26.4 Choosing a Power Supply
Use the
following guidelines to choose a power supply appropriate for your
system:
- Choose the correct form factor
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Above all, make sure the power supply you buy fits your case and has
the proper connectors for your motherboard. If your motherboard
includes the ATX Optional Power Supply Connector, buy a power supply
that provides that connector. Consider buying such a power supply
even if your current motherboard does not require that connector so
that if you upgrade, the connector will be available.
- Match power supply to system configuration
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Some sources recommend adding up maximum current draws for all system
components and sizing the power supply on that basis. The problem
with that method is that it can be nearly impossible to determine
those draws for all components, especially motherboards and expansion
cards. We recommend using the KISS method instead, as follows:
- Basic system
-
For a desktop or mini-tower system with a low-end processor, 128 MB
or less of RAM, one ATA hard disk, one ATAPI DVD/CD-ROM drive, and
zero or one expansion card, install a 300W ATX12V power supply.
- Mainstream system
-
For a desktop or mini/mid-tower system with a midrange processor, 256
MB of RAM, one or two ATA hard disks, a DVD/CD-ROM drive, a DVD/CD
writer, perhaps a tape drive, and one or two expansion cards, install
a 350W power supply.
- High-performance system
-
For a mid- or full-tower system with a fast processor, 512 MB of RAM,
one or two fast SCSI or ATA hard disks, a DVD/CD-ROM drive, a DVD/CD
writer, a tape drive, and several expansion cards, install a 400W to
450W power supply.
- Heavily loaded system
-
For a full-tower system with fast dual processors, 1 GB or more of
RAM, two or three fast SCSI or ATA hard disks, a CD/DVD-ROM drive, a
DVD/CD writer, a tape drive, and all expansion slots filled, install
a 450W to 600W power supply.
Obviously, individual configurations vary, but generally following
these guidelines ensures that the power supply is adequate for the
current configuration and has some room for growth if you add
components. If in doubt, buy the next size up.
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If you build a dual-CPU system, make sure the power supply you choose
is rated for dual-CPU motherboards. Even power supplies that have
relatively high overall wattage ratings may not be adequate for
dual-CPU motherboards. Dual processors may draw 150W or more, which
may exceed the maximum allowable current at the required voltage. Our
best advice for those building dual-CPU systems is to check the PC
Power & Cooling web site (http://www.pcpowercooling.com) to locate a
power supply appropriate for your configuration.
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- Match power supply capacity to case style
-
Regardless of your current configuration, take case style into
account. It is senseless, for example, to install a 200W power supply
in a full-tower case. You might just as well buy a smaller case
because that power supply will never support even a fraction of the
number of devices the case can hold. Neither does it make sense to
install a 600W power supply in a mini-tower case, which simply does
not have room for enough drives to require all that power.
- Avoid ATX power supplies that are not Pentium 4-compatible
-
Any ATX power supply you buy should be ATX12V-compliant. Even if
you're not using a Pentium 4 in that system, buying
an ATX12V power supply protects your upgrade path if you decide later
to install a newer motherboard. We expect that an increasing number
of newly introduced motherboards, both for Pentium 4 and other
processors, will begin including the 4-pin +12VDC connector. If your
power supply doesn't have this connector, you may
need to replace the power supply when you upgrade the motherboard.
(You can use an ATX12V adapter cable to pull +12VDC from a drive
connector to the motherboard, but that doesn't
guarantee that your original power supply will work with motherboards
that require +5VSB current higher than
required by ATX 2.03.) Buying an ATX12V unit now avoids that needless
expense later.
- Avoid cheap power supplies
-
Don't assume that the power supply bundled with an
inexpensive case or a $20 unit you find on the sale table at the
computer store is adequate. It probably isn't. A
good power supply costs at least $35 for a basic system, $50 to $75
for a mainstream mini/mid-tower system, and $100 or more for heavily
loaded, full-tower systems.
- Avoid replacing proprietary power supplies
-
Some big-name computer makers (notably Compaq and Dell) have used
proprietary power supplies in their systems, although this practice
is fortunately less common nowadays. If a proprietary power supply
fails, the only source for a replacement is the manufacturer, who may
charge literally $500 for a power supply equivalent to a standard $50
unit. If you need to replace a proprietary power supply in a system
that cannot physically accept a standard power supply, it may be
cheaper to replace the PC than to buy a new power supply for it.
Rather than pay proprietary prices, consider buying a new case and
power supply and migrating the components from the failed unit to the
new case. You may also have to buy a new motherboard, but you should
be able to migrate the processor, memory, and other components.
A similar problem exists with some of the inexpensive systems sold by
such retailers as Circuit City and Best Buy. Many of those use
inexpensive, low-output SFX power supplies that are prone to fail
quickly. For some of these systems, manufacturer policies make
replacing a failed power supply quite expensive, even if the system
is still under warranty.
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Instead of overpaying for a replacement power supply
that's no better than the original, purchase a
replacement power supply from PC Power & Cooling (http://www.pcpowercooling.com), which makes
both a standard SFX power supply and a special SFX model for
eMachines and HP 67XX models. Better still, replace the power supply
before it fails. Doing so improves the
system's reliability, stability, and life
expectancy. Put the original, still-functioning power supply on the
shelf as a spare, although you probably won't need
it.
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