(^320) PC Hardware: A Beginner’s Guide
filtering, regulation, isolation, cooling, and power management. Here is an explanation of
each of these functions:
Rectification A rectifier converts AC power to DC power. The primary task
of the power supply is to rectify the AC power of the power source into the DC
power used by the computer.
Filtering When electricity is rectified, electrical ripples are occasionally
introduced in the DC voltage. These ripples are smoothed out through
electrical filtering.
Voltage conversion The PC uses only a small range of voltages, including
+/–5V, +/–12V, and +3.3V. The 110V AC primary power source must be
converted into the +12V and +5V DC used by many older PCs and the +3.3V
DC used by most newer PCs.
Regulation Voltage regulation, along with filtering, removes any line or load
variations in the DC power produced by the power supply.
Isolation The AC power must be kept separate, insulated, and isolated from
the DC power.
Cooling The main system cooling fan that controls the airflow into or out of
the system case is typically located inside the power supply. Some systems also
have auxiliary fans located outside of the power supply.
Power Management Nearly all newer PCs have energy efficiency tools and
power management functions to help reduce the amount of electrical power
consumed by the PC.
Outside of North America, where the primary power source is already DC, the power
supply performs all of the same tasks, except rectification. Most power supplies have the
ability to take 110V AC or 220V DC and have a two-position slide switch on back of the
power supply near the fan grill that is used to select the power source voltage. If this switch is
on 220V, it will not harm the system to plug it into a 110V source. The power supply will not
be getting as much power as it believes it should be and will not function at all. So, if a PC
seems to not have power the first time you plug it in, the first thing to check is the voltage
selector.Ifitisonthewrongsetting,itislikelythatthefactory,whichisprobablynotinNorth
America, tested the system on 220V and forgot to change the switch before shipment.
Good Power Signal
After rectification, one of the power supply’s most important functions is sending the
POWER_GOOD (or PWR_OK) signal to the motherboard. The POWER_GOOD signal tells
themotherboardthatthepowersupplyhascompleteditscycle-upprocessandisnowableto
provide clean power in the voltages needed by the PC. Should there be a problem with the
power or with the power supply, no signal is sent and the boot process never completes.
The power supply performs a self-test when the power is switched on that checks the
incoming power for the required voltages. If all is well, the POWER_GOOD signal wire is