Power Supply 101: A Reference Of Specifications

Power Supply 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 (www.corsair.com).

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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