Hardware Reference
In-Depth Information
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 +12V 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.
Table 18.23 shows the ratings and calculations for various single +12V rail ATX12V/EPS12V power
supplies from Corsair ( www.corsair.com ) .
Table 18.23. Typical ATX12V/EPS12V Power Supply Output Ratings
Virtually all power supplies place limits on the maximum combined draw for the +3.3V and +5V.
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
650W-rated units that had less actual power output than honestly rated 200W 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
240V, 50-cycle current used in Europe and many other parts of the world. Many power supplies that
can switch from 120V to 240V 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.
 
 
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