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{{Short description|Motherboard and power supply configuration}}
{{About|the computer form factor}}
{{Lead too short|date=January 2025}}
{{Main|Motherboard form factor}}
[[File:Abit-kt7-large.jpg|thumb|An ATX motherboard]]
[[File:VIA Mini-ITX Form Factor Comparison.jpg|right|thumb|400px|Comparison of some common [[motherboard]] form factors (pen for scale)]]

'''ATX''' ('''Advanced Technology Extended''') is a motherboard and power supply configuration specification developed by [[Intel]] to improve on previous [[de facto standard|''de facto'' standards]] such as the [[AT (form factor)|AT design]].  Originally released in July 1995, it was the first major change in [[computer case|desktop computer enclosure]], [[motherboard]] and [[power supply unit (computer)|power supply]] design in many years, improving standardization and interchangeability of parts. The specification defines the dimensions; the mounting points; the I/O panel; and the power and connector interfaces among a [[computer case]], a [[motherboard]], and a [[power supply unit (computer)|power supply]].

== Overview ==
ATX is the most common motherboard design.<ref>{{Cite book|title = Authorized Cert Guide: CompTIA A+|last1 = Mark|first1 = Soper|publisher = Pearson Education|date = September 2012|isbn = 978-0-7897-4850-8|last2 = Prowse|first2 = David|last3 = Mueller|first3 = Scott}}</ref>{{page number|date=September 2025}} Other standards for smaller boards (including [[microATX]], [[FlexATX]], [[nano-ITX]], and [[mini-ITX]]) usually keep the basic rear layout but reduce the size of the board and the number of expansion slots.  Dimensions of a full-size ATX board are {{convert|12|xx|9.6|in|mm|0|abbr=on}} which means [[microATX]] boards fit in many ATX chassies.  The ATX specifications were patented by David Dent at Intel and co-developed with [[Astec|Astec America]], a major manufacturer of [[power supply unit (computer)|power supplies]].<ref>{{cite patent |country=US |number=5555158}}</ref><ref>{{cite magazine | last=Mankikar | first=Mohan | date=August 14, 1995 | url=https://link.gale.com/apps/doc/A17296569/GPS?u=wikipedia&sid=bookmark-GPS&xid=96e97861 | title=Baby AT grows up | magazine=Electronic Engineering Times | publisher=UBM LLC | issue=861 | page=86 | via=Gale}}</ref> The inaugural version 1.0 of the ATX specification was unveiled by Intel in July 1995.<ref>{{cite book | last=Mueller | first=Scott | date=2003 | url=https://books.google.com/books?id=eV1_LjW3pTkC&dq=%22ATX%22+%22intel%22+%22motherboard%22+july+1995&pg=PA211 | title=Upgrading and Repairing PCs | publisher=Que | edition=14th | page=211 | isbn=9780789727459 | via=Google Books}}</ref> Since then, ATX has been revised numerous times. The most recent ATX motherboard specification is version 2.2.<ref name="formfactors_org_atx22">{{cite web |url=http://www.formfactors.org/developer/specs/atx2_2.pdf |title=ATX Specification - Version 2.2 |publisher=Intel Corporation |website=formfactors.org |access-date=April 4, 2014 |archive-url=https://web.archive.org/web/20120725150314/http://www.formfactors.org/developer/specs/atx2_2.pdf |archive-date=2012-07-25 |url-status=dead }}</ref> The most recent ATX12V power supply unit specification is ATX 3.0 released in February 2022.<ref>{{cite web|url=https://edc.intel.com/content/www/us/en/design/ipla/software-development-platforms/client/platforms/alder-lake-desktop/atx-version-3-0-multi-rail-desktop-platform-power-supply-design-guide/2.0/revision-history/ |title=ATX Version 3.0 Multi Rail Desktop Platform Power Supply Design Guide|work=[[Intel]]|access-date=October 8, 2024}}</ref><ref>{{cite news|url=https://www.anandtech.com/show/21367/the-xpg-core-reactor-ii-ve-850w-atx-31-psu-review/2|archive-url=https://web.archive.org/web/20240509084349/https://www.anandtech.com/show/21367/the-xpg-core-reactor-ii-ve-850w-atx-31-psu-review/2|url-status=dead|archive-date=May 9, 2024|title=Intel's ATX v3.0 PSU Standard Has More Power for GPUs|first=Aris|last=Mpitziopoulos|work=[[Tom's Hardware]]|date=March 4, 2022|access-date=October 8, 2024|quote=On February 1st, 2022, the ATX12V v3.0 spec became final...}}</ref><ref>{{cite news|url=https://www.anandtech.com/show/21367/the-xpg-core-reactor-ii-ve-850w-atx-31-psu-review/2|archive-url=https://web.archive.org/web/20240509084349/https://www.anandtech.com/show/21367/the-xpg-core-reactor-ii-ve-850w-atx-31-psu-review/2|url-status=dead|archive-date=May 9, 2024|title=The XPG Core Reactor II VE 850W PSU Review: Our First ATX 3.1 Power Supply|author=E. Fylladitakis|work=[[Anandtech]]|date=May 2, 2024|access-date=October 8, 2024|quote=The ATX v3.1 specification was officially launched on 13 September 2023, following the release of the ATX v3.0 in February 2022.}}</ref>

EATX (Extended ATX) is a bigger version of the ATX motherboard with {{convert|12|xx|13|in|mm|0|abbr=on}} dimensions. While some dual-CPU-socket motherboards have been implemented in ATX, the extra size of EATX makes it the typical form factor for dual-socket systems and for single-socket systems with a large number of memory slots (or more than two memory channels).

In 2004, Intel announced the [[BTX (form factor)|BTX]] (Balanced Technology eXtended) standard, intended as a replacement for ATX. While some manufacturers adopted the new standard, Intel discontinued any future development of BTX in 2006. {{As of|2025}}, the ATX design still remains the de facto standard for personal computers.

== Connectors ==
{{unreferenced section|date=May 2020}}
[[File:Several atx io shields (smial).jpg|thumb|ATX I/O plates for motherboard rear connectors]]

On the back of the computer case, some major changes were made to the AT standard. Originally AT style cases had only a [[computer keyboard|keyboard]] connector and expansion slots for add-on card backplates. Any other onboard interfaces (such as [[serial port|serial]] and [[parallel port]]s) had to be connected via [[Ribbon cable|flying leads]] to connectors which were mounted either on spaces provided by the case or brackets placed in unused expansion slot positions.<ref>{{Cite news |author1=Alex Campbell |date=2016-07-26 |title=A brief history of PC case design |language=en |work=PC Gamer |url=https://www.pcgamer.com/a-brief-history-of-pc-case-design/ |access-date=2023-12-09}}</ref>

The ATX standard specified a rectangular area on the back of the system where motherboard manufacturers can arrange ports any way they want. A number of general patterns (depending on which ports the motherboard offers) have been used by most manufacturers. Cases are usually fitted with a snap-out panel, also known as an I/O plate or I/O shield, in one of the common arrangements. I/O plates are usually included with retail motherboards for installation in any suitable case. The computer will operate correctly without a plate fitted, but there will be open gaps in the case which may compromise the EMI/RFI screening and allow the ingress of dirt and random foreign bodies. Panels were made that fit an AT motherboard in an ATX case. Some ATX motherboards come with an integrated I/O plate.

ATX also made the [[PS/2 port|PS/2-style]] [[mini-DIN]] keyboard and mouse connectors ubiquitous. AT systems used a 5-pin [[DIN connector]] for the keyboard and were generally used with serial port mice (although PS/2 mouse ports were also found on some systems). Many modern motherboards are phasing out the PS/2-style keyboard and mouse connectors in favor of the more modern [[USB|Universal Serial Bus]]. Other legacy connectors that are slowly being phased out of modern ATX motherboards include 25-pin [[parallel port]]s and 9-pin [[RS-232]] [[serial port]]s. In their place are onboard peripheral ports such as [[Ethernet]], [[IEEE 1394|FireWire]], [[eSATA]], [[sound card|audio ports]] (both analog and [[S/PDIF]]), video (analog [[D-sub]], [[Digital Visual Interface|DVI]], [[HDMI]], or [[DisplayPort]]), extra [[USB]] ports, and Wi-Fi.

A notable issue with the ATX specification was that it was last revised when power supplies were normally placed at the top of many old computer cases rather than at the bottom, as with many modern computer cases. This has led to some problematic standard locations for ports, in particular the 4/8 pin CPU power, which is normally located along the top edge of the board, convenient for top-mounted power supplies. This makes it very difficult for cables from bottom-mounted power supplies to reach, and commonly requires a special cutout in the back plane for the cable to come in from behind and bend around the board, making insertion and wire management very difficult. Many power supply cables barely or fail to reach, or are too stiff to make the bend, and extensions are sometimes required due to this placement. Modern power supplies often have longer cables to alleviate this issue.

== Variants ==
{{redirect|EATX|the electronic automatic transaxle|automatic transmission}}
{{Disputed section | E-ATX is not a form factor|date=April 2020}}
[[File:ATX ITX AT Motherboard Compatible Dimensions.svg|thumb|ATX, [[Mini-ITX]], and AT motherboard compatible dimensions and bore positions]]
[[File:Form Factor dimension.png|thumb|ATX motherboard size comparison; rear is on left.
{{legend|#57ccff|[[microATX]] {{nowrap|({{convert|9.6|xx|9.6|in|mm|0|disp=output only|abbr=on}})}}}}
{{legend|#ffffff|Standard ATX {{nowrap|({{convert|12|xx|9.6|in|mm|0|disp=output only|abbr=on}})}}}}
{{legend|#e6ff24|Ultra ATX {{nowrap|({{convert|14.4|xx|9.6|in|mm|0|disp=output only|abbr=on}})}}}}
{{legend|#336aff|Extended ATX (EATX) {{nowrap|({{convert|12|xx|13|in|mm|0|disp=output only|abbr=on}})}}}}
{{legend|#f02700|[[WTX (form factor)|WTX]] {{nowrap|({{convert|14|xx|16.75|in|mm|0|disp=output only|abbr=on}})}}}}
{{legend|#3aff48|[[SSI CEB|SSI MEB]] {{nowrap|({{convert|16.2|xx|13|in|mm|0|disp=output only|abbr=on}})}}}}]]
Several ATX-derived designs have been specified that use the same power supply, mountings and basic back panel arrangement, but set different standards for the size of the board and number of expansion slots. Standard ATX provides seven slots at {{convert|0.8|in|mm|0|abbr=on}} spacing; the popular [[microATX]] size removes {{convert|2.4|in|mm}} and three slots, leaving four. Here width refers to the distance along the external connector edge, while depth is from front to rear. Note each larger size inherits all previous (smaller) colors area.

[[AOpen]] has conflated the term [[Mini ATX]] with a more recent{{when|date=September 2022}} {{convert|15|xx|15|cm|in|1|abbr=on}} design. Since references to Mini ATX have been removed from ATX specifications since the adoption of microATX, the AOpen definition is the more contemporary term and the one listed above is apparently only of historical significance. This sounds contradictory to the now common Mini-ITX standard ({{convert|17|xx|17|cm|in|1|abbr=on}}), which is a potential source of confusion. A number of manufacturers have added one to three additional expansion slots (at the standard 0.8 inch spacing) to the standard 12-inch ATX motherboard width.

