Operational Ratings
Manufacturers list a number of operational ratings in the specification lists of their power
supplies.Theseitemsareverytechnical,buttheyhelpyoutodetermineifapowersupply
is compatible with the form factor and operational ratings of your PC’s case, mother-
board, processor, and chipset. The ratings you should find in most operational rating
specificationsincludeitsoperatingrange,frequency,efficiency,EMI,outputcurrent,reg-
ulation, ripple percent, hold time, PG delay, agency approval, noise, and mean time be-
fore failure. The list should also include the voltage outputs the power supply produces
and any testing laboratory safety approvals (such as UL or TUV) it has been awarded or
conformities to FCC (Federal Communications Commission) or other regulatory agency
radio frequency (RF) emission standards.
Here is a brief explanation of the more common items you may find in a manufac-
turer’s specification list: Operating range States the minimum and maximum of its input and output
voltages. This is the least and most input voltage a power supply can take and
be able to produce its designated output values. An operating range that is
wide indicates that the power supply is able to produce reliable output
voltages from even a fluctuating, unreliable power source.
Efficiency The amount of output power that is produced from its input
power source. This number is usually stated as a percentage.
EMI (electromagnetic interference) All power supplies produce some
electromagnetic noise, but the FCC limits the amount of EMI noise a power
supply can produce. Most power supplies on the market today meet or exceed
the FCC requirements.
Output current The maximum volts that the power supply can consistently
produce and supply to the motherboard and the disk drives.
Line regulation Measures the amount of change passed through to the
output voltage from fluctuations in the input voltage.
Load regulation Measures the voltage change caused by increases in voltage
demands on the power supply.
Ripple percent A certain amount of variance, called aripple, occurs in the
output voltage as the result of incomplete conversion of the AC power source.
The ripple percent indicates the percentage of output voltage affected.
Hold-up time The amount of time the power supply will continue to provide
operating levels of power after its input power source is lost. This time should
be matched to the cutover time of the PCs UPS.
PG delay The amount of time needed by the power supply to cycle up before
it can send the POWER_GOOD signal.(^328) PC Hardware: A Beginner’s Guide