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Motherboard Power Connectors: AT/LPX And ATX

Power Supply 101: A Reference Of Specifications
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Every PC power supply has connectors that attach to the motherboard, providing power to the motherboard, processor, memory, chipset, integrated components (such as video, LAN, universal serial bus [USB], and FireWire), and any cards plugged into bus slots. These connectors are important; not only are these the main conduit through which power flows to your system, but attaching these connectors improperly can have a devastating effect on your PC, including burning up both your power supply and motherboard. Just as with the mechanical shape of the power supply, these connectors are usually designed to conform to one of several industry-standard specifications, which dictate the types of connectors used as well as the pinouts of the individual wires and terminals. Unfortunately, just as with the mechanical form factors, some PC manufacturers use power supplies with custom connectors or, worse yet, use standard connector types but with modified (incompatible) pinouts (meaning the signals and voltages are rearranged from standard specifications). Plugging a power supply with an incompatible pinout into a motherboard that uses a standard pinout (or vice versa) usually results in the destruction of either the board or the power supply—or both.

Just as I insist on industry-standard mechanical form factors in my systems, I also want to ensure that they use industry-standard connectors and pinouts. By only purchasing components that conform to industry standards, I can ensure the greatest flexibility and lowest cost for future upgrades and repairs.

Two main sets of motherboard power connectors have been used over the years: what I would call AT/LPX type and the ATX type. Each of these has minor variations; for example, the ATX type has evolved over the years, with new connectors coming (and some going) and modifications to existing connectors. This section details the motherboard power connectors used by various types of industry-standard (and some not-so-standard) power supplies.

AT/LPX Power Supply Connectors

Industry-standard PC, XT, AT, Baby-AT, and LPX motherboards use the same type of main power supply connectors. AT/LPX power supplies feature two main power connectors (P8 and P9), each with six pins that attach the power supply to the motherboard. The terminals used in these connectors are rated to handle up to five amps at up to 250 V (even though the maximum used in a PC is +12 V). These two connectors are shown in the following figure.

AT/LPX main P8/P9 (also called P1/P2) power connectors, side and terminal end view.AT/LPX main P8/P9 (also called P1/P2) power connectors, side and terminal end view.

All AT/LPX power supplies that use the P8 and P9 connectors have them installed end to end so that the two black wires (ground connections) on both power connectors are next to each other when properly plugged in. Note the designations P8 and P9 are not fully standardized, although most use those designations because that is what IBM stamped on the originals. Some power supplies have them labeled as P1/P2 instead. Because these connectors usually have a clasp that prevents them from being inserted backward on the motherboard’s pins, the major concern is getting the two connectors in the correct orientation side by side and also not offsetting by one or more pins side to side. Following the black-to-black rule and ensuring they are on-center keeps you safe. You must take care to ensure that no remaining unconnected motherboard pins exist between or on either side of the two connectors after you install them. A properly installed connector connects to and covers every motherboard power pin. If any power pins are showing on either side of or between the connectors, the entire connector assembly is installed incorrectly, which can result in catastrophic failure for the motherboard and everything plugged into it at the time of power-up. This next image shows the P8 and P9 connectors (sometimes also called P1/P2) in their proper orientation when connected to a motherboard.

The P8/P9 power connectors (sometimes also called P1/P2) that connect an AT/LPX power supply to the motherboard.The P8/P9 power connectors (sometimes also called P1/P2) that connect an AT/LPX power supply to the motherboard.

This table shows typical AT/LPX power supply connections.

AT/LPX Power Supply Connectors (Wire End View)
ConnectorPin
Signal
Color2Connector
Pin
Signal
Color2
P8 (or P1)1
Power_Good (+5V)OrangeP9 (or P2)1
GroundBlack

2
+5V1Red
2
GroundBlack

3
+12VYellow

3
-5 V
White

4
-12V
Blue

4
+5 V
Red

5
GroundBlack
5
+5 VRed

6
GroundBlack
6
+5 VRed
1. First-generation PC/XT motherboards and power supplies did not require this voltage, so the pin might be missing from the motherboard and terminal and the wire might be missing from the connector (P8 pin 2).
2. I have seen some supplies where the manufacturer did not follow industry-standard wire color-codes even though the signals were correct.


