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Power Supply Specifications

Power Supply 101: A Reference Of Specifications

Power supplies have several specifications that define their input and output capabilities as well as their operational characteristics. This section defines and examines most of the common specifications related to power supplies.

Power Supply Loading

PC power supplies are of a switching rather than a linear design. The switching type of design uses a high-speed oscillator circuit to convert the higher wall-socket AC voltage to the much lower DC voltage used to power the PC and PC components. Switching-type power supplies are noted for being efficient in size, weight, and energy compared to the linear design, which uses a large internal transformer to generate various outputs. This type of transformer-based design is inefficient in at least three ways:

  • The output voltage of the transformer linearly follows the input voltage (hence the name linear), so any fluctuations in the AC power going into the system can cause problems with the output.
  • The high current-level (power) requirements of a PC system require the use of heavy wiring in the transformer.
  • The 60 Hz frequency of the AC power supplied from your building is difficult to filter out inside the power supply, requiring large and expensive filter capacitors and rectifiers.

The switching supply, on the other hand, uses a switching circuit that chops up the incoming power at a relatively high frequency. This enables the use of high-frequency transformers that are much smaller and lighter. Also, the higher frequency is much easier and cheaper to filter out at the output, and the input voltage can vary widely. Input ranging from 90 V to 135 V still produces the proper output levels, and many switching supplies can automatically adjust to 240 V input.

One characteristic of all switching-type power supplies is that they do not run without a load. Therefore, you must have something such as a motherboard and hard drive plugged in and drawing power for the supply to work. If you simply have the power supply on a bench with nothing plugged into it, either the supply burns up or its protection circuitry shuts it down. Most power supplies are protected from no-load operation and shut down automatically. Some of the cheapest supplies, however, lack the protection circuit and relay and can be destroyed after a few seconds of no-load operation. A few power supplies have their own built-in load resistors, so they can run even though there isn’t a normal load (such as a motherboard or hard disk) plugged in.

Some power supplies have minimum load requirements for both the +5 V and +12 V sides. According to IBM specifications for the 192-watt power supply used in the original AT, a minimum load of 7.0 amps was required at +5 V and a minimum of 2.5 amps was required at +12 V for the supply to work properly. As long as a motherboard was plugged into the power supply, the motherboard would draw sufficient +5 V at all times to keep those circuits in the supply happy. However, +12 V is typically used only by motors (and not motherboards), and the floppy or CD/DVD drive motors are usually off. Because floppy or optical (CD/DVD) drives don’t present +12 V load unless they are spinning, systems without a hard disk drive could have problems because there wouldn’t be enough load on the +12 V circuit in the supply.

To alleviate problems, when IBM used to ship the original AT systems without a hard disk, it plugged the hard disk drive power cable into a large 5-ohm, 50-watt sandbar resistor that was mounted in a small metal cage assembly where the drive would have been. The AT case had screw holes on top of where the hard disk would go, specifically designed to mount this resistor cage.

Note: Several computer stores I knew of in the mid-1980s ordered the diskless AT and installed their own 20 MB or 30 MB drives, which they could get more cheaply from sources other than IBM. They were throwing away the load resistors by the hundreds! I managed to grab a couple at the time, which is how I know the type of resistor they used.

This resistor would be connected between pin one (+12 V) and pin two (Ground) on the hard disk power connector. This placed a 2.4-amp load on the supply’s +12 V output, drawing 28.8 watts of power (it would get hot!) and thus enabling the supply to operate normally. Note that the cooling fan in most power supplies draws approximately 0.1–0.25 amps, bringing the total load to 2.5 amps or more. If the load resistor were missing, the system would intermittently fail to start.

Most of the power supplies in use today do not require as much of a load as the original IBM AT power supply. In most cases, a minimum load of 0–0.3 amps at +3.3 V, 2.0–4.0 amps at +5 V, and 0.5–1.0 amps at +12 V is considered acceptable. Most motherboards easily draw the minimum +5 V current by themselves. The standard power supply cooling fan draws only 0.1–0.25 amps, so the +12 V minimum load might still be a problem for a diskless workstation. Generally, the higher the rating on the supply, the more minimum load that is required. However, exceptions do exist, so this is a specification you should check when evaluating power supplies.

