Power Supply 101: A Reference Of Specifications

Power Switches

Three main types of power switches are used on PCs. They can be described as follows:

  • Front panel motherboard-controlled switch (ATX and newer)
  • Front panel power supply AC switch (AT/LPX; obsolete)
  • Integral power supply AC switch (PC/XT/AT; obsolete)


ATX and Newer

All ATX and newer power supplies that employ the 20- or 24-pin motherboard connector use the PS_ON signal to power up the system. In this design, the power supply runs in standby mode when plugged in with the system off. The PS_ON signal is routed from the power supply through the motherboard to a low-voltage momentary contact DC switch on the front panel. As a result, the remote power switch does not physically control the power supply’s access to the 120 V AC power, as in older-style power supplies. Instead, the power supply’s on or off status is toggled by the PS_ON signal received on the ATX Main power connector. This is sometimes called a soft-off switch because this is the name of the Advanced Configuration Power Interface (ACPI) state when the system is off but still receiving standby power.

The PS_ON signal can be manipulated physically by the computer’s power switch or electronically by the motherboard under software control. PS_ON is an active low signal, meaning the power supply voltage outputs are disabled (the system is off) when the PS_ON is high (greater than or equal to 2.0 V). This excludes the +5 VSB (Standby) on pin nine of the ATX main power connector, which is active whenever the power supply is connected to an AC power source. The power supply maintains the PS_ON signal at either 3.3 V or +5 V. This signal is then routed through the motherboard to the remote switch on the front of the case. When the switch is pressed, the PS_ON signal is grounded. When the power supply sees the PS_ON signal drop to 0.8 V or less, the power supply (and system) is turned on. Thus, the remote switch in a system using an ATX or newer power supply carries up to only +5 V of DC power, rather than the full 120 V–240 V AC current like that of the older AT/LPX form factors.

The actual power switch used in ATX systems is normally a tiny momentary contact push button switch, which is connected to the motherboard front panel header via a tiny two-pin connector. When the button is pushed, the motherboard then grounds the PS_ON signal in the main 20/24-pin power connector, causing the power supply to turn on.

Caution: The continuous presence of the +5 VSB power on pin nine of the ATX main connector means the motherboard is always receiving standby power from the power supply when connected to an AC source, even when the computer is turned off. As a result, it is even more important to unplug an ATX system from the power source before working inside the case than it is on earlier model systems.

The remote switch on ATX and newer designs can only put the system in a soft-off state, in which the system appears off but is still receiving standby power. Some ATX and newer power supplies include a hard override AC power switch on the back, which essentially disconnects AC power from the system when turned off. With the AC switch off, the system no longer receives standby power and is essentially the same as being completely unplugged from an AC outlet.

Tip: The design of the ATX power switch is such that the motherboard actually controls the status of the power supply. On systems with full support for ACPI, when you press the power switch, the motherboard informs the OS to perform an orderly shutdown before the power is actually turned off. However, if the system is locked up or corrupted, it can remain running when you press the switch. In that situation, you can manually override the ACPI control by pressing the switch continuously for more than four seconds, which overrides the software control and forcibly turns off the system.

PC/XT/AT and LPX Power Switches

The earliest systems had power switches integrated or built directly into the power supply, which turned the main AC power to the system on and off. This was a simple design, but because the power supply was mounted in the rear right of the system, it required reaching around to the right side near the back of the system to actuate the switch. Also, switching the AC power directly meant the system couldn’t be remotely started without special hardware.

Starting in the late 1980s, systems with LPX power supplies began using remote front panel switches. These were still AC switches; the only difference was that the AC switch was now mounted remotely (usually on the front panel of the chassis), rather than integrated in the power supply unit. The switch was connected to the power supply via a four-wire cable, and the ends of the cable were fitted with spade connector lugs, which plugged onto the spade connectors on the power switch. The cable from the power supply to the switch in the case contained four color-coded wires. In addition, a fifth wire supplying a ground connection to the case was sometimes included. The switch was usually included with the power supply and heavily shrink-wrapped or insulated where the connector lugs attached, to prevent electric shock.

This solved the ergonomic problem of reaching the switch, but it still didn’t enable remote or automated system power-up without special hardware. Plus, you now had a 120 V AC switch mounted in the chassis, with wires carrying dangerous voltage through the system. Some of these wires are hot anytime the system is plugged in (all are hot when the system’s turned on), creating a dangerous environment for the average person when messing around inside the system.

Caution: At least two of the remote power switch leads to a remote-mounted AC power switch in an AT/LPX supply are energized with 120 V AC at all times. You can be electrocuted if you touch the ends of these wires with the power supply plugged in, even if the unit is turned off! For this reason, always make sure the power supply is unplugged before connecting or disconnecting the remote power switch or touching any of the wires connected to it.

The four or five wires are usually color-coded as follows:

  • Brown and blue—These wires are the live and neutral feed wires from the 120 V power cord to the power supply. These are always hot when the power supply is plugged in.
  • Black and white—These wires carry the AC feed from the switch back to the power supply. These leads should be hot only when the power supply is plugged in and the switch is turned on.
  • Green or green with a yellow stripe—This is the ground lead. It should be connected to the PC case and should help ground the power supply to the case.


On the switch, the tabs for the leads are usually color-coded; if not, you’ll find that most switches have two parallel tabs and two angled tabs. If no color-coding is on the switch, plug the blue and brown wires onto the tabs that are parallel to each other and the black and white wires to the tabs that are angled away from each other. If none of the tabs are angled, simply make sure the blue and brown wires are plugged into the most closely spaced tabs on one side of the switch and the black and white wires on the most closely spaced tabs on the other side (see the following image).

Caution: Although these wire color-codings and parallel/angled tabs are used on most power supplies, they are not necessarily 100% universal. I have encountered power supplies that do not use the same coloring or tab placement scheme described here. One thing is sure: two of the wires will be hot with potentially fatal AC voltage anytime the power supply is plugged in. No matter what, always disconnect the power supply from the wall socket before handling any of these wires. Be sure to insulate the connections with electrical tape or heat-shrink tubing so you won’t be able to touch the wires when working inside the case in the future.

As long as the blue and brown wires are on one set of tabs and the black-and-white leads are on the other, the switch and supply will work properly. If you incorrectly mix the leads, you will likely blow the circuit breaker for the wall socket because mixing them can create a direct short circuit.

This thread is closed for comments
33 comments
    Your comment
  • 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...