Tip: Although older PC/XT power supplies do not have a connection at connector P8 pin 2, you still can use them on AT-type motherboards, or vice versa. The presence or absence of the +5 V on that pin has little or no effect on system operation because the remaining +5 V wires can usually carry the load.

Note that all the AT/LPX-type power supplies use the same connectors and pin configurations; to my knowledge there were never nonstandard variations.

ATX and ATX12V Motherboard Power Connectors

Power supplies conforming to the original ATX and ATX12V 1.x form factor standards or variations thereof use the following three motherboard power connectors:

  • 20-pin main power connector
  • Six-pin auxiliary power connector
  • Four-pin +12 V power connector


The main power connector is always required, but the other two are optional depending on the application. Consequently, a given ATX or ATX12V power supply can have up to four combinations of connectors, as listed here:

  • Main power connector only
  • Main and auxiliary
  • Main and +12 V
  • Main, auxiliary, and +12 V


The most common varieties are those including the main only and those with the main and +12 V connectors. Most motherboards that use the +12 V connector do not use the auxiliary connector, and vice versa.

20-Pin Main Power Connector

The 20-pin main power connector is standard for all power supplies conforming to the ATX and ATX12V 1.x power supply form factors and consists of a Molex Mini-Fit Jr. connector housing with female terminals. For reference, the connector is Molex part number 39-01-2200 (or equivalent), and the standard terminals are part number 5556 (see the following figure). This is a 20-pin keyed connector with pins configured as shown in the next table. The colors for the wires listed are those the ATX standard recommends; however, to enable them to vary from manufacturer to manufacturer, they are not required for compliance to the specification. I like to show these connector pinouts in a wire end view, which shows how the pins are arranged looking at the back of the connector (from the wire and not the terminal end). This way, you can see how they would be oriented if you were back-probing the connector with the connector plugged in.

ATX 20-pin motherboard main power connector, perspective view.ATX 20-pin motherboard main power connector, perspective view.

ATX 20-pin main power connector, side and terminal end view.ATX 20-pin main power connector, side and terminal end view.

ATX 20-pin Main Power Supply Connector Pinout (Motherboard Connector)
Color
Signal
Pin
Pin
Signal
Color
Orange+3.3 V1111
+3.3 VOrange
Blue–12 V12
2
+3.3 VOrange
BlackGND
13
3
GND
Black
Green
PS_On
14
4
+5 V
Red
Black
GND
15
5
GND
Black
BlackGND
16
6
+5 V
Red
BlackGND
17
7
GND
Black
White
-5 V
1828
Power_Good
Gray
Red
+5 V
19
9
+5 VSB (Standby)
Purple
Red
+5 V
20
10
+12 V
Yellow
1. Might have a second orange or brown wire, used for +3.3 V sense feedback. The power supply uses this wire to monitor 3.3 V regulation.
2. Pin 18 will be N/C (no connection, absent) on some later model supplies or motherboards because –5 V was removed from the ATX12V 1.3 and later specifications. Supplies with no connection at pin 18 should not be used with older motherboards that incorporate ISA Bus slots.


Note: The ATX supply features several voltages and signals not seen on earlier AT/LPX designs, such as the +3.3 V, PS_On, and +5V_Standby. Therefore, adapting a standard LPX form factor supply to make it work properly in an ATX system is impossible, even though the shapes of the power supplies are virtually identical.

However, because ATX is a superset of the older LPX power supply standard, you can use a connector adapter to allow an ATX power supply to connect to an older motherboard using AT/LPX connectors.

