want the efficiency to move them. Edition 17 was huge and very heavy. These books are already to thick for many to pick up with one hand. Scott Mueller's has published 20 editions of this book and most come with CD/DVD which may guide you to online information about the subject.

    Here is a link to his online forum.
    http://forum.scottmueller.com/
  • 0 Hide
    digiex , December 14, 2011 3:48 PM
    I'm just wondering what is the use of the floppy connector...

    Until unexpected glitch ruined the flashing if my motherboard, beyond this, I think the floppy connector is useless.
  • 4 Hide
    mayankleoboy1 , December 14, 2011 3:49 PM
    PSU: the most overlooked and underrated component
  • 4 Hide
    Anonymous , December 14, 2011 4:14 PM
    I fixed one of those non-compatible Dells way back with a standard PSU. Dell wanted £120 for a new PSU, I was suspicious, "how could they get away with that?". Checked online, found the incompatibility, dodged the bullet bought a PSU for £20 and an adapter for £5. Never bought Dell again nor recommend them.
  • 6 Hide
    A Bad Day , December 14, 2011 6:07 PM
    This reminded me of a friend who bought a $5 no-name "600 watt" PSU for a +$900 rig.

    As soon as he turned on the computer, the PSU failed so badly that it exploded into flames and took out everything: motherboard, RAM, CPU, GPU, hard drive, CD drive, you name it.
  • 0 Hide
    grantmcconnaughey , December 14, 2011 7:49 PM
    I've been reading this book lately. To me, this is absolutely the bible of PC hardware.
  • 0 Hide
    newbie_mcnoob , December 14, 2011 8:19 PM
    I remember working on an old Dell Dimension 4100 series with the proprietary power supply and RIMM memory. I'm glad those got phased out.
  • 2 Hide
    hunter315 , December 14, 2011 10:50 PM
    Quote:
    In other words, it is far better to have a single 12 V rail that can supply 40 amps than two 12 V rails supplying 20 amps each because with the single rail you don’t have to worry which connectors derive power from which rail and then try to ensure that you don’t overload one or the other.


    Im quite disappointed to see tom's fell for the marketing BS of "a single rail is better than multiple rails". On a well designed unit it does not matter one bit, the design engineers already split the connectors so the rails were reasonably balanced, and the OCP threshold is set such that added together their theoretical current limit is more than the total limit of the 12 V source so you don't have to have your rails perfectly balanced to get the full power out of your unit.

    I wrote up a post on this a while ago, if anyone has any questions or anything they think should be added to it let me know.
    Single 12V rail or multiple 12V rails? The eternal question answered


    Also, you guys left the CPU off the +12 V part of your chart of what requires what voltages.
  • 1 Hide
    PreferLinux , December 14, 2011 10:58 PM
    ChesteracorgiVery informative and interesting. The part about single 12V vs. multiple 12V rails is important reading for system builders who opt for "safer" multiple 12V PSUs. With the current state of design of PSUs anyone planning a sli or Xfire rig is well advised to opt for the single 12V design rather than risk an imbalanced PSU that overvolts a component.

    I guess it is better to be able to use the 12 V rail as an arc welder then? Because you could if you have a >1000 W single-rail PSU. Not to mention that it won't overvolt anything – how does a high power draw cause high voltages? It generally causes low voltages. And if the PSU is a decent one, the rails will be pretty well balanced, especially for SLI or Crossfire.
  • 2 Hide
    iam2thecrowe , December 14, 2011 11:15 PM
    ChesteracorgiVery informative and interesting. The part about single 12V vs. multiple 12V rails is important reading for system builders who opt for "safer" multiple 12V PSUs. With the current state of design of PSUs anyone planning a sli or Xfire rig is well advised to opt for the single 12V design rather than risk an imbalanced PSU that overvolts a component.

    you couldn't be more wrong.
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