Form factors considered obsolete in 1999 included Baby-AT, full size AT, and the semi-proprietary LPX for low-profile cases. Proprietary motherboard designs such as those by Compaq, Packard-Bell, Hewlett Packard and others existed, and were not interchangeable with multi-manufacturer boards and cases. Portable and notebook computers and some 19-inch rackmount servers have custom motherboards unique to their particular products.<ref>Scott Mueller, ''Upgrading and Repairing PCs, Eleventh Edition'', Que Books, 1999, {{ISBN|0-7897-1903-7}}, page 1255</ref>
{{sort-under}}
{| class="wikitable sortable sort-under" style="text-align:center;"
|-
! Form factor
! Originated
! Date
! Max. size{{efn|For boards which take expansion slots, the length of the expansion card aligns with the depth of the system board. The case may support cards longer than the depth of the mainboard.}}<br />depth × width
! {{verth|va=middle|Slots}}
! Notes<br />(typical usage, market adoption, etc.)
|-
! ATX
| Intel
| 1995
| 12&nbsp;×&nbsp;9.6&nbsp;in (305&nbsp;×&nbsp;244&nbsp;mm)
| 7{{efn|Most common ATX motherboards have 6 slots despite having the physical sizing for 7 slots.}}
| Original, successor to AT motherboard
|-
| {{Proprietary}}, ''specific to crypto-mining specific motherboards''{{citation needed|date=July 2025}}
| {{unk}}
| 2011
| 12&nbsp;×&nbsp;8&nbsp;in (305&nbsp;×&nbsp;203&nbsp;mm)
| 3
| 3 double-slot add-in cards with 1 slots of free space in between
|-
! [[SSI CEB]]
| [[SSI CEB|SSI]]
| {{dunno}}
| 12&nbsp;×&nbsp;10.5 in (305&nbsp;×&nbsp;267&nbsp;mm)
| 7
| Compact Electronics Bay
|-
! [[SSI CEB|SSI MEB]]
| SSI
| 2011
| 16.2&nbsp;×&nbsp;13&nbsp;in (411&nbsp;×&nbsp;330&nbsp;mm)
| 12
| Midrange Electronics Bay
|-
! [[SSI CEB|SSI EEB]]
| SSI
| {{dunno}}
| 12&nbsp;×&nbsp;13&nbsp;in (305&nbsp;×&nbsp;330&nbsp;mm)
| 7
| Enterprise Electronics Bay
|-
! [[SSI CEB|SSI TEB]]
| SSI
| {{dunno}}
| 12&nbsp;×&nbsp;10.5&nbsp;in (305&nbsp;×&nbsp;267&nbsp;mm)
| 7
| Thin Electronics Bay, for rack-mount, has board component height specification
|-
! [[microATX]]
| Intel
| 1997
| 9.6&nbsp;×&nbsp;9.6&nbsp;in (244&nbsp;×&nbsp;244&nbsp;mm)
| 4
| Fits in ATX, and EATX cases.
|-
! [[FlexATX]]
| Intel
| 1997
| 9&nbsp;×&nbsp;7.5&nbsp;in (229&nbsp;×&nbsp;191&nbsp;mm)
| 3
|
|-
! Extended ATX (standard)
| Supermicro / [[Asus]]
| {{dunno}}
| 12&nbsp;×&nbsp;13&nbsp;in (305&nbsp;×&nbsp;330&nbsp;mm)
| 7
| Screw holes not completely compatible with some ATX cases. Intended for two CPU sockets.
|-
! Extended ATX (commonly)
| {{unk}}
| {{dunno}}
| 12&nbsp;×&nbsp;10.1&nbsp;in (305&nbsp;×&nbsp;257&nbsp;mm)<br />12&nbsp;×&nbsp;10.4&nbsp;in (305&nbsp;×&nbsp;264&nbsp;mm)<br />12&nbsp;×&nbsp;10.5&nbsp;in (305&nbsp;×&nbsp;267&nbsp;mm)<br />12&nbsp;×&nbsp;10.7&nbsp;in (305&nbsp;×&nbsp;272&nbsp;mm)
| 7
| ATX pattern screw holes
|-
! EE-ATX
|Supermicro
| {{dunno}}
| 13.68&nbsp;×&nbsp;13&nbsp;in (347&nbsp;×&nbsp;330&nbsp;mm)
| 7
| Enhanced Extended ATX
|-
! Ultra ATX
| Foxconn
| 2008
| 14.4&nbsp;×&nbsp;9.6&nbsp;in (366&nbsp;×&nbsp;244&nbsp;mm)<ref>{{Cite web|url=https://www.tomshardware.com/news/ces-foxconn-x48-shimano,4677.html|title=Foxconn Reveals X48, Ultra ATX, and Shamino|date=13 January 2008}}</ref>
| 10
| Intended for multiple double-slot video cards, and dual CPUs.
|-
! XL-ATX
| [[EVGA Corporation|EVGA]] 
| 2009
| 13.5&nbsp;×&nbsp;10.3&nbsp;in (343&nbsp;×&nbsp;262&nbsp;mm)
| 9
|
|-
! XL-ATX
| Gigabyte 
| 2010
| 13.58&nbsp;x&nbsp;10.31&nbsp;in (345&nbsp;x&nbsp;262&nbsp;mm)
| 7
|
|-
! XL-ATX
| [[Micro-Star International|MSI]] 
| 2010
| 13.6&nbsp;× 10.4&nbsp;in (345&nbsp;×&nbsp;264&nbsp;mm)
| 7
|
|-
! [[WTX (form factor)|WTX]]
| Intel
| 1998
| 14&nbsp;×&nbsp;16.75&nbsp;in (356&nbsp;×&nbsp;425&nbsp;mm).
| 9
| Discontinued 2008
|-
! [[Mini-ITX]]
| [[VIA Technologies|VIA]]
| 2001
| 6.7&nbsp;x&nbsp;6.7in (170&nbsp;×&nbsp;170&nbsp;mm).
| 1
| Originally designed for home theatre or other fanless applications
|-
! [[DTX (form factor)|Mini-DTX]]
| [[Advanced Micro Devices|AMD]]
| 2007
| 8&nbsp;×&nbsp;6.7&nbsp;in (203&nbsp;×&nbsp;170&nbsp;mm)
| 2
| Derived from Mini-ITX and DTX
|-
! [[BTX (form factor)|BTX]]
| Intel
| 2004
| 12.8&nbsp;×&nbsp;10.5&nbsp;in (325&nbsp;×&nbsp;267&nbsp;mm)
| 7
| Canceled 2006. Also micro, nano, and pico variants. Not generally compatible with ATX mounting.
|-
! HPTX
| EVGA
| 2010
| 13.6&nbsp;×&nbsp;15&nbsp;in (345&nbsp;×&nbsp;381&nbsp;mm)
| 6
| Dual processors, 12 RAM slots
|-
! [[SWTX]]
| Supermicro
| 2006
| 16.48&nbsp;×&nbsp;13&nbsp;in (419&nbsp;×&nbsp;330&nbsp;mm)<br />and others
| 5
| Quad processors, not compatible with ATX mounting
|-
! YTX
| [[Asus]], [[Micro-Star International|MSI]] <br />& Maxsun
| 2023
| 6.89&nbsp;×&nbsp;9.65&nbsp;in (175&nbsp;×&nbsp;245&nbsp;mm)
| 1
| New form factor supporting the DIY-APE initiative
|}

Although true E-ATX is {{convert|12|xx|13|in|mm|0|abbr=on}} most motherboard manufacturers also refer to motherboards with measurements {{convert|12|xx|10.1|in|mm|0|abbr=on}}, {{convert|12|xx|10.4|in|mm|0|abbr=on}}, {{convert|12|xx|10.5|in|mm|0|abbr=on}} and {{convert|12|xx|10.7|in|mm|0|abbr=on}} as E-ATX. While E-ATX and SSI EEB (Server System Infrastructure (SSI) Forum's Enterprise Electronics Bay (EEB)) share the same dimensions, the screw holes of the two standards do not all align; rendering them incompatible.{{Citation needed|date=April 2020}}

In 2008, [[Foxconn]] unveiled a Foxconn F1 motherboard prototype, which has the same width as a standard ATX motherboard, but an extended 14.4" length to accommodate 10 slots.<ref>{{cite web |url=http://www.hardwaresecrets.com/article/513 |title=Foxconn F1 Motherboard Prototype |publisher=Hardwaresecrets.com |access-date=18 November 2014 |url-status=dead |archive-url=https://web.archive.org/web/20141024000823/http://www.hardwaresecrets.com/article/513 |archive-date=24 October 2014 }}</ref> The firm called the new {{convert|14.4|xx|9.6|in|mm|0|abbr=on}} design of this motherboard "Ultra ATX"<ref>{{cite web |url=https://www.tomshardware.com/news/ces-foxconn-x48-shimano,4677.html |title=Foxconn Reveals X48, Ultra ATX, and Shamino |author=Thomas Soderstrom|work=Tom's Hardware|date=13 January 2008 |access-date=18 November 2014}}</ref> in its CES 2008 showing. Also unveiled during the January 2008 CES was the [[Lian Li]] Armorsuit PC-P80 case with 10 slots designed for the motherboard.<ref>{{cite web |url=http://www.techpowerup.com/reviews/LianLi/Armorsuit_PC-P80R_Spider_Edition |title=Lian Li Armorsuit PC-P80R Spider Edition|work=TechPowerUp |date=11 May 2008 |access-date=18 November 2014}}</ref>

The name "XL-ATX" has been used by at least three companies in different ways:

* In September 2009, [[EVGA Corporation]] had already released a {{convert|13.5|xx|10.3|in|mm|0|abbr=on}} "XL-ATX" motherboard as its ''EVGA X58 Classified 4-Way SLI'',<ref>{{cite web |url=https://www.evga.com/articles/00501/ |title=The New 4-Way SLI Platform Has Arrived! |publisher=Evga.com |access-date=18 November 2014}}</ref> which requires an Ultra-ATX sized case to support four "double-slot" graphics cards.<ref>{{Cite web| url=http://www.evga.com/products/moreInfo.asp?pn=170-BL-E762-A1&family=Motherboard | archive-url=https://web.archive.org/web/20110716165637/http://www.evga.com/products/prodlist.asp | title=Product list GeForce 500 Series | archive-date=2011-07-16}}</ref>
* Gigabyte Technology launched another XL-ATX motherboard, with model number GA-X58A-UD9 in 2010 measuring at {{convert|345|xx|262|mm|in|1|order=flip|abbr=on}}, and GA-X79-UD7 in 2011 measuring at {{convert|324|xx|253|mm|in|1|order=flip|abbr=on}}. In April 2010, Gigabyte announced its {{convert|12.8|xx|9.6|in|mm|0|abbr=on}} GA-890FXA-UD7 motherboard that allowed all seven slots to be moved downward by one slot position. The added length could have allowed placement of up to eight expansion slots, but the top slot position is vacant on this particular model.
* [[Micro-Star International|MSI]] released MSI X58 Big Bang in 2010, MSI P67 Big Bang Marshal in 2011, MSI X79 Xpower Big Bang 2 in 2012 and MSI Z87 Xpower in 2013 all of them are {{convert|345|xx|264|mm|in|1|order=flip|abbr=on}}. Although these boards have room for additional expansion slots (9 and 8 total, respectively), all three provide only seven expansion connectors; the topmost positions are left vacant to provide more room for the CPU, chipset and associated cooling.