Some switching power supplies have built-in load resistors and can run in a no-load situation. Most power supplies don’t have internal load resistors but might require only a small load on the +5 V line to operate properly. Some supplies, however, might require +3.3 V, +5 V, and +12 V loads to work; the only way to know is by checking the documentation for the particular supply in question.

No matter what, if you want to properly and accurately bench test a power supply, be sure you place a load on at least one (or preferably all) of the positive voltage outputs. This is one reason it is best to test a supply while it is installed in the system instead of testing it separately on the bench. For impromptu bench testing, you can use a spare motherboard and one or more hard disk drives to load the outputs.

Power Supply Ratings

A system manufacturer should be able to provide you the technical specifications of the power supplies it uses in its systems. This type of information can be found in the system’s technical reference manual, as well as on stickers attached directly to the power supply. Power supply manufacturers can also supply this data, which is preferable if you can identify the manufacturer and contact it directly or via the Web.

The input specifications are listed as voltages, and the output specifications are listed as amps at several voltage levels. You can convert amperage to wattage by using the following simple formula:

watts = volts × amps

For example, if a component is listed as drawing 8 amps of +12 V current, that equals 96 watts of power according to the formula.

By multiplying the voltage by the amperage available at each main output and then adding the results, you can calculate the total capable output wattage of the supply. Note that only positive voltage outputs are normally used in calculating outputs; the negative outputs, Standby, Power_Good, and other signals that are not used to power components are usually exempt.

The following table shows the ratings and calculations for various single +12 V rail ATX12V/EPS12V power supplies from Corsair (

Typical ATX12V/EPS12V Power Supply Output Ratings
+12 V (A)33
–12 V (A)0.8
+5 VSB (A)
+5 V (A)20
+3.3 V (A)20
Max +5 V/+3.3 V (W)
Rated Max. (W)450
Calculated Max. (W)548

Virtually all power supplies place limits on the maximum combined draw for the +3.3 V and +5 V.

The calculated maximum output assumes the maximum draw from all outputs simultaneously and is generally not sustainable. For this reason, the (sustainable) rated maximum output is normally much less.

Although store-bought PCs often come with lower-rated power supplies of 350 watts or less, higher output units are often recommended for fully optioned desktops or tower systems. Unfortunately, the ratings on cheap or poorly made power supplies cannot always be trusted. For example, I’ve seen 650 W-rated units that had less actual power output than honestly rated 200 W units. Another issue is that few companies actually make power supplies. Most of the units you see for sale are made under contract by a few manufacturers and sold under a variety of brands, makes, and models. Because few people have the time or equipment to actually test or verify output, it is better to stick to brands that are known for selling quality units.

Most power supplies are considered to be universal, or worldwide. That is, they also can run on the 240 V, 50-cycle current used in Europe and many other parts of the world. Many power supplies that can switch from 120 V to 240 V input do so automatically, but a few require you to set a switch on the back of the power supply to indicate which type of power you will access.

Note: In North America, power companies are required to supply split-phase 240 V (plus or minus 5%) AC, which equals two 120 V legs. Resistive voltage drops in the building wiring can cause the 240 V to drop to 220 V or the 120 V to drop to 110 V by the time the power reaches an outlet at the end of a long circuit run. For this reason, the input voltage for an AC-powered device might be listed as anything between 220 V and 240 V, or 110 V and 12 0V. I use the 240/120 V numbers throughout this chapter because those are the intended standard figures.

Caution: If your supply does not switch input voltages automatically, make sure the voltage setting is correct. If you plug the power supply into a 120 V outlet while it’s set in the 240 V setting, no damage will result, but the supply won’t operate properly until you correct the setting. On the other hand, if you plug into a 240 V outlet and have the switch set for 120 V, you can cause damage.

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

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

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

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

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

    Overall very well written.

  • 0 Hide
    kd0frg , December 14, 2011 2:39 PM
    awesome information! nice work!
  • 0 Hide
    elbert , December 14, 2011 3:28 PM
    cmcghee358But the last part isn't for PSUs. It's just the last part in the series of PC components.
    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.
  • 0 Hide
    digiex , December 14, 2011 3:48 PM
    I'm just wondering what is the use of the floppy connector...

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

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

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

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

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

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

    you couldn't be more wrong.
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