This Instructable describes how to replace the fan inside a standard PC power supply. You may want to do this because the fan is defective, or to install a different type of fan, for example, an illuminated one.

In my case, I decided to replace the fan because my cheap power supply's fan started making enough noise to drive me to distraction...

WARNINGS

Power supplies have dangerous voltages inside, even when completely disconnected. Capacitors on the line side usually retain their full charge even when unplugged, and can inflict a painful or even lethal shock. Please proceed only if you know what you are doing.
Disassembling the power supply will void its warranty.
Opening your PC may void its warranty, though I haven't come across such a computer so far. Also, fiddling with the insides may damage other components, so go ahead only if you are sure of yourself.

Update
- 2011-05-02 : Corrected explanation of fan dimensions. Thanks to KanyonKris for the correction (see comment at bottom)

Step 1: Before You Start

You will need to know what kind of fan you need to replace. Obviously, the only way to know this is to open up the power supply and see what type is required. In my case, I had to open it up twice; once to find out the type of fan, and the second time to replace it.

For your safety:
Before opening the power supply, try to discharge the capacitors inside as much as possible. I did this by switching on the PC and unplugging the power cord. Of course, there is no guarantee that this will completely discharge the capacitors.

Another method is to use a 1 megaohm resistor to