Power Supply 101: A Reference Of Specifications

Power Supply Form Factors

The shape and general physical layout of a component is called the form factor. Items that share a form factor are generally interchangeable, at least as far as their sizes and fits are concerned. When designing a PC, the engineers can choose to use one of the popular standard PSU form factors, or they can elect to build their own custom design. Choosing the former means that a virtually inexhaustible supply of inexpensive replacement parts will be available in a variety of quality and power output levels. Going the custom route means additional time and expense for development. In addition, the power supply is unique to the system and generally available for replacement only from the original manufacturer. This precludes any upgrades as well, such as installing higher-output replacement models.

If you can’t tell already, I am a fan of the industry-standard form factors! Having standards and then following them allows us to upgrade and repair our systems by easily replacing physically (and electrically) interchangeable components. Having interchangeable parts means that we have a better range of choices for replacement items, and the competition makes for better pricing, too.

In the PC market, IBM originally defined the form factor standards, and everybody else copied them; this included power supplies. All the popular PC power supply form factors up through 1995 were based on one of three IBM models, including the PC/XT, AT, and PS/2 Model 30. The interesting thing is that these three original IBM power supply form factors had the same motherboard connectors and pinouts; where they differed was mainly in shape, maximum power output, the number of peripheral power connectors, and switch mounting. PC systems using knock-offs of one of those three designs were popular up through 1996 and beyond; in fact, even the current industry standard ATX12V models are based on the PS/2 Model 30 physical form factor, but with different connectors.

Intel defined a new power supply form factor in 1995 with the introduction of the ATX form factor. ATX became popular in 1996 and started a shift away from the previous IBM-based standards. ATX and the standards that have followed since use different connectors with additional voltages and signals that allow systems with greater power consumption and additional features that would otherwise not be possible with the AT-style supplies.

Note: Although two power supplies can share the same basic design and form factor, they can differ greatly in quality and efficiency. Later in this chapter, you’ll learn about some of the features and specifications to look for when evaluating PC power supplies.

More than 10 different power supply form factors have existed that can be called industry standards. Many of these are or were based on designs IBM created in the 1980s, whereas the rest are based on Intel designs from the 1990s to the present. The industry-standard form factors can be broken down into two main categories: those that are currently in use in modern systems and those that are largely obsolete.

Note that although the names of some of the power supply form factors seem to be the same as those of motherboard form factors, the power supply form factor relates more to the system chassis (case) than to the motherboard. That is because all the form factors use one of only two main types of connector designs: either AT or ATX, with subtle variations on each. So, although a particular power supply form factor might be typically associated with a particular motherboard form factor, many other power supplies would plug in as well.

For example, all modern ATX form factor motherboards with PCI Express slots have two main power connectors, including a 24-pin ATX main connector along with a four-pin +12 V connector. All the modern power supply form factors include these same connectors and therefore are capable of plugging into the same motherboards. In other words, no matter what the form factor of the motherboard (ATX, BTX, or any of the smaller variants of either), virtually any of the modern industry-standard power supplies will plug in.

Plugging the power supply connectors into the motherboard is one thing, but for the power supply to work in the system, it must physically fit inside the chassis or case—and that is what the different modern power supply form factors are all about. The bottom line is that it is up to you to ensure that the power supply you purchase not only plugs in to your motherboard, but also fits into the chassis or case you plan to use.

The following two tables show the industry-standard power supply form factors, their connector types, and the motherboard form factors with which they are usually associated.

Modern Industry-Standard Power Supply Form Factors
Modern Power Supply
Form Factors
Year
Introduced
Normally Associated Motherboard
Connector Types
Form
Factors
ATX/ATX12V1995
20/24-pin Main, 4-pin +12VATX, microATX, BTX, microBTX
SFX/SFX12V*/PS31997
20/24-pin Main, 4-pin +12VmicroATX, FlexATX, microBTX, picoBTX, Mini-ITX, DTX
EPS/EPS12V1998
24-pin Main, 8-pin +12VATX, extended ATX
TFX12V2002
20/24-pin Main, 4-pin +12VmicroATX, FlexATX, microBTX,

picoBTX, Mini-ITX, DTX

CFX12V2003
20/24-pin Main, 4-pin +12VmicroBTX, picoBTX, DTX
LFX12V2004
24-pin Main, 4-pin +12VpicoBTX, nanoBTX, DTX
Flex ATX2007
24-pin Main, 4-pin +12V‑microATX, FlexATX, microBTX,
picoBTX, nanoBTX, Mini-ITX, DTX
*SFX12V also includes the PS3 form factor, which is a shortened version of ATX12V.


12 V versions include a four-pin or eight-pin +12 V connector.

You may encounter power supplies using obsolete form factors if you work on PCs built in the 1980s through the mid-1990s. I cover them in more detail in the 18th and earlier editions of this book.

