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Switching Power Supplies A - Z, Second Edition
Switching Power Supplies A - Z, Second Edition, by Sanjaya Maniktala (Newnes), starting at $55.25
Home » Power
Everything You Need to Know About Power Supplies
Author: Gabriel Torres 723,648 views
Type: Tutorials Last Updated: May 29, 2008
Page: 7 of 14

The power supply plays a crucial role in the process of cooling the PC. Its exact function is to remove hot air out of the case. The air flow inside the PC works as follows. The cold air enters through the grooves found in the frontal part of the case. The air is heated by devices such as the processor, video card, chipset, etc. As hot air is less dense than cold air, the natural tendency is to rise. Consequently, hot air is retained in the top of the case. The power supply cooling fan works as an exhaust fan, pulling hot air from this area and blowing it out of the PC. See how this works in Figure 24. Hi-end power supplies have two or three cooling fans. Some cases have appropriate space for installing an extra fan at the rear.

Airflow on a PC
click to enlarge
Figure 24: Airflow inside the PC case.

Traditionally, PC power supplies use an 80 mm fan on their rear side, as you can see in Figure 25. Some years ago, power supply manufacturers started using a 120 mm or bigger fan on the bottom of the power supply, replacing the rear panel of the power supply with a mesh. Usually, the use of a bigger fan provides a higher airflow and a lower noise level, because a bigger fan can rotate at a lower speed in order to produce the same airflow as a smaller fan.

PSU with 80 mm fan
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Figure 25: Power supply with an 80 mm on the rear.

PSU with 120 mm fan
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Figure 26: Power supply with a 120 mm on the bottom.

Some power supplies may have more than one fan, while a few manufacturers provide speed control to the power supply fan or a cable for you to monitor the fan speed through your favorite monitoring program. This cable must be installed on an empty fan header on the motherboard. (These features are not so common.)

The problem with a power supply fan and/or extra fans is the noise produced by them. Sometimes, it's such an irritating noise that simply working with the computer causes us stress.  In order to reduce noise, currently, most power supplies use a circuit to control the fan speed according to the power supply internal temperature. When the power supply is cold the fan spins at a lower speed, thus producing less noise.

In order to provide a better airflow and organization inside the PC, some power supplies use a modular cabling system, where instead of being permanently attached to the power supply, peripheral cables are attached to the unit using connectors. You can remove the cables you won’t use. Some manufacturers also sell extra cables for their power supplies' modular cabling system, helping users for future upgrades. Usually on power supplies using modular cabling systems the main motherboard cable and the ATX12V/EPS12V cables are permanently attached to the unit, as shown in the power supply portrayed in Figure 27.

PSU modular cabling system
click to enlarge
Figure 27: Modular cabling system.

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