In 2010, [[EVGA Corporation]] released a new motherboard, the "Super Record 2", or SR-2, whose size surpasses that of the "EVGA X58 Classified 4-Way SLI". The new board is designed to accommodate two Dual QPI LGA1366 socket CPUs (e.g. Intel [[Xeon]]), similar to that of the [[Intel Skulltrail]] motherboard that could accommodate two Intel Core&nbsp;2 Quad processors and has a total of seven PCI-E slots and 12 DDR3 RAM slots. The new design is dubbed "HPTX" and is {{convert|13.6|xx|15|in|mm|0|abbr=on}}.<ref>{{cite web|url=https://www.evga.com/articles/00537/ |title=EVGA Corporation Super Record 2 |publisher=Evga.com |access-date=18 November 2014}}</ref>

== Power supply ==
{{See also|Power supply unit (computer)}}

The ATX specification requires the power supply to produce three main outputs, +3.3&nbsp;V, +5&nbsp;V and +12&nbsp;V. Low-power −12&nbsp;V and +5&nbsp;V<sub>SB</sub> (standby) supplies are also required. The −12&nbsp;V supply is primarily used to provide the negative supply voltage for [[RS-232]] ports and is also used by one pin on [[conventional PCI]] slots primarily to provide a reference voltage for some models of [[sound card]]s. The 5&nbsp;V<sub>SB</sub> supply is used to produce trickle power to provide the soft-power feature of ATX when a PC is turned off, as well as powering the [[real-time clock]] to conserve the charge of the [[CMOS battery]]. A −5&nbsp;V output was originally required because it was supplied on the [[Industry Standard Architecture|ISA]] bus; it was removed in later versions of the ATX standard, as it became obsolete with the removal of the ISA bus expansion slots (the ISA bus itself is still found in any computer which is compatible with the old IBM PC specification).

Originally, the motherboard was powered by one 20-pin connector. An ATX power supply provides a number of peripheral power connectors and (in modern systems) two connectors for the motherboard: an 8-pin (or 4+4-pin) auxiliary connector providing additional power to the CPU and a main 24-pin power supply connector, an extension of the original 20-pin version. 20-pin Molex 39-29-9202 at the motherboard. 20-pin Molex 39-01-2200 at the cable. The connector pin pitch is 4.20&nbsp;mm<ref name="molex">{{cite web |last1=Molex |title=Mini-fit Jr |url=https://www.molex.com/pdm_docs/ps/PS-5556-001-001.pdf |website=www.molex.com |access-date=30 January 2023}}</ref> (about one sixth of an inch).

[[File:ATX PS signals.svg|thumb|right|upright=1.1|Pinouts of ATX&nbsp;2.x motherboard power connectors, 24-pin (top) and four-pin "P4" (bottom), as viewed into mating side of the plugs<ref>{{Cite web
 |url=http://cache-www.intel.com/cd/00/00/52/37/523796_523796.pdf#page=26
 |title=Power Supply Design Guide for Desktop Platform Form Factors, Revision 1.31
 |date=April 2013
 |access-date=February 6, 2015
 |publisher=[[Intel]]
 |format=[[PDF]]
 |page=26
 |url-status=dead
 |archive-url=https://web.archive.org/web/20141021123124/http://cache-www.intel.com/cd/00/00/52/37/523796_523796.pdf
 |archive-date=October 21, 2014
}}</ref>]]
[[File:ATX 20-PIN.png|thumb|ATX 20-PIN]]
[[File:24-pin ATX power plug from a Seasonic PSU 20131105.jpg|thumb|right|upright=1.4|24-pin ATX motherboard power plug; pins 11, 12, 23 and 24 form a detachable separate four-pin plug, making it backward-compatible with 20-pin ATX receptacles]]

{| class="wikitable" style="text-align:center; width:410px;"
|+ 24-pin ATX12V 2.x power supply connector
! Color
! Signal{{efn-ua|name=control|{{color box|lightblue}} Light-blue background denotes control signals.}}
! Pin{{efn-ua|name=conn24|{{color box|lightgreen}} Light-green background denotes the pins present only in the 24-pin connector.}}
! Pin{{efn-ua|name=conn24}}{{efn-ua|In the 20-pin connector, pins 13–22 are numbered 11–20 respectively.}}
! Signal{{efn-ua|name=control}}
! Color
|-
| style="background:orange; color:black" rowspan="2"         | Orange
| style="text-align:right;" rowspan="2"                      | +3.3&nbsp;V
| rowspan="2"                                                | 1
| rowspan="2"                                                | 13
| style="text-align:left;"                                   | +3.3&nbsp;V
| style="background:orange; color:black"                     | Orange
|-
| style="background:lightblue; color:black; text-align:left;"| +3.3&nbsp;V&nbsp;sense{{efn-ua|Supplies +3.3&nbsp;V power and also has a second low-current wire for [[Sense (electronics)|remote sensing]].<ref>{{cite web |title=ATX Specification Version 2.1|url=http://www.formfactors.org/developer/specs/atx2_1.pdf|archive-url=https://web.archive.org/web/20030924224738/http://www.formfactors.org/developer/specs/atx2_1.pdf|archive-date=2003-09-24|date=June 2002|publisher=Intel Corporation|website=formfactors.org}}</ref>}}
| style="background:brown; color:white"                     | Brown
|-
| style="background:orange; color:black"                     | Orange
| style="text-align:right;"                                  | +3.3&nbsp;V
|                                                              2
|                                                              14
| style="text-align:left;"                                   | −12&nbsp;V
| style="background:blue; color:white"                       | Blue
|-
| style="background:black; color:white"                      | Black
| style="text-align:right;"                                  | Ground
|                                                              3
|                                                              15
| style="text-align:left;"                                   | Ground
| style="background:black; color: white"                     | Black
|-
| style="background:red; color:white"                        | Red
| style="text-align:right;"                                  | +5&nbsp;V
|                                                              4
|                                                              16
| style="background:lightblue; color:black; text-align:left;" | Power&nbsp;on{{efn-ua|A control signal that is [[pull-up resistor|pulled up]] to +5&nbsp;V by the PSU and must be driven low to turn on the PSU.}}
| style="background:green; color:white"                      | Green
|-
| style="background:black; color:white"                      | Black
| style="text-align:right;"                                  | Ground
|                                                              5
|                                                              17
| style="text-align:left;"                                   | Ground
| style="background:black; color:white"                      | Black
|-
| style="background:red; color:white"                        | Red
| style="text-align:right;"                                  | +5&nbsp;V
|                                                              6
|                                                              18
| style="text-align:left;"                                   | Ground
| style="background:black; color:white"                      | Black
|-
| style="background:black; color:white"                      | Black
| style="text-align:right;"                                  | Ground
|                                                              7
|                                                              19
| style="text-align:left;"                                   | Ground
| style="background:black; color:white"                      | Black
|-
| style="background:gray; color:black"                       | Grey
| style="text-align:right;background:lightblue; color:black" | [[Power Good Signal|Power&nbsp;good]]{{efn-ua|A control signal that is low when other outputs have not yet reached, or are about to leave, correct voltages.}}
|                                                              8
|                                                              20
| style="text-align:left;"                                   | Reserved{{efn-ua|Formerly −5&nbsp;V ({{color box|white}} white wire), absent in modern power supplies; it was optional in ATX and ATX12V v1.2 and deleted since v1.3.}}
|                                                              {{n/a|None}}
|-
| style="background:purple; color:white"                     | Purple
| style="text-align:right;"                                  | +5&nbsp;V&nbsp;standby
|                                                              9
|                                                            | 21
| style="text-align:left;"                                   | +5&nbsp;V
| style="background:red; color:white"                        | Red
|-
| style="background:yellow; color:black"                     | Yellow
| style="text-align:right;"                                  | +12&nbsp;V
|                                                              10
|                                                              22
| style="text-align:left;"                                   | +5&nbsp;V
| style="background:red; color:white"                        | Red
|-
| style="background:yellow; color:black"                     | Yellow
| style="text-align:right;"                                  | +12&nbsp;V
| style="background:lightgreen; color:black;"                | 11
| style="background:lightgreen; color:black;"                | 23
| style="text-align:left;"                                   | +5&nbsp;V
| style="background:red; color:white"                        | Red
|-
| style="background:orange; color:black"                     | Orange
| style="text-align:right;"                                  | +3.3&nbsp;V
| style="background:lightgreen; color:black;"                | 12
| style="background:lightgreen; color:black;"                | 24
| style="text-align:left;"                                   | Ground
| style="background:black; color:white"                      | Black
|-
| colspan="6" style="text-align:left;" |
{{notelist-ua}}
|}

{{ATX12VO power connector}}

{| class="wikitable" style="text-align:center"
|+ Molex connector part numbers
! Pins !! Female/receptacle <br /> on PS cable !! Male/vertical header <br /> on PCB !! Male/plug <br /> extender cable
|-
| 4-pin || 39-01-2040 || 39-28-1043 || 39-01-2046
|-
| 20-pin || 39-01-2200 || 39-28-1203 || 39-01-2206
|-
| 24-pin || 39-01-2240 || 39-28-1243 || 39-01-2246
|}

Four wires have special functions:
* '''PS_ON#''' (''power on'') is a signal from the motherboard to the power supply. When the line is connected to ground (by the motherboard), the power supply turns on. It is internally pulled up to +5&nbsp;V inside the power supply.<ref name="formfactors_org_atx22"/><ref name="wikihow_com-Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply">{{cite web |title= How to Convert a Computer ATX Power Supply to a Lab Power Supply (with video) - wikiHow |url= http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply |access-date=2013-08-17}} wikihow.com</ref>
* '''PWR_OK''' (''"[[Power Good Signal|power good]]"'') is an output from the power supply that indicates that its output has stabilized and is ready for use. It remains low for a brief time (100–500&nbsp;[[Millisecond|ms]]) after the PS_ON# signal is pulled low.<ref name="pcguide_com-func_SoftPower">{{cite web |work= The PC Guide |first=Charles M. |last=Kozierok |title=Soft Power (Power On and 5V Standby Signals) |url= http://www.pcguide.com/ref/power/sup/func_SoftPower.htm |date=17 April 2001 |access-date= 2013-08-17 |archive-url= https://web.archive.org/web/20130828233419/http://www.pcguide.com/ref/power/sup/func_SoftPower.htm |at=Power Supply Functions and Signals |archive-date= 2013-08-28 |url-status= dead }}</ref>
* '''+5&nbsp;V<sub>SB</sub>''' (''+5&nbsp;V standby'') supplies power even when the rest of the supply wire lines are off. This can be used to power the circuitry that controls the power-on signal.
* '''+3.3&nbsp;V sense''' should be connected to the +3.3&nbsp;V on the motherboard or its power connector. This connection allows [[Four-terminal sensing|remote sensing]] of the voltage drop in the power-supply wiring. Some manufacturers also provided a '''+5&nbsp;V sense''' wire (typically colored pink) connected to one of the red +5&nbsp;V wires on some models of power supply; however, the inclusion of such wire was a non-standard practice and was never part of any official ATX standard.

Generally, supply voltages must be within ±5% of their nominal values at all times. The little-used negative supply voltages, however, have a ±10% tolerance. There is a specification for ripple in a 10&nbsp;Hz–20&nbsp;MHz bandwidth:<ref name="formfactors_org_atx22" />

{| class="wikitable" style="text-align:center"
! Supply (V) !! Tolerance            !! Range, min. to max. (V)  !! Ripple, p. to p., max. (mV)
|-
| +5         ||  ±5% (±0.25&nbsp;V)  ||  +4.75&nbsp;V  to  +5.25 || {{0}}50
|-
| −5         || ±10% (±0.50&nbsp;V)  ||  −4.50&nbsp;V  to  −5.50 || {{0}}50
|-
| +12        ||  ±5% (±0.60&nbsp;V)  || +11.40&nbsp;V  to +12.60 ||     120
|-
| −12        || ±10% (±1.20&nbsp;V)  || −10.80&nbsp;V  to −13.20 ||     120
|-
| +3.3       ||  ±5% (±0.165&nbsp;V) ||  +3.135&nbsp;V to +3.465 || {{0}}50
|-
| +5 standby ||  ±5% (±0.25&nbsp;V)  ||  +4.75&nbsp;V  to  +5.25 || {{0}}50
|}

The 20–24-pin Molex Mini-Fit Jr. has a power rating of, 9&nbsp;amperes maximum per pin).<ref>{{cite web |url=http://www.molex.com/molex/products/family?key=minifit_jr&channel=products&chanName=family&pageTitle=Introduction |title=Mini-Fit Jr. Power Connectors - Molex |publisher=Molex.com |access-date=18 November 2014 |archive-date=21 October 2014 |archive-url=https://web.archive.org/web/20141021133640/http://www.molex.com/molex/products/family?key=minifit_jr&channel=PRODUCTS&chanName=family&pageTitle=Introduction |url-status=dead }}</ref> As large server motherboards and 3D graphics cards have required progressively more and more power to operate, it has been necessary to revise and extend the standard beyond the original 20-pin connector, to allow more current using multiple additional pins in parallel. The low circuit voltage is the restriction on power flow through each connector pin; at the maximum rated voltage, a single Mini-Fit Jr pin would be capable of 4800&nbsp;watts.

