Hardware Reference
In-Depth Information
• 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 para-
meters 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 failing. Power supplies typically have MTBF ratings (such as 100,000 hours
or more) that are clearly not the result of real-time empirical testing. In fact, manu-
facturers use published standards to calculate the results based on the failure rates of
thepowersupply'sindividual components. MTBFfiguresforpowersupplies oftenin-
clude the load to which the power supply was subjected (in the form of a percentage)
and the temperature of the environment in which the tests were performed.
•
Input Range (or Operating Range)
—The range of voltages that the power supply is
prepared to accept from the AC power source. For 120V AC power, an input range of
90V-135V is common; for 240V power, a 180V-270V range is typical.
•
Peak Inrush Current
—The greatest amount of current drawn by the power supply at
a given moment immediately after it is turned on, expressed in terms of amps at a par-
ticular voltage. The lower the current, the less thermal shock the system experiences.
•
Hold-Up Time
—The amount of time (in milliseconds) that a power supply can main-
tainoutputwithinthespecifiedvoltagerangesafteralossofinputpower.Thisenables
your PC to continue running without resetting or rebooting if a brief interruption in
AC power occurs. Values of 15-30 milliseconds are common for today's power sup-
plies, and the higher (longer), the better. The Power Supply Design Guide for Desktop
