Hardware Reference
In-Depth Information
Note
In North America, power companies are required to supply split-phase 240V (plus or minus
5%) AC, which equals two 120V legs. Resistive voltage drops in the building wiring can
cause the 240V to drop to 220V or the 120V to drop to 110V 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 220V and 240V, or 110V and 120V. I use the
240/120V 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 120V outlet while it's set in the 240V 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 240V outlet and have the switch set for 120V, you can cause
damage.
Other Power Supply Specifications
In addition to power output, many other specifications and features go into making a high-quality
power supply. I have had many systems over the years. My experience has been that if a brownout
occurs in a room with several systems running, the systems with higher-quality power supplies and
higher output ratings are far more likely to make it through the power disturbances unscathed, whereas
others choke.
High-quality power supplies also help protect your systems. A high-quality power supply from a
vendor such as PC Power & Cooling will not be damaged if any of the following conditions occur:
• A 100% power outage of any duration
• A brownout of any kind
• A spike of up to 2,500V applied directly to the AC input (for example, a lightning strike or a
lightning simulation test)
Decent power supplies have an extremely low current leakage to ground of less than 500 microamps.
This safety feature is important if your outlet has a missing or an improperly wired ground line.
As you can see, these specifications are fairly tough and are certainly representative of a high-quality
power supply. Make sure that your supply can meet these specifications.
You can also use many other criteria to evaluate a power supply. The power supply is a component
many users ignore when shopping for a PC, so it is one that some system vendors choose to skimp on.
After all, a dealer is far more likely to be able to increase the price of a computer by spending money
on additional memory or a larger hard drive than by installing a better power supply.
When buying a computer (or a replacement power supply), learn as much as possible about the power
supply. Many consumers are intimidated by the vocabulary and statistics found in a typical power
supply specification. Here are some of the most common parameters found on power supply
specification sheets, along with their meanings:
•
Mean Time Between Failures (MTBF) or Mean Time To Failure (MTTF)
—The
(calculated) average interval, in hours, that the power supply is expected to operate before