=== Physical characteristics ===
ATX power supplies generally have the dimensions of {{convert|150|xx|86|xx|140|mm|in|abbr=on}},<ref name="atx12v"/>{{rp|23–24}} with the width and height being the same as the preceding [[LPX (form factor)|LPX]] (Low Profile eXtension) form factor (which are often incorrectly referred to as "AT" power supplies due to their ubiquitous use in later AT and Baby AT systems, even though the actual AT and Baby AT power supply form factors were physically larger) and share a common mounting layout of four screws arranged on the back side of the unit. That last dimension, the 140&nbsp;mm depth, is frequently varied, with depths of 160, 180, 200 and 230&nbsp;mm used to accommodate higher power, larger fan and/or modular connectors.

=== Main changes from AT and LPX designs ===

==== Power switch ====
Original AT cases (flat case style) have an integrated power switch that protruded from the power supply and
sits flush with a hole in the AT chassis. It utilizes a paddle-style DPST switch and is similar to the PC and PC-XT style power supplies.

Later AT (so-called "Baby AT") and LPX style computer cases have a power button that is directly connected to the system [[computer power supply]] (PSU). The general configuration is a double-pole latching mains voltage switch with the four pins connected to wires from a four-core cable. The wires are either [[solder]]ed to the power button (making it difficult to replace the power supply if it failed) or [[blade connector|blade receptacles]] were used.

[[File:ATX-Netzteil.jpg|thumb|Typical ATX 1.3 [[computer power supply|power supply]]. From left to right, the connectors are 20-pin motherboard, 4-pin "P4 connector", fan RPM monitor (note the lack of a power wire), SATA power connector (black), "Molex connector" and floppy connector.]]
[[File:ATX power supply interior.jpg|thumb|Interior view in an ATX power supply]]
An ATX power supply is typically controlled by an electronic switch connected to the power button on the computer case and allows the computer to be turned off by the [[operating system]]. In addition, many ATX power supplies have a manual switch on the back that also ensures no power is being sent to the components. When the switch on the power supply is turned off, the computer cannot be turned on with the front power button.

==== Power connection to the motherboard ====
The power supply's connection to the motherboard was changed from the older AT and LPX standards; AT and LPX had two similar connectors that could be accidentally interchanged by forcing the different keyed connectors into place, usually causing short-circuits and irreversible damage to the motherboard (the rule of thumb for safe operation was to connect the side-by-side connectors with the black wires together). ATX uses one large, keyed connector which can not be connected incorrectly. The new connector also provides a 3.3&nbsp;volt source, removing the need for motherboards to derive this voltage from the 5&nbsp;V rail. Some motherboards, particularly those manufactured after the introduction of ATX but whilst LPX equipment was still in use, support both LPX and ATX PSUs.<ref>{{cite web |url=http://www.createch.com.tw/images/Backplane/cbp-14p7-2.jpg |archive-url=https://web.archive.org/web/20131213101935/http://www.createch.com.tw/images/Backplane/cbp-14p7-2.jpg |url-status=dead |archive-date=13 December 2013 |title=Example of a motherboard that can support connecting AT and ATX PSUs |publisher=Createch.com |access-date=18 November 2014}}</ref>

If using an ATX PSU for purposes other than powering an ATX motherboard, power can be fully turned on (it is always partly on to operate "wake-up" devices) by shorting the "power-on" pin on the ATX connector (pin 16, green wire) to a black wire (ground), which is what the power button on an ATX system does. A minimum load on one or more voltages may be required (varies by model and vendor); the standard does not specify operation without a minimum load and a conforming PSU may shut down, output incorrect voltages, or otherwise malfunction, but will not be hazardous or damaged.<ref>{{cite web |title=PC Power Supply Requirements and Troubleshooting Problems |url=https://reprap.org/wiki/PC_Power_Supply |publisher=[[RepRap project]] |archive-url=https://web.archive.org/web/20180919080407/https://reprap.org/wiki/PC_Power_Supply |archive-date=September 19, 2018 |date=September 19, 2018 |url-status=live}}</ref> An ATX power supply is not a replacement for a current-limited bench laboratory DC power supply; instead it is better described as a bulk DC [[power supply]].<ref>J. B. Calvert. [https://web.archive.org/web/20171101033821/https://mysite.du.edu/~etuttle/electron/elect62.htm "Using a Computer PSU"]; University of Denver.</ref>

==== Airflow ====
The original ATX specification called for a power supply to be located near to the CPU with the power supply fan drawing in cooling air from outside the chassis and directing it onto the processor. It was thought that in this configuration, cooling of the processor would be achievable without the need of an active heatsink.<ref>{{cite web | url=http://www.intel.com/support/processors/pentium/sb/cs-011025.htm | title=Support for Intel® Processors | access-date=2010-07-17 | archive-date=2008-04-05 | archive-url=https://web.archive.org/web/20080405220142/http://www.intel.com/support/processors/pentium/sb/cs-011025.htm | url-status=dead }}</ref> This recommendation was removed from later specifications; modern ATX power supplies usually exhaust air from the case.

=== {{anchor|0.9|1.0|1.1|2.0|2.01|2.02|2.03|2.1|2.2|}}ATX power supply revisions ===

==== Original ATX ====
ATX, introduced in late 1995, defined three types of power connectors:
* 4-pin "[[Molex connector#Disk drive connector (Molex 8981 Series Power Connector)|Molex connector]]" – transferred directly from AT standard: +5&nbsp;V and +12&nbsp;V for [[Parallel ATA|P-ATA]] hard disks, CD-ROMs, 5.25&nbsp;inch floppy drives and other peripherals.<ref name="pinouts_ru-BigPower_pinout">{{cite web |title=PC peripheral power connector pinout and signals @ pinouts.ru |url=https://pinouts.ru/Power/BigPower_pinout.shtml|publisher=Pinouts.ru |access-date=18 November 2014}}</ref>
* 4-pin [[Berg connector|Berg floppy connector]] – transferred directly from AT standard: +5&nbsp;V and +12&nbsp;V for 3.5&nbsp;inch floppy drives and other peripherals.<ref name="pinouts_ru-SmallPower_pinout">{{cite web |title=PC floppy power connector pinout and signals @ pinouts.ru |url=https://pinouts.ru/Power/SmallPower_pinout.shtml |publisher=Pinouts.ru |access-date=18 November 2014}}</ref>
* 20-pin [[Molex connector#Motherboard|Molex Mini-fit Jr.]] ATX motherboard connector – new to the ATX standard.
* A supplemental 6-pin AUX connector providing additional 3.3&nbsp;V and 5&nbsp;V supplies to the motherboard, if needed. This was used to power the CPU in motherboards with CPU [[voltage regulator module]]s which required 3.3&nbsp;volt and/or 5&nbsp;volt rails and could not get enough power through the regular 20-pin [[Pin header|header]].

The power distribution specification defined that most of the PSU's power should be provided on 5&nbsp;V and 3.3&nbsp;V rails, because most of the electronic components (CPU, RAM, chipset, PCI, AGP and ISA cards) used 5&nbsp;V or 3.3&nbsp;V for power supply. The 12&nbsp;V rail was only used by [[computer fan]]s and motors of peripheral devices (HDD, FDD, CD-ROM, etc.)

==== ATX12V 1.x ====
While designing the Pentium&nbsp;4 platform in 1999/2000, the standard 20-pin ATX power connector was found insufficient to meet increasing power-line requirements; the standard was significantly revised into ATX12V 1.0 (ATX12V 1.x is sometimes inaccurately called ATX-P4). ATX12V 1.x was also adopted by AMD Athlon&nbsp;XP and Athlon&nbsp;64 systems. However, some early model Athlon XP and MP boards (including some server boards) and later model lower-end motherboards do not have the 4-pin connector as described below.

Numbering of the ATX revisions may be a little confusing: ATX refers to the design, and goes up to version 2.2 in 2004 (with the 24 pins of ATX12V 2.0), while ATX12V describes only the PSU.
For instance, ATX 2.03 is quite commonly seen on PSUs from 2000 and 2001 and often includes the P4 12V connector, even if the norm itself did not define it yet.<ref name="formfactors_org_atx22"/>

=====ATX12V 1.0 =====
The main changes and additions in ATX12V 1.0 (released in February 2000) were:
* Increased the power on the 12&nbsp;V rail (power on 5&nbsp;V and 3.3&nbsp;V rails remained mostly the same).
* An extra 4-pin Mini Fit Jr. (Molex 39-01-2040), 12-volt connector to power the CPU.<ref name="atx12v">
{{cite web
|url        = http://www.formfactors.org/developer%5Cspecs%5CATX_ATX12V_PS_1_1.pdf
|title      = AT / ATX12V Power Supply Design Guide Version 1.1
|pages      = 28
|publisher  = Intel Corporation
|access-date = 2011-03-11
|date       = August 2000
|archive-url = https://web.archive.org/web/20101207170858/http://www.formfactors.org/developer/specs/ATX_ATX12V_PS_1_1.pdf
|url-status   = live
|archive-date= 2010-12-07
}}</ref>
* Minimum 68% efficiency at full load.
Formally called the ''+12&nbsp;V Power Connector'', this is commonly referred to as the '''P4 connector''' because this was first needed to support the [[Pentium 4]] processor.

Before the Pentium&nbsp;4, processors were generally powered from the 5&nbsp;V rail. Later processors operate at much lower voltages, typically around 1&nbsp;V and some draw over 100&nbsp;A. It is infeasible to provide power at such low voltages and high currents from a standard system power supply, so the Pentium&nbsp;4 established the practice of generating it with a [[DC-to-DC converter]] on the motherboard next to the processor, powered by the 4-pin 12&nbsp;V connector.

=====ATX12V 1.1=====
This is a minor revision from August 2000. The power on the 3.3&nbsp;V rail was slightly increased and other smaller changes were made.

=====ATX12V 1.2=====
A relatively minor revision from January 2002. The only significant change was that the −5&nbsp;V rail was no longer required (it became optional). This voltage was required by the ISA bus, which is no longer present on almost all modern computers.