Obsolete Industry Standard Power Supply Form Factors
Obsolete Power Supply
Form Factors
Year
Introduced
Normally Associated Motherboard
Connector Types
Form
Factors
*PC/XT1981
PC/XT
PC/XT, Baby-AT
AT/Desk
1984
ATFull-size AT, Baby-AT
AT/Tower1984
ATFull-size AT, Baby-AT
Baby-AT1984
ATFull-size AT, Baby-AT
**LPX (PS/2)1987
ATBaby-AT, Mini-AT, LPX
*PC/XT connectors were the same as AT connectors, except one +5 V pin (P8 pin 2) was not used.
**LPX is also sometimes called PS/2 or Slimline.


Each of these power supply form factors is, or has been, available in numerous configurations and power output levels. The obsolete LPX form factor supply originated in the IBM PS/2 Model 30 in April 1987 and was the standard used on most systems from the late 1980s to mid-1996, when the ATX form factor started to gain in popularity. Since then, ATX and the many variants based on ATX have become by far the dominant form factors for power supplies. It is interesting to note that IBM’s legacy lives on even now because ATX, PS3, and EPS are all based on the LPX (PS/2) physical form factor. Any power supply that does not conform to one of these standards is considered proprietary. In general, avoid systems that use proprietary power supply designs because replacements are difficult to obtain and upgrades are generally not available. When you consider that the power supply is one of the most failure-prone components, purchasing systems that use proprietary designs can be a significant liability in the future. If you need a replacement for a proprietary form factor supply, one of the best sources is ATXPowerSupplies.com. They maintain replacement models that cover a huge number of both proprietary and industry standard designs.

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33 comments
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  • joytech22
    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.
  • cmcghee358
    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...
  • cangelini
    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!
  • cmcghee358
    But the last part isn't for PSUs. It's just the last part in the series of PC components.
  • nikorr
    Thanx ...
  • neiroatopelcc
    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?
  • chesteracorgi
    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.
  • JohnnyLucky
    Great article. It's not just for beginners.
  • Reynod
    Compatibility Issues was a useful section.

    Overall very well written.

    Cheers,
  • kd0frg
    awesome information! nice work!
  • elbert
    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/
  • digiex
    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.
  • mayankleoboy1
    PSU: the most overlooked and underrated component
  • 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.
  • A Bad Day
    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.
  • grantmcconnaughey
    I've been reading this book lately. To me, this is absolutely the bible of PC hardware.
  • newbie_mcnoob
    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.
  • hunter315
    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.
  • PreferLinux
    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.
  • iam2thecrowe
    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.
  • Onus
    Thank you, Hunter315. Maybe that was an issue 6-8 years ago, maybe? It no longer is; I cannot imagine any possibility of creating sufficiently unbalanced +12V rail loading creating the least difficulty for any reasonable PSU.
    Anyway, there were some interesting tidbits in there, like the specifics on the capacities of individual connectors. The connector type used is another area where I'd be certain the cheap PSUs skimp.
    Some of it was a trip down memory lane; I remember those load resistors in the hard drive cages of the IBM PC-AT.

    Edit: Oh, and your answer to the Eternal Question was very good, as was the short discussion which followed, touching on the safety issues associated with overly large +12V rails. Although I have said many of those things, I doubt I've expressed myself as clearly. Those who have read this chapter and this far into the comments would probably also like to click on your link and read that thread.
  • aidynphoenix
    It really doesn't matter weather you have a psu with a single 12v rail or several.
    what matters is the quality of the power supply and how well it performs.
    viewing a powesupply's voltages on a graph and seeing how far the voltages fluctuate and how smooth the ripple currents are is the most important thing.
    even if your powersupply is 1200w id rather have a psu that can get the job done, but stays as close as possible to the 12v specification, and the power is clean and has little spikes.
    i am a firm believer that poorly designed powersupply's is the #1 cause of premature hardisk failure aside from shipping damage and excessive heat.
    you know that when company's calculate their MTBF they are useing the cleanest power possible.
    I have had alot of experience with poor powersupply's and many HDD deaths.
    now im running a powersupply with a very smooth 12v ripple and high tolorance (thanks corsair)
    and have not had a hardisk die on me since.

    i order recommend 1 seasonic, 2 corsair, 3 antec.. i do not recommend cooler-master.. or any other brand.
  • neiroatopelcc
    neiroatopelcc"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.


    To answer my own question, as nobody else probably will, I tested it. Installed a reset switch yesterday - well two wires I could shortcircuit anyway. Only effect was that my mainboard thought overclocking had failed. It didn't fix the memory issue. So this is a fat NO. PSU power good timing isn't my issue.
  • ojas
    Chris/Don,

    Any idea when the paperback comes out??? Looking to buy it...