58 MAXIMUMPC JUNE 2004
POWER SUPPLIES
What terms should I know be-
fore purchasing a power supply?
There are three technical terms used in
describing electricity that you should
understand: voltage, current, and power.
Voltage, expressed in volts (V), is the
potential difference of electrical energy
between two points. In simpler terms, it’s
electrical pressure. Current, expressed in
amperes (A), is the rate of flow of elec-
trons (one ampere of electrical current
is the flow of roughly 6.24x10^18 electrons
a second—you’ll find out why this scary
number is important in a moment). Power
is simply voltage multiplied by current,
and is expressed in watts (W).
Here’s a classic metaphor for how volt-
age, current, and power are interrelated.If you picture electricity as water in a
garden hose, voltage is akin to the pres-
sure in the hose and current is the rate of
water flowing through it. A watt of power
is the ability to move 6.24x10^18 —there’s
that number again!—electrons a second
at one volt.What should I look for on a label
when I buy a power supply?
All power supplies have a power rating
expressed in watts (W). This is a measure-
ment of the maximum electrical power
the unit can deliver. In general, a high
power rating is preferable, and a PC built
of contemporary parts should use a PSU
of at least 350W.
The label on a power supply unit will
also state the input voltage. For the US
market, this should be 120V AC at 50/60Hz,
although many units will be capable of
switching between 120V and 240V AC
(either automatically or via a manual
switch). If you plan to take your system
abroad, be sure to get a unit that offers this
universal voltage support.
Other than the power and input voltage
specifications, many PSU labels will state
the maximum current load supported by
each of the individual output lines—the
connectors hanging from the PSU like
dreadlocks that you connect to each of your
PC’s components. To arrive at the power
rating—the maximum amount of consis-
tently sustainable wattage—for each of the
output lines, simply multiply the voltage
(V) rating by the current rating (A).
For example, if a PSU’s output for a 5V
connector is stated as +5V @ 40A, it means
the connector can maintain 200W (5x40)
of consistent power (although few compo-
nents are that gluttonous). In general you
should only have to worry about the posi-
tive (+) voltages because this is where your
components draw their power from. Buteven here there are degrees of precision.
Intel’s ATX 12V specification requires that
output voltages be held to a +/-5 percent
tolerance. But some manufacturers go a
step further and limit their high-end units to
+/-3 percent or even +/-1 percent tolerance.
This is sometimes indicated on the label,
and the stricter the tolerance the better.
A power consumption chart at the end of
this article will help you assess the power
requirements of each of your components
and ensure that the projected power draw
on each of the power lines does not exceed
what the PSU can provide.In what ways do manufacturers
misrepresent wattage claims?
A seldom publicized fact is that the watt-
age rating posted for a power supply is
only meaningful for a particular operating
temperature. The reason is that all con-
sumer PSUs lose efficiency as tempera-
ture increases. One of the most common
ways shameless vendors cheat on their
wattage claims is by testing their power
supplies at a temperature far below a
reasonable operating temperature. It’s not
unusual for such power supplies to lose
25 percent or more of their rated ability
at 100° F—the heat level where they typi-
cally operate.
The problem is that there is no regula-
tory or conventional requirement for the
power rating to be accompanied by a
temperature rating. A “450W” power sup-
ply could very well be capable of 450 watts
only at 40° F. The manufacturer is technically
not lying, but even if you use your PC in
a meat locker, the inside of your system
never gets this cold. At a normal operating
temperature of 100° F, this same PSU would
likely be capable of just 280 watts!
A word of warning: Don’t give much
credence to the official-looking indepen-
dent laboratory emblems and certifications
on PSU boxes. Again, there are no clearly
defined standards regarding the ambient
or operating temperatures at which these
products were tested. In addition, extremely
unscrupulous vendors have been known to
fraudulently tack on certification logos.
Unfortunately, there is no practical
way of testing the PSU for its perfor-
mance before you buy it, and that’s why
we have always recommended (to the
irritation of some readers who think
we’re butt-kissing) that you stick with
popular brands that have solid reputa-
tions to uphold and excellent-quality
brands to protect, such as PC Power &
Cooling, Enermax, and Antec.The Power
Supply Demystified
13 important questions about your PC’s
most veiled component
Quality power supplies will state the
current load that each output line can
handle.