One of the most important issues with respect to power supply connectors is the capability to deliver sufficient power to the motherboard without overheating. It doesn’t help to have a 500-watt power supply if the cables and connectors supplying power to the motherboard can handle only 250 watts before they start to melt. When talking about specific connectors, the current rating is stated in amperes per circuit, which is a measure of the amount of current that can be passed through a mated terminal that will allow no more than a 30°C (86°F) temperature rise over ambient 22°C (72°F). In other words, at a normal ambient temperature of 22°C (72°F), when operating under the maximum rated current load, the temperature of the mated terminals will not exceed 52°C (126°F). Because the ambient temperature inside a PC can run 40°C (104°F) or higher, running power connectors at maximum ratings can result in extremely high temperatures in the connectors.

The maximum current level is further de-rated or adjusted for the number of circuits in a given connector housing due to the heat of any adjacent terminals. For example, a power connector might be able to carry eight amps per circuit in a four-position connector, but the same connector and terminal design might be able to handle only six amps per circuit in a 20-position connector.

All the modern form factor power supplies since ATX have standardized on the use of Molex Mini-Fit Jr. connectors for the main and +12 V connectors. A number of connector housings are used with anywhere from four to 24 positions or terminals. Molex makes three types of terminals for these connectors: a standard version, an HCS version, and a Plus HCS version. The current ratings for these terminals are shown below.

Current Ratings for Mini-Fit Jr. Connectors
Mini-Fit Jr.
Terminal Type/No.
2–3 Pins
(Amps/Pin)
4–6 Pins
(Amps/Pin)
7–10 Pins
(Amps/Pin)
12–24 Pins
(Amps/Pin)
Standard/55569
8
7
6
HCS/44476 
12
11
10
9
Plus HCS/4575012
12
12
11
All ratings assume Mini-Fit Jr. connectors with 12–24 circuits using 18-gauge wire under standard temperature conditions.


The ATX main power connector is either a 20-pin or 24-pin connector, which, if standard terminals are used, is rated for up to six amps of current per terminal. If the connector were upgraded to HCS terminals, the rating would increase to nine amps per terminal, and if upgraded to Plus HCS terminals, the rating would increase further to 11 amps per terminal. Prior to March 2005, all the power supply form factor specifications called for using standard terminals, but all the ratings from March 2005 to the present have changed to require HCS terminals instead. If your power supply connector has been overheating, you can easily install HCS or Plus HCS terminals to increase the power-handling capability of your connector by 50% or more.

By counting the number of terminals for each voltage level, you can calculate the power-handling capability of the connector, as shown below.

ATX 20-pin Main Power Connector Maximum Power Handling Capabilities
VoltsNo. Pins Using Std. Terminals (W)Using HCS Terminals (W)
Using Plus HCS Terminals (W)
+3.3 V3
59.489.1108.9
+5 V4
120180
220
+12 V1
72108
132
Total watts:251.4
377.1460.9
Standard terminals are rated six amps.
HCS terminals are rated nine amps.
Plus HCS terminals are rated 11 amps.
All ratings assume Mini-Fit Jr. connectors with 12–24 circuits using 18-gauge wire under standard temperature conditions.


This means the total power-handling capacity of this connector is only 251 watts if standard terminals are being used, which is lower than most systems need today. Unfortunately, drawing more power than this maximum rating through the connector causes it to overheat. I’m sure you can appreciate how inadequate this has become today; for example, it certainly doesn’t make sense to manufacture a 400- or 500-watt power supply if the main power connector can handle only 251 watts without melting! That would be like building a car that could go 200 MPH and then equipping it with tires rated for only 100 MPH. Everything would be fine until you exceeded the tires’ rated speed, after which the results would not be pretty.

This is why the official power supply form factor specifications were updated in March 2005 to include HCS terminals, which have 50% greater power-handling capability than the standard terminals. Using HCS terminals, the power-handling capability of the 20-pin main connector alone increases to 377 watts, which is more than most systems need to run the entire system through all the connectors combined.

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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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