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Power Supply Fan
One of the more important components in the power supply is one that seems tangential to it: the power supply fan. Since the earliest PCs, the power supply fan has been the primary cooling source for the entire PC. Today's PCs of course incorporate additional cooling methods, including auxiliary fans and CPU cooling devices, but the power supply fan remains an important factor in the overall cooling equation. You can find out more on the general subject of system cooling in this section of the System Care Guide.
The fan is traditionally located at the rear of the power supply, and special vents are provided for it in the case of the supply itself to allow for it to exhaust (though the newer ATX form factor changes things; see below). Most fans use +12 V power to operate, despite the fact that the wires that run to them are normally red for the +12 V line, and black for the ground (not yellow and black as you might expect from the wire color standards elsewhere in the PC).
In addition to the regular fan found in the power supply, most newer systems include auxiliary fans for improved air flow and system cooling. These are typically mounted at various venting locations around the outside of the system case. The standard size of a PC cooling fan is about 3.25" or 80 mm square, but they come in other sizes as well.
A very important quality consideration when it comes to PC cooling fans is the quality of construction of their motors, and in particular, the motor bearings. Cheaper fans use sleeve bearings that are much less durable than their ball bearing counterparts. While "sleeve vs. ball" isn't the only dimension upon which to measure cooling fan quality, it is an important one. Sleeve bearing fans can lock up after as little as a year of use, while ball bearing fans typically last many years.
Another quality consideration of a fan is how much air it can move. This is normally measured in cubit feet per minute (CFM). The higher the rating, the more work the fan is accomplishing. Fan speed can be controlled on some systems through the use of the FanC, FanM and/or Fan On/Off signals. Many power supplies also have automatic thermal control of the power supply fan: they reduce or increase its speed based on internal temperature without any intervention required by the rest of the system.
The fan is the component most likely to go first in a power supply. The usual cause of this is dirt that gets into the motor of the fan and gums up the works. The average time until failure is greatly increased if the PC is used in a very dirty or dusty environment, or if the PC is never cleaned. When the fan stops working, overheating of the components within the power supply as well as components in the rest of the PC are likely. A PC that makes use of the optional fan monitoring signal FanM can detect a fan failure and sound an alarm to the user, or shut down the PC. Another way of detecting an overheating condition is through hardware that monitors the internal temperature of the system.
The power supply fan is probably the only component that can be replaced by an end-user (although I still recommend against it for most users since it requires opening the power supply case.) If a technician replaces the fan, an easy way is to use one of the fans shown above, with the drive connector snipped off and the wires spliced into the ones that the old fan used. A possible solution to a bad fan that does not involve opening the power supply is an add-on external fan. These fans plug into the wall directly and are typically heavier-duty and higher-capacity than a standard power supply fan. They are actually designed for improving the cooling of the existing fan even when it is running.
The final issue to consider regarding the power supply fan is the direction in which it circulates air. Older PC/XT, AT, Baby AT and LPX form factor power supplies are designed to exhaust air out the back of the PC. For older machines using older, slower CPUs, this worked fine, but starting with the Intel 486, separate cooling started to become required for the processor. In response, Intel designed the ATX form factor to reverse the flow of air and move the power supply fan to the inside edge of the supply case, with the goal of using the power supply fan to also cool the CPU. Later, when it became obvious that the newest CPUs still needed their own cooling and having the power supply fan blow already-heated air on them wouldn't get the job done, Intel made the fan direction (and location) optional (see the ATX form factor section for details).
One advantage of a power supply fan that blows into the case is that it provides much better control over the air that enters the system. Instead of being drawn in through all the holes and cracks in the case as with a fan that pushes air out the back of the system, the air entering the case all comes in from the power supply fan intake. This intake can be filtered to dramatically reduce the amount of dirt that accumulates within the system.
Next: Power Supply Fuse