=====ATX12V 1.3=====
Introduced in April 2003 (a month after 2.0). This standard introduced some changes, mostly minor. Some of them are:
* Slightly increased the power on 12&nbsp;V rail.
* Increased minimal efficiency for full load from 68% to 70% and defined minimal required PSU efficiencies for light (50% minimal efficiency) and typical (60% minimal efficiency) loads.
* Defined acoustic levels.
* Introduction of Serial ATA power connector (but defined as optional).
* Guidance for the −5&nbsp;V rail was removed (but it was not prohibited).<ref name="ATX12V_1.3">
{{cite web
|url       = http://www.formfactors.org/developer/specs/ATX12V_1_3dg.pdf
|url-status = dead
|archive-url = https://web.archive.org/web/20180417153752/http://www.formfactors.org:80/developer/specs/ATX12V_1_3dg.pdf
|archive-date = 2018-04-17
|title     = ATX12V Power Supply Design Guide Version 1.3
|pages     = 38
|publisher = Intel Corporation
|access-date= 2013-03-24
|date      = April 2003
}}</ref>

==== ATX12V 2.x ====
ATX12V 2.x brought a significant design change regarding power distribution. By analyzing the power demands of then-current PCs, it was determined that it would be much cheaper and more practical to power most PC components from 12&nbsp;V rails, instead of from 3.3&nbsp;V and 5&nbsp;V rails.

In particular, PCI Express expansion cards take much of [[PCI Express#Power|their power]] from the 12&nbsp;V rail (up to 5.5&nbsp;A), while the older [[Accelerated Graphics Port#Power consumption|AGP graphics cards took only]] up to 1&nbsp;A on 12&nbsp;V and up to 6&nbsp;A on 3.3&nbsp;V. The CPU is also driven by a 12&nbsp;V rail, while it was done by a 5&nbsp;V rail on older PCs (before the Pentium&nbsp;4).

=====ATX12V 2.0=====
[[File:Power Supply ATX-450PNF.jpg|thumb|right|ATX-450PNF by FSP Group]]
The power demands of PCI Express were incorporated in ATX12V 2.0 (introduced in February 2003), which defined quite different power distribution from ATX12V 1.x:
* Most power is now provided on 12&nbsp;V rails. The standard specifies that two independent 12&nbsp;V rails (12&nbsp;V<sub>2</sub> for the four-pin connector and 12&nbsp;V<sub>1</sub> for everything else) with independent overcurrent protection are needed to meet the power requirements safely (some very high power PSUs have more than two rails; recommendations for such large PSUs are not given by the standard).
* The power on 3.3&nbsp;V and 5&nbsp;V rails was significantly reduced.
* The ATX motherboard connector was extended to 24 pins.<ref>{{cite book |chapter=4. Server Motherboards and BIOS § Power Supplies and Connectors § ATX Power Supply Standards § ATX/ATX12V Power Supplies and Connectors |title=Upgrading and Repairing Servers |isbn=0-7897-2815-X |date=May 2006 |publisher=Que Publishing |location=Indianapolis |first1=Scott |last1=Mueller |first2=Mark Edward |last2=Soper |first3=Barrie |last3=Sosinsky |quote=The ATX12V version 2.0 power supply standard [...] and the main power connector was enlarged from 20 pins to 24 pins }}</ref> The extra four pins provide one additional 3.3&nbsp;V, 5&nbsp;V and 12&nbsp;V circuit.
* The six-pin AUX connector from ATX12V 1.x was removed because the extra 3.3&nbsp;V and 5&nbsp;V circuits which it provided are now incorporated in the 24-pin ATX motherboard connector.
* The power supply is required to include a [[Serial ATA#Standard connector|Serial ATA power cable]].
* Minimum efficiency of 60% for light load, 70% for typical load, and 70% for full load
* Many other specification changes and additions

=====ATX12V 2.01=====
This is a minor revision from June 2004. An errant reference for the −5&nbsp;V rail was removed. Other minor changes were introduced.<ref name="ATX12V_2.01">{{cite web
 |url          = http://www.formfactors.org/developer/specs/ATX12V%20PSDG2.01.pdf
 |title        = ATX12V Power Supply Design Guide Version 2.01
 |pages        = 44
 |publisher    = Intel Corporation
 |access-date  = 2013-03-24
 |date         = June 2004
 |archive-url  = https://web.archive.org/web/20091122121933/http://www.formfactors.org/developer/specs/ATX12V%20PSDG2.01.pdf
 |archive-date = 2009-11-22
 |url-status   = dead
}}</ref>

=====ATX12V 2.1=====
This is a minor revision from March 2005. The power was slightly increased on all rails. Efficiency requirements changed: minimum 65% at light load, 72% at typical load, and 70% at full load. Introduced recommended minimum values of 75% at light load, 80% at typical load, and 77% at full load.

=====ATX12V 2.2=====
Also released in March 2005<ref name="formfactors_org_atx22" /> it includes corrections and specifies High Current Series wire terminals for 24-pin ATX motherboard and 4-pin +12&nbsp;V power connectors.

=====ATX12V 2.3=====
Effective March 2007. Recommended efficiency was increased to 80% (with at least 70% required) and the 12&nbsp;V minimum load requirement was lowered. Higher efficiency generally results in less power consumption (and less [[waste heat]]) and the 80% recommendation brings supplies in line with new [[Energy star#Home office|Energy Star 4.0]] mandates.<ref>{{cite web |url=http://www.formfactors.org/developer%5Cspecs%5CPSU_DG_rev_1_1.pdf |title=Power Supply Design Guide for Desktop Platform Form Factors, Revision 1.1 |date=March 2007 |author=Intel Corporation |website=Formfactors.org |access-date=18 November 2014 |archive-url=https://web.archive.org/web/20150114105013/http://www.formfactors.org/developer/specs/PSU_DG_rev_1_1.pdf |archive-date=2015-01-14 |url-status=dead }}</ref> The reduced load requirement allows compatibility with processors that draw very little power during startup.<ref>{{cite web |url=https://www.youtube.com/watch?v=_yiNEZWpLZ0 |archive-url=https://ghostarchive.org/varchive/youtube/20211221/_yiNEZWpLZ0 |archive-date=2021-12-21 |url-status=live|title=#138 - Question/Answer: ATX 12V 2.2 vs. ATX 12V 2.3|work=YouTube|date=3 January 2009 |access-date=18 November 2014}}{{cbignore}}</ref> The absolute over-current limit of 240&nbsp;VA per rail was removed, allowing 12&nbsp;V lines to provide more than 20&nbsp;A per rail.

=====ATX12V 2.31=====
This revision became effective in February 2008. It added a maximum allowed ripple/noise specification of 400 millivolts to the PWR_ON and PWR_OK signals, requires that the DC power must hold for more than 1 millisecond after the PWR_OK signal drops, clarified country-specific [[Harmonics (electrical power)|input line harmonic content]] and [[electromagnetic compatibility]] requirements, added a section about Climate Savers, updated recommended power supply configuration charts, and updated the cross-regulation graphs.

=====ATX12V 2.32=====
This is the unofficial name given to the later revisions of the v2.31 specifications, published in May 2020<ref>{{cite web|url=https://www.techpowerup.com/reviews/Antec/HCG-750M/2.html|title=Antec High Current Gamer Modular 750 W Review|date=28 June 2013 }}</ref>

=====ATX12V 2.4=====
The ATX12V 2.4 specifications were published in August 2012.{{citation needed|date=August 2025|reason=rev 1.31 of spec doc is dated April 2013; whence Aug 2012?}} It is specified in Revision 1.31 of the ''Power Supply Design Guide for Desktop Platform Form Factors'', which names this as ATX12V version 2.4.<ref name="2.4">{{cite web |title=Power Supply Design Guide for Desktop Platform Form Factors, Revision 1.31 |url=https://www.intel.com/content/dam/www/public/us/en/documents/guides/power-supply-design-guide.pdf |publisher=[[Intel]] |access-date=April 1, 2018 |archive-url=https://web.archive.org/web/20180401025946/https://www.intel.com/content/dam/www/public/us/en/documents/guides/power-supply-design-guide.pdf |archive-date=April 1, 2018 |date=April 2013 |url-status=dead}}</ref>

In July 2012, revision 1.3 of the ''Power Supply Design Guide for Desktop Platform Form Factors'' made inclusion of the [[Berg connector|floppy drive connector]] optional, instead of mandatory.<ref name="floppy-opt">{{cite book |page=8 |chapter=Revision History |title=Power Supply Design Guide for Desktop Platform Form Factors, Revision 1.31 |date=April 2013 |publisher=[[Intel Corporation]] |url-status=dead |url=https://www.intel.com/content/dam/www/public/us/en/documents/guides/power-supply-design-guide.pdf |archive-url=https://web.archive.org/web/20170718011357/https://www.intel.com/content/dam/www/public/us/en/documents/guides/power-supply-design-guide.pdf |archive-date=2017-07-18 |access-date=2025-07-27 }}</ref>

====={{anchor|ATX12V_2.5}}ATX12V 2.51=====
The specifications for ATXV12 2.51 were released in September 2021 and introduced support for ''Alternative Low Power Mode (ALPM)'' which supersedes the traditional [[Advanced Configuration and Power Interface#Global states|S3 power state]]. Windows 10 implements this functionality as ''[[Modern Standby]]''.<ref name="2.53">{{cite web |title=ATX Multi Rail Desktop Power Supply Design Guide, Revision 003 |url=https://cdrdv2.intel.com/v1/dl/getContent/336521|publisher=[[Intel]]|access-date=February 22, 2021|date=June 2020 |archive-url=https://web.archive.org/web/20210222194448/https://d2pgu9s4sfmw1s.cloudfront.net/UAM/Prod/Done/a062E00001XMVETQA5/b2f2f54c-e980-7c06-4032-9bd4e037c83b?Expires=1614023388&Key-Pair-Id=APKAJKRNIMMSNYXST6UA&Signature=ajw60wVgjLf6SYOG6yYeVAfTpP~KQrb6Op4~6-Wj6qRuqLFh~8vdlMcNZLx3p0M3snMx4lWtMYnTUiAflAAtAD9gU04NhgO5hCmt4O-6IR5Q9ovGG6gupEbWcWkMN5XwxdH8keSILQWnAMv5-JSxfgNS23WVbfwRW45WUPjXe-PQd1aXN-3i6VwftsNuONo52RHrNm8gmFO5vZHHntEqkDlmPbOlxIS8nizMIO6OQCc27APnSl0npO3C-wINfnStA8eLUSdO1ACwwNEBcPjjzqlzh-wz5EhsLwaskKhOmQoGwJRULAaw0T45OyXsHc2dAYk4hpWdhdjPu0FezOVjow__ |archive-date=2021-02-22 |format=PDF }}</ref>

=====ATX12V 2.52=====
The specifications for ATXV12 2.52 were released in October 2021 introduces minor changes to the standard, most notably it requires power supply manufacturers to ensure power supplies with ''Alternative Low Power Mode (ALPM)'' support are able to withstand power cycles every 180 seconds (480 times per day or 175,200 per year). Power supply fans are also recommended to turn on with at least a two second delay for an improved user experience.<ref name="2.52">{{cite web |title=Power Supply Design Guide for Desktop Platform Form Factors Revision 002|url=https://www.intel.com/content/dam/www/public/us/en/documents/guides/power-supply-design-guide-june.pdf |publisher=[[Intel]] |access-date=September 15, 2020 |archive-url=https://web.archive.org/web/20200313220428/https://www.intel.com/content/dam/www/public/us/en/documents/guides/power-supply-design-guide-june.pdf |archive-date=March 13, 2020 |date=June 2018 |url-status=live}}</ref>

This version also stated that the [[Berg connector|floppy drive connector]] should be excluded from PSUs, as opposed to merely being optional, and marked that connector's section in the specification as "For Historical Reference Only".<ref>{{cite book |page=35 |chapter=4.2.2.3 Floppy Drive Connector – Do Not Included{{sic}} (For Historical Reference Only) |title=Desktop Platform Form Factors Power Supply Design Guide, Revision 002 |publisher=[[Intel Corporation]] |date=June 2018 |id=Intel Document Number: 336521 - 002 |url-status=dead |url=https://www.intel.com/content/dam/www/public/us/en/documents/guides/power-supply-design-guide-june.pdf |archive-url=https://web.archive.org/web/20200313220428/https://www.intel.com/content/dam/www/public/us/en/documents/guides/power-supply-design-guide-june.pdf |archive-date=2020-03-13 |access-date=2025-07-27 }}</ref>

=====ATX12V 2.53=====
The specifications for ATXV12 2.53 were released in December 2021 and constitute another minor update to the ATX standard. ATXV12 2.53 makes further recommendations on efficiency and references the [[Energy Star]] ''Computers Specification Version 8.0'' which was finalized in April 2020.<ref name="2.53" /><ref name="energystar8">{{cite web |title=Computers Specification Version 8.0|url=https://www.energystar.gov/products/spec/computers_version_8_0_pd |publisher=[[Energy Star]] |access-date=February 22, 2021 | date=April 2020 |url-status=live|archive-url=https://web.archive.org/web/20180524205744/https://www.energystar.gov/products/spec/computers_version_8_0_pd |archive-date=2018-05-24 }}</ref>

==== {{anchor|3.0|3.01|3.1}}ATX 3.x ====
{{See also|16-pin 12VHPWR connector#Reliability and design changes}}
The specifications for ATX 3.0 were released in February 2022. It includes the new [[16-pin 12VHPWR connector]] that can deliver up to 600&nbsp;W for H+ and up to 675 W in H++ to graphics cards. These incorporate data lines for components to negotiate power capabilities with the PSU so they do not draw more power than the PSU is capable of delivering. The specification also has more strict requirements for handling spikes, two times the nominal output for 100 microseconds.

The −12&nbsp;V supply is now specified as optional on the ATX motherboard connector,<ref>{{cite web | title=DC Voltage Regulation – REQUIRED - 2.1a - ID:336521 &#124; ATX Version 3 Multi Rail Desktop Platform Power Supply | url=https://edc.intel.com/content/www/us/en/design/ipla/software-development-platforms/client/platforms/alder-lake-desktop/atx-version-3-0-multi-rail-desktop-platform-power-supply-design-guide/2.0/2.1a/dc-voltage-regulation-required/ }}</ref> and some PSU manufacturers (e.g. Corsair) have begun removing the −12&nbsp;V supply from their PSUs. However, as of 2025, some motherboards (e.g. recent designs from ASRock) still use the −12&nbsp;V supply - typically to power the amplifier for on-board audio - and functions that use it will not work as expected<ref>{{cite web | title=ATX 3.1 PSU causes low sound volume | date=26 November 2024 | url=https://forum.corsair.com/forums/topic/192392-atx-31-psu-causes-low-sound-volume/ }}</ref> when paired with a PSU that does not provide a −12&nbsp;V supply.{{citation needed|date=August 2025}}

ATX 3.1 replaces the 12VHPWR power connector with the 12V-2x6 power connector and reduces required power loss hold-up time from 17 milliseconds to 12 milliseconds.<ref>{{cite web |author=<!-- not stated --> |date= |title=ATX 3.0 vs ATX 3.1: What's the Difference? |url=https://www.corsair.com/us/en/explorer/diy-builder/power-supply-units/atx-30-vs-atx-31-whats-the-difference/ |website= |location= |publisher=[[Corsair Gaming]] |access-date=January 25, 2025}}</ref>

=== ATX power supply derivatives ===

==== ATX12VO{{Anchor|ATX12VO}} ====
{{Further|Power supply unit (computer)#12 V-only supplies}}
Standing for '''ATX 12-volt-only''', this is a new specification published by Intel in 2019, aimed at pre-built systems in the first run, and stricter power efficiency requirements by the [[California Energy Commission]] going into effect in 2021.<ref>{{Cite web|url=https://www.pcworld.com/article/3518831/how-intels-changing-the-future-of-power-supplies-with-its-atx12vo-spec.html|title=How Intel is changing the future of power supplies with its ATX12VO spec|date=2020-03-09|website=[[PC World]]|first=Gordon |last=Mah Ung|language=en|access-date=2020-04-13}}</ref> Several OEMs were already using a similar design with proprietary connectors and this effectively standardizes those.<ref name=":0">{{Cite web|url=https://www.gamersnexus.net/guides/3568-intel-atx-12vo-spec-explained-what-manufacturers-think|title=Intel ATX12VO vs. 12V Spec Explained & What Manufacturers Think|last=Lathan|first=Patrick|date=2020-04-10|website=GamersNexus|access-date=2020-04-13}}</ref>

Under this standard, power supplies provide only a 12&nbsp;V output. ATX12VO introduces a new 10-pin connector to supply the motherboard, replacing the 24-pin ATX12V connector. This greatly simplifies power supplies, but moves [[DC-to-DC conversion]] and some connectors to the motherboard instead. Notably, [[SATA power connector]]s, which include 3.3&nbsp;V and 5&nbsp;V pins, need to move to the motherboard instead of being connected directly to the power supply.<ref name=":0" />

a. Connector pinout per Intel ATX12VO (12V Only) Desktop Power Supply Design Guide.<ref>{{Cite web |title=DC Connectors - 2.0 - ID:613768 {{!}} ATX12VO (12V Only) Desktop Power Supply |url=https://edc.intel.com/content/www/us/en/design/products-and-solutions/processors-and-chipsets/alder-lake-s/atx12vo-12v-only-desktop-power-supply-design-guide/2.0/dc-connectors/ |access-date=2022-11-24 |website=edc.intel.com}}</ref>
{| class="wikitable"
|-
! Pin !! Signal      !! Colour
! Pin !! Signal      !! Colour
|-
|  5  || I_PSU%  || style="background-color:blue; color:white;" | Blue
| 10  || 12&nbsp;V [12V Sensing Pin]   || style="background-color:yellow; color:black;" | Yellow
|-
|  4  || GND         || style="background-color:black; color:white;"  | Black
|  9  || 12&nbsp;V   || style="background-color:yellow; color:black;" | Yellow
|-
|  3  || GND         || style="background-color:black; color:white;"  | Black
|  8  || 12&nbsp;V   || style="background-color:yellow; color:black;"  | Yellow
|-
|  2  || GND         || style="background-color:black; color:white;" | Black
|  7  || 12&nbsp;V SB || style="background-color:purple; color:white;"  | Purple
|-
|  1  || PS_ON#      || style="background-color:green; color:white;" | Green
|  6  || PWR_OK      || style="background-color:gray; color:white;" | Gray
|}

==== SFX ====
[[File:SFX PC-PSU Delta IMG 9760.JPG|thumb|An SFX power supply unit]]

SFX (Small Form-Factor eXtended)<ref>{{cite web |title=How to choose the best PSU for a Gaming Computer IN 2023? |url=https://tictech.co.in/how-to-choose-the-best-psu-for-a-gaming-computer-in-2023/ |website=Tic Tech |date=27 April 2023 |access-date=12 August 2023}}</ref> is merely a design for a [[Small form factor (desktop and motherboard)|small form factor]] (SFF) power supply casing (such as those using microATX, FlexATX, nano-ITX, mini-ITX, and [[NLX (motherboard form factor)|NLX]]), with the power specifications almost identical to ATX. Thus, an SFX power supply is mostly [[pin-compatible]] with the ATX power supply as the main difference is its reduced dimensions; the only electrical difference is that the SFX specifications do not require the −5&nbsp;V rail. Since −5&nbsp;V is required only by some ISA-bus expansion cards, this is not an issue with modern hardware and decreases productions costs. As a result, ATX pin 20, which carried −5&nbsp;V, is absent in current power supplies; it was optional in ATX and ATX12V version 1.2 and deleted as of ATX version 1.3.

SFX has dimensions of 125&nbsp;× 63.5&nbsp;× 100&nbsp;mm (width&nbsp;× height&nbsp;× depth), with a 60&nbsp;mm fan, compared with the standard ATX dimensions of 150&nbsp;× 86&nbsp;× 140&nbsp;mm. Optional 80 or 40&nbsp;mm fan replacement increases or decreases the height of an SFX unit.<ref>{{cite web |url=http://www.pcguide.com/ref/power/sup/formSFX-c.html |archive-url=https://web.archive.org/web/20010211212656/http://www.pcguide.com/ref/power/sup/formSFX-c.html |url-status=dead |archive-date=11 February 2001 |title=SFX Form Factor |website=The PC Guide |first=Charles M. |last=Kozierok |date=18 December 2000 |access-date=18 November 2014}}</ref>

Some manufacturers and retailers incorrectly market SFX power supplies as μATX or MicroATX power supplies.<ref>{{Cite web |url=https://silverstonetek.com/techtalk_cont.php?tid=10055 |title=List of computer PSU form factors - SilverStone Technology Co., Ltd. |access-date=2019-02-09 |archive-date=2019-02-09 |archive-url=https://web.archive.org/web/20190209180534/https://silverstonetek.com/techtalk_cont.php?tid=10055 |url-status=dead }}</ref>

Some manufacturers make SFX-L dimensions of 125&nbsp;× 63.5&nbsp;× 130&nbsp;mm to accommodate a 120&nbsp;mm fan.<ref>[https://smallformfactor.net/news/sfx-l-computex-2017-roundup SFX-L Computex 2017 roundup - Small Form Factor Network]</ref>

==== TFX ====
[[File:TFX power supply unit.jpg|thumb|A TFX power supply unit]]

Thin Form Factor is another small power supply design with standard ATX specification connectors. Its dimensions are generally (W × H × D): 85&nbsp;× 64&nbsp;× 175&nbsp;mm (3.34&nbsp;× 2.52&nbsp;× 6.89&nbsp;in).<ref>[https://web.archive.org/web/20120112021532/http://www.tomshardware.co.uk/power-supply-specifications-atx-reference,review-32338-5.html Modern Form Factors: EPS, TFX, CFX, LFX, And Flex ATX - Power Supply 101: A Reference Of Specifications]</ref><ref>[https://scan.co.uk/tekspek/power-supplies/psus-(power-supply-units) Power Supplies TekSpek Guide - SCAN UK]</ref>

==== WTX ====
WTX power supplies provide a [[WTX (form factor)|WTX]] style motherboard connector which is incompatible with the standard ATX motherboard connector.

