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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 them directly or via the Web.

The input specifications are listed as voltages, and the output specifications are listed as amps at several voltage levels. IBM reports output wattage level as "specified output wattage." If your manufacturer does not list the total wattage, you can convert amperage to wattage by using the following simple formula:

watts = volts x amps

For example, if a motherboard is listed as drawing 6 amps of +5v current, that would be 30 watts of power, according to the formula.

By multiplying the voltage by the amperage available at each output and then adding the results, you can calculate the total capable output wattage of the supply.

Table 3.12 shows the rated outputs at each of the voltage levels for supplies with different manufacturer-specified output ratings. To compile the table, I referred to the specification sheets for supplies from Astec Standard Power and PC Power and Cooling. Although most of the ratings are accurate, they are somewhat misleading for the higher wattage units.

Table 3.12 Typical Non-ATX Power Supply Output Ratings

Rated Output (Watts)

100W

150W

200W

250W

300W

375W

450W

Output current (amps):

+5V

10.0

15.0

20.0

25.0

32.0

35.0

45.0

+12V

3.5

5.5

8.0

10.0

10.0

13.0

15.0

–5V

0.3

0.3

0.3

0.5

1.0

0.5

0.5

–12V

0.3

0.3

0.3

0.5

1.0

0.5

1.0

Calc. output (watts)

97.1

146.1

201.1

253.5

297.0

339.5

419.5


Adding a +3.3v output to the power supply modifies the equation significantly. Table 3.13 contains data for various ATX power supplies from PC Power and Cooling that include a +3.3v output.

Table 3.13 PC Power and Cooling ATX Power Supply Output Ratings

Rated Output

235W

275W

300W

350W

400W

425W

Output current (amps):

+5V

22.0

30.0

30.0

32.0

30.0

50.0

+3.3V

14.0

14.0

14.0

28.0

28.0

40.0

Max watts +5V and +3.3V:

125W

150W

150W

215W

215W

300W

+12V

8.0

10.0

12.0

10.0

14.0

15.0

–5V

0.5

0.5

0.5

0.3

1.0

0.3

–12V

1.0

1.0

1.0

0.8

1.0

1.0


If you compute the total output using the formula described earlier, these power supplies seem to produce outputs that are much higher than their ratings. The 300W model, for example, comes out at 354.7 watts. However, notice that the supply also has a maximum combined output for the +3.3v and +5v of 150 watts. This means you cannot draw the maximum rating on both the 5v and 3.3v lines simultaneously but must keep the total combined draw between them at 150W. This brings the total output to a more logical 308.5 watts.

Most PC power supplies have ratings between 150 and 300 watts. Although lesser ratings are not usually desirable, you can purchase heavy-duty power supplies for most systems that have outputs as high as 600 watts or more.

The 300-watt and larger units are recommended for fully optioned desktops or tower systems. These supplies run any combination of motherboard and expansion card, as well as a large number of disk drives and other peripherals. In most cases, you cannot exceed the ratings on these power supplies—the system will be out of room for additional items first!

Most power supplies are considered to be universal, or worldwide. That is, they also can run on the 220v, 50-cycle current used in Europe and many other parts of the world. Many power supplies that can switch from 110v to 220v 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.

CAUTION

If your supply does not switch input voltages automatically, be sure the voltage setting is correct. If you plug the power supply into a 110v outlet while it's set in the 220v setting, no damage will result, but the supply won't operate properly until you correct the setting. On the other hand, if you are in a foreign country with a 220v outlet and have the switch set for 110v, you might cause damage.

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