==== AMD GES ====
This is an ATX12V power supply derivative made by [[AMD]] to power its Athlon MP (dual processor) platform. It was used only on high-end Athlon MP motherboards. It has a special 8-pin supplemental connector for motherboard, so an AMD GES PSU is required for such motherboards (those motherboards will not work with ATX(12&nbsp;V) PSUs).

a. ATX12V-GES 24-pin P1 motherboard connector. The pinout on the motherboard connector is as follows when viewing the motherboard from above:
{| class="wikitable"
|-
! Pin !! Signal      !! Colour
! Pin !! Signal      !! Colour
|-
| 12  || 12&nbsp;V   || style="background-color:yellow; color:black;" | Yellow
| 24  || 12&nbsp;V   || style="background-color:yellow; color:black;" | Yellow
|-
| 11  || 12&nbsp;V   || style="background-color:yellow; color:black;" | Yellow
| 23  || GND         || style="background-color:black; color:white;"  | Black
|-
| 10  || GND         || style="background-color:black; color:white;"  | Black
| 22  || GND         || style="background-color:black; color:white;"  | Black
|-
|  9  || GND         || style="background-color:black; color:white;"  | Black
| 21  || 3.3&nbsp;V  || style="background-color:orange; color:black;" | Orange
|-
|  8  || 3.3&nbsp;V  || style="background-color:orange; color:black;" | Orange
| 20  || 3.3&nbsp;V  || style="background-color:orange; color:black;" | Orange
|-
|  7  || 3.3&nbsp;V  || style="background-color:orange; color:black;" | Orange
| 19  || 3.3&nbsp;V  || style="background-color:orange; color:black;" | Orange
|-
|  6  || GND         || style="background-color:black; color:white;"  | Black
| 18  || GND         || style="background-color:black; color:white;"  | Black
|-
|  5  || PS_ON_N     || style="background-color:green; color:white;"  | Green
| 17  || −12&nbsp;V  || style="background-color:blue; color:white;"   | Blue
|-
|  4  || GND         || style="background-color:black; color:white;"  | Black
| 16  || 5&nbsp;V SB || style="background-color:purple; color:white;" | Purple
|-
|  3  || GND         || style="background-color:black; color:white;"  | Black
| 15  || GND         || style="background-color:black; color:white;"  | Black
|-
|  2  || 5&nbsp;V    || style="background-color:red; color:white;"    | Red
| 14  || 5&nbsp;V    || style="background-color:red; color:white;"    | Red
|-
|  1  || 5&nbsp;V    || style="background-color:red; color:white;"    | Red
| 13  || 5&nbsp;V    || style="background-color:red; color:white;"    | Red
|}

b. ATX12V-GES 8-pin P2 motherboard connector. This pinout on the motherboard connector is as follows when viewing the motherboard from above:
{| class="wikitable"
|-
! Pin !! Signal    !! Colour
! Pin !! Signal    !! Colour
|-
| 4   || GND       || style="background-color:black; color:white;"  | Black
| 8   || 12&nbsp;V || style="background-color:yellow; color:black;" | Yellow striped black
|-
| 3   || GND       || style="background-color:black; color:white;"  | Black
| 7   || 12&nbsp;V || style="background-color:yellow; color:black;" | Yellow striped black
|-
| 2   || PWR_OK    || style="background-color:gray; color:black;"   | Gray
| 6   || 12&nbsp;V || style="background-color:yellow; color:black;" | Yellow striped black
|-
| 1   || 5&nbsp;V  || style="background-color:red; color:white;"    | Red
| 5   || GND       || style="background-color:black; color:white;"  | Black
|}

==== EPS12V ====
[[EPS12V]] is defined in Server System Infrastructure (SSI) and used primarily by [[symmetric multiprocessing|SMP]]/multi-core systems such as [[Core 2]], [[Core i7]], [[Opteron]] and [[Xeon]]. It has a 24-pin ATX motherboard connector (same as ATX12V v2.x), an 8-pin secondary connector and an optional 4-pin tertiary connector. Rather than include the extra cable, many power supply makers implement the 8-pin connector as two combinable 4-pin connectors to ensure backwards compatibility with ATX12V motherboards.

=== Specification changes and additions ===
High-performance video card power demands dramatically increased during the 2000s and some high-end graphics cards have power demands that exceed [[Accelerated Graphics Port|AGP]] or [[PCI Express|PCIe]] slot capabilities. For these cards, supplementary power was delivered through a standard 4-pin peripheral or floppy power connector. Midrange and high-end PCIe graphics cards manufactured after 2004 typically use a standard [[PCI Express#6- and 8-pin power connectors|6- or 8-pin PCIe power connector]] directly from the PSU.

=== Interchanging PSUs ===
Although the ATX power supply specifications are mostly vertically compatible in both ways (both electrically and physically), there are potential issues with mixing old motherboards/systems with new PSUs and vice versa. The main issues to consider are the following:
* The power allocation between 3.3&nbsp;V, 5&nbsp;V and 12&nbsp;V [[power supply rail|rails]] is very different between older and newer ATX PSU designs, as well as between older and newer PC system designs.
* Older PSUs may not have connectors which are required for newer PC systems to properly operate.
* Newer systems generally have higher power requirements than older systems.

This is a practical guidance what to mix and what not to mix:
* Older systems (before Pentium&nbsp;4 and Athlon&nbsp;XP platforms) were designed to draw most power from 5&nbsp;V and 3.3&nbsp;V rails.
* Because of the DC-DC converters on the motherboard that convert 12&nbsp;V to the low voltages required by the Intel Pentium 4 and AMD Athlon&nbsp;XP (and subsequent) processors, such systems draw most of their power from the 12&nbsp;V rail.
* Original ATX PSUs have power distribution designed for pre-P4/XP PCs. They lack the supplemental 4-pin 12-volt CPU power connector, so they most likely cannot be used with P4/XP or newer motherboards. Adapters do exist but power drain on the 12&nbsp;V rail must be checked very carefully. There is a chance it can work without connecting the 4-pin 12&nbsp;V connector, but caution is advised.<ref>{{cite web |url=http://www.neoseeker.com/Hardware/faqs/kb/12,17.html |title= Hardware Knowledgebase - do I absolutely HAVE to plug in the ATX12V 4pin connector on P4 motherboards? - HardwareFAQs: Powered by neofaq|website=www.neoseeker.com |archive-url=https://web.archive.org/web/20091003092553/http://www.neoseeker.com/Hardware/faqs/kb/12%2C17.html |archive-date=October 3, 2009}}</ref>
* ATX12V 1.x PSUs have power distribution designed for P4/XP PCs, but they are also greatly suitable for older PCs, since they give plenty of power (relative to old PCs' needs) both on 12&nbsp;V and on 5&nbsp;V/3.3&nbsp;V. It is not recommended to use ATX12V 1.x PSUs on ATX12V 2.x motherboards because those systems require much more power on 12&nbsp;V than ATX12V 1.x PSUs provide.
* ATX12V 2.x PSUs have power distribution designed for late P4/XP PCs and for Athlon 64 and Core Duo PCs. They can be used with earlier P4/XP PCs, but the power distribution will be significantly suboptimal, so a more powerful ATX12V 2.0 PSU should be used to compensate for that discrepancy. ATX12V 2.x PSUs can also be used with pre-P4/XP systems, but the power distribution will be greatly suboptimal (12&nbsp;V rails will be mostly unused, while the 3.3&nbsp;V/5&nbsp;V rails will be overloaded), so this is not recommended.
* Systems that use an ISA bus should have a PSU that provides the −5&nbsp;V rail, which became optional in ATX12V 1.2 and was subsequently phased out by manufacturers.

Some proprietary brand-name systems require a matching proprietary power supply, but some of them may also support standard and interchangeable power supplies.

=== Efficiency ===
{{See also|Green computing|80 Plus}}

Efficiency in power supplies means the extent to which power is not wasted in converting [[electricity]] from a household supply to regulated [[direct current|DC]]. Computer power supplies vary from around 70% to over 90% efficiency.

Various initiatives exist to improve the efficiency of computer power supplies. [[Climate Savers Computing Initiative]] promotes energy saving and reduction of greenhouse gas emissions by encouraging development and use of more efficient power supplies. [[80 PLUS]] certifies a variety of efficiency levels for power supplies and encourages their use via financial incentives. Efficient power supplies also save money by wasting less power; as a result they use less electricity to power the same computer, and they emit less waste heat which results in significant energy savings on central air conditioning in the summer. The gains of using an efficient power supply are more substantial in computers that use a lot of power.

Although a power supply with a larger than needed power rating will have an extra margin of safety against overloading, such a unit is often less efficient and wastes more electricity at lower loads than a more appropriately sized unit. For example, a 900-watt power supply with the [[80 Plus Silver]] efficiency rating (which means that such a power supply is designed to be at least 85 percent efficient for loads above 180&nbsp;W) may only be 73% efficient when the load is lower than 100&nbsp;W, which is a typical idle power for a desktop computer. Thus, for a 100&nbsp;W load, losses for this supply would be 27&nbsp;W; if the same power supply was put under a 450&nbsp;W load, for which the supply's efficiency peaks at 89%, the loss would be only 56&nbsp;W despite supplying 4.5 times the useful power.<ref>{{cite web
 | url = http://www.anandtech.com/show/2624/3
 | archive-url = https://web.archive.org/web/20110623044340/http://www.anandtech.com/show/2624/3
 | url-status = dead
 | archive-date = June 23, 2011
 | title = Debunking Power Supply Myths
 | date = 2008-09-22 | access-date = 2014-10-07
 | author = Christoph Katzer | publisher = [[AnandTech]]
 | page = 3
}}</ref><ref>{{cite web
 | url = http://www.coolermaster.com/xresserver01-DLFILE-P130218025925ba-F13032500212140.html
 | title = Cooler Master UCP Product Sheet
 | year = 2008
 | access-date = 2014-10-11
 | publisher = [[Cooler Master]]
 | format = PDF
 | archive-date = 2014-07-28
 | archive-url = https://web.archive.org/web/20140728095533/http://www.coolermaster.com/xresserver01-DLFILE-P130218025925ba-F13032500212140.html
 | url-status = dead
 }}</ref> For a comparison, a 500-watt power supply carrying the [[80 Plus Bronze]] efficiency rating (which means that such a power supply is designed to be at least 82-percent efficient for loads above 100&nbsp;W) may provide an 84 percent efficiency for a 100&nbsp;W load, wasting only 19&nbsp;W.<ref>{{cite web
 | url = http://www.anandtech.com/show/4908/silverstone-strider-plus-500w-modular-power/4
 | archive-url = https://archive.today/20141011031907/http://www.anandtech.com/show/4908/silverstone-strider-plus-500w-modular-power/4
 | url-status = dead
 | archive-date = October 11, 2014
 | title = SilverStone Strider Plus – 500&nbsp;W Modular Power
 | date = 2011-10-10 | access-date = 2014-10-11
 | author = Martin Kaffei | publisher = [[AnandTech]]
 | page = 4
}}</ref>

== See also ==
{{Portal|Electronics}}
* [[AT (form factor)]]
* [[BTX (form factor)]]
* [[Computer form factor]]
* [[Mini-ITX]]
* [[PC 97]]
* [[Power supply unit (computer)]]
* [[SSI CEB]]

== Explanatory notes ==
{{notelist}}

== References ==
{{Reflist|colwidth=30em}}

== Further reading ==
* {{cite web |title=Power Supply Form Factors: ATX (NLX) Form Factor |first=Charles M. |last=Kozierok |date=August 26, 2000 |work=The PC Guide |url-status=dead |url=http://www.pcguide.com/ref/power/sup/form_ATX.htm |archive-url=https://web.archive.org/web/20000901041651/http://www.pcguide.com/ref/power/sup/form_ATX.htm |archive-date=2000-09-01 }}

=== ATX specifications ===
* {{cite web |date=February 1996 |title=ATX specification, Release 1.1 |publisher=[[Intel Corporation]] |via=Datasheet Archive |url=https://www.datasheetarchive.com/datasheet/4ff28002d806bffa |url-status=live |archive-url=https://web.archive.org/web/20250726023511/https://datasheet.datasheetarchive.com/originals/library/Datasheet-078/DSAE0073611.pdf |archive-date=2025-07-26 }}
* {{cite web |date=December 1998 |title=ATX Specification, Version 2.03 |publisher=[[Intel Corporation]] |url-status=dead |url=http://www.formfactors.org/developer/specs/atx/atx2_03p1.pdf |archive-url=https://web.archive.org/web/20030405103115/http://www.formfactors.org/developer/specs/atx/atx2_03p1.pdf |archive-date=2003-04-05 }} Has ECR# P1 appended.
* {{cite web |date=May 8, 2000 |title=ECR# P1, ATX12V Connector Requirement |website=Platform Development Support Web Site |url-status=dead |url=http://www.teleport.com/~ffsupprt/spec/atx/ecr_p1.pdf |archive-url=https://web.archive.org/web/20000816024333/http://www.teleport.com/~ffsupprt/spec/atx/ecr_p1.pdf |archive-date=2000-08-16 }} Engineering change request applicable to ''ATX Specification, Version 2.03''.
* {{cite web |date=June 2002 |title=ATX Specification - Version 2.1 |publisher=[[Intel Corporation]] |url=http://www.formfactors.org/developer/specs/atx2_1.pdf |url-status=dead |archive-url=https://web.archive.org/web/20030924224738/http://www.formfactors.org/developer/specs/atx2_1.pdf |archive-date=2003-09-24 }}
* {{cite web |date=2004 |title=ATX Specification - Version 2.2 |publisher=[[Intel Corporation]] |url=http://www.formfactors.org/developer/specs/atx2_2.pdf |url-status=dead |archive-url=https://web.archive.org/web/20120725150314/http://www.formfactors.org/developer/specs/atx2_2.pdf |archive-date=2012-07-25 }}

=== ATX-related power supply design guides ===
* {{cite web |date=August 2000 |title=ATX/ATX12V Power Supply Design Guide, Version 1.1 |publisher=[[Intel Corporation]] |url-status=dead |url=http://www.formfactors.org/developer/specs/atx/ATX_ATX12V_PS_1_1.pdf |archive-url=https://web.archive.org/web/20030424061333/http://www.formfactors.org/developer/specs/atx/ATX_ATX12V_PS_1_1.pdf |archive-date=2003-04-24 }} Complies with ''ATX Specification, Version 2.03''.
* {{cite web |date=April 2003 |title=ATX12V Power Supply Design Guide, Version 1.3 |publisher=[[Intel Corporation]] |url-status=live |url=https://community.intel.com/cipcp26785/attachments/cipcp26785/intel-optane-ssd/2893/1/developer_specs_ATX12V_1_3dg.pdf |archive-url=https://web.archive.org/web/20240922205710/https://community.intel.com/cipcp26785/attachments/cipcp26785/intel-optane-ssd/2893/1/developer_specs_ATX12V_1_3dg.pdf |archive-date=2024-09-22 }} Complies with ''ATX Specification, Version 2.03''.
* {{cite web |date=June 2004 |title=ATX12V Power Supply Design Guide, Version 2.01 |publisher=[[Intel Corporation]] |url=http://www.formfactors.org/developer/specs/ATX12V%20PSDG2.01.pdf |url-status=dead |archive-url=https://web.archive.org/web/20091122121933/http://www.formfactors.org/developer/specs/ATX12V%20PSDG2.01.pdf |archive-date=2009-11-22 }} Complies with ''ATX Specification, Version 2.03''.
* {{cite web |date=March 2005 |title=ATX12V Power Supply Design Guide, Version 2.2 |publisher=[[Intel Corporation]] |url=http://www.formfactors.org/developer/specs/ATX12V_PSDG_2_2_public_br2.pdf |url-status=dead |archive-url=https://web.archive.org/web/20080920191034/http://www.formfactors.org/developer/specs/ATX12V_PSDG_2_2_public_br2.pdf |archive-date=2008-09-20 }} Complies with ''ATX Specification, Version 2.03''.
* {{cite web |date=March 2007 |title=Power Supply Design Guide for Desktop Platform Form Factors, Revision 1.1 |publisher=[[Intel Corporation]] |url=http://www.formfactors.org/developer/specs/PSU_DG_rev_1_1.pdf |url-status=dead |archive-url=https://web.archive.org/web/20150114105013/http://www.formfactors.org/developer/specs/PSU_DG_rev_1_1.pdf |archive-date=2015-01-14 }} Includes ''ATX12V Specific Guidelines 2.3''.
* {{cite web |date=February 2008 |title=Power Supply Design Guide for Desktop Platform Form Factors, Revision 1.2 |publisher=[[Intel Corporation]] |url=http://www.formfactors.org/developer/specs/Power_Supply_Design_Guide_Desktop_Platform_Rev_1_2.pdf |url-status=dead |archive-url=https://web.archive.org/web/20160414133738/http://www.formfactors.org/developer/specs/Power_Supply_Design_Guide_Desktop_Platform_Rev_1_2.pdf |archive-date=2016-04-14 }} Includes ''ATX12V Specific Guidelines 2.31''.
* {{cite web |date=April 2013 |title=Power Supply Design Guide for Desktop Platform Form Factors, Revision 1.31 |publisher=[[Intel Corporation]] |url=https://www.intel.com/content/dam/www/public/us/en/documents/guides/power-supply-design-guide.pdf |url-status=dead |archive-url=https://web.archive.org/web/20170718011357/https://www.intel.com/content/dam/www/public/us/en/documents/guides/power-supply-design-guide.pdf |archive-date=2017-07-18 }} Includes ''ATX12V Specific Guidelines 2.4''.
* {{ cite web |date=June 2018 |title=Desktop Platform Form Factors Power Supply Design Guide, Revision 002 |publisher=[[Intel Corporation]] |url=https://www.intel.com/content/dam/www/public/us/en/documents/guides/power-supply-design-guide-june.pdf |url-status=dead |archive-url=https://web.archive.org/web/20200313220428/https://www.intel.com/content/dam/www/public/us/en/documents/guides/power-supply-design-guide-june.pdf |archive-date=2020-03-13 |id=Intel Document Number 336521 - 002 }} Includes ''ATX12V Specific Guidelines 2.52''.
* {{cite web |date=June 2020 |title=ATX Multi Rail Desktop Power Supply Design Guide, Revision 003 |publisher=[[Intel Corporation]] |via=[[Wayback Machine]] |url=https://d2pgu9s4sfmw1s.cloudfront.net/UAM/Prod/Done/a062E00001XMVETQA5/b2f2f54c-e980-7c06-4032-9bd4e037c83b?Expires=1614026599&Key-Pair-Id=APKAJKRNIMMSNYXST6UA&Signature=iVsGBI7emsEdrFsQ67~1S2M90n5TYe3~QRV4TUX1VC-EseakcayGKQeIrDqA~ls21nyFEuSEcp9TwYA2uYXHQRfrjsYAhPvPaKLNivtX25XWNG2Bqr1eajUeZY6SobcaUhqU8I1coBOXz6TzmLMmgRiy4awrNQnBFtagJsJRME50wE8zd8HrBmgITOGB7dXv9IFX3YCMPpx1PXaubRAcfm9tsS71Sn6xf2n2rSizwWA-JjXuYK6sZhXKNNzn-7FtBtRJoyx~uOnWPtvxE2i-O1X1cFFpnhq3wDn1Kubh-qttzKiRYs0pnXElqDpH4i9TFOFBCR-g2dmtm6USEUgBBA__ |id=Intel Document Number 336521 - 003 |archive-url=https://web.archive.org/web/20210222203819/https://d2pgu9s4sfmw1s.cloudfront.net/UAM/Prod/Done/a062E00001XMVETQA5/b2f2f54c-e980-7c06-4032-9bd4e037c83b?Expires=1614026599&Key-Pair-Id=APKAJKRNIMMSNYXST6UA&Signature=iVsGBI7emsEdrFsQ67~1S2M90n5TYe3~QRV4TUX1VC-EseakcayGKQeIrDqA~ls21nyFEuSEcp9TwYA2uYXHQRfrjsYAhPvPaKLNivtX25XWNG2Bqr1eajUeZY6SobcaUhqU8I1coBOXz6TzmLMmgRiy4awrNQnBFtagJsJRME50wE8zd8HrBmgITOGB7dXv9IFX3YCMPpx1PXaubRAcfm9tsS71Sn6xf2n2rSizwWA-JjXuYK6sZhXKNNzn-7FtBtRJoyx~uOnWPtvxE2i-O1X1cFFpnhq3wDn1Kubh-qttzKiRYs0pnXElqDpH4i9TFOFBCR-g2dmtm6USEUgBBA__ |archive-date=2021-02-22 }} Includes ''ATX12V Specific Guidelines 2.53''.
* {{cite web |date=February 2022 |title=ATX Version 3.0 Multi Rail Desktop Platform Power Supply Design Guide, Revision 2.0 |publisher=[[Intel Corporation]] |url-status=live |url=https://www.cybenetics.com/attachs/52.pdf |via=Cybenetics Ltd |archive-url=https://web.archive.org/web/20250528152752/https://www.cybenetics.com/attachs/52.pdf |archive-date=2025-05-28 |id=Intel Document Number 336521 }} Includes ''ATX12V Specific Guidelines 3.0''; aligns to ''ATX Specification Version 3.0''.
* {{cite web |date=November 1, 2023 |title=ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide, Revision 2.1a |publisher=[[Intel Corporation]] |url-status=live |url=https://edc.intel.com/content/www/us/en/design/ipla/software-development-platforms/client/platforms/alder-lake-desktop/atx-version-3-0-multi-rail-desktop-platform-power-supply-design-guide/2.1a/ |archive-url=https://web.archive.org/web/20250214053259/https://cdrdv2-public.intel.com/336521/336521_Rev2p1a.pdf |archive-date=2025-02-14 |id=Intel Document Number 336521-2.1a }} Includes ''ATX12V Specific Guidelines 3.1''; aligns to ''ATX Specification Version 3.1''.

=== EPS12V ===
* {{cite web |title=EPS12V Power Supply Design Guide, Version 2.0 |date=24 September 2002 |publisher=Server System Infrastructure Forum |via=Power Sources Unlimited, Inc. |url-status=dead |url=http://www.psui.com/1upower/pdf/892_ref.pdf |archive-url=https://web.archive.org/web/20061114023234/http://www.psui.com/1upower/pdf/892_ref.pdf |archive-date=2006-11-14 }}
* {{cite web |title=EPS12V Power Supply Design Guide, Version 2.91 |date=2004 |publisher=Server System Infrastructure Forum |via=PC Power and Cooling, Inc. |url-status=dead |url=http://www.pcpower.com/downloads/EPS12VSpec2_91.pdf |archive-url=https://web.archive.org/web/20110707162514/http://www.pcpower.com/downloads/EPS12VSpec2_91.pdf |archive-date=2011-07-07 }}
* {{cite web |title=EPS12V Power Supply Design Guide, Version 2.92 |date=2004 |publisher=Server System Infrastructure Forum |url-status=dead |via=Enermax |url=http://www.enermax.cn/enermax_pdf/EPS12V%20Spec2_92.pdf |archive-url=https://web.archive.org/web/20201205140420/http://www.enermax.cn/enermax_pdf/EPS12V%20Spec2_92.pdf |archive-date=2020-12-05 }}
* {{cite web |title=Power Supply Design Guideline for 2008 Dual-Socket Servers and Workstations, Version 1.2.1 |date=2012 |publisher=Server System Infrastructure Forum |via=[[Storage Networking Industry Association]] |url-status=live |url=https://www.snia.org/sites/default/files/SSIF/2018-05-31/SSI%20PSDG%202008%20Version%201.2.1.pdf |archive-url=https://web.archive.org/web/20240405093523/https://www.snia.org/sites/default/files/SSIF/2018-05-31/SSI%20PSDG%202008%20Version%201.2.1.pdf |archive-date=2024-04-05 }} Includes ''EPS12V Specific Guidelines''.

== External links ==
{{Commons category}}

* [https://www.playtool.com/pages/psuconnectors/connectors.html Various power supply cables and connectors]
* [https://www.playtool.com/pages/psurailhistory/rails.html A short history of power supply voltage rails]
* [https://pinouts.ru/pin_Power.shtml ATX power supply connectors with pinouts]
* [http://www.hardwarebook.info/Category:Connector#Power_Supply More ATX power supply connectors with pinouts]{{dead link|date=August 2025}}
* [https://www.nipron.co.jp/english/product_info/encyclopedia/1_5.htm ATX Power Supply Terminology]

{{Computer form factors}}
{{Foxconn}}

{{DEFAULTSORT:Atx}}
[[Category:IBM PC compatibles]]
[[Category:Motherboard form factors]]
[[Category:Computer-related introductions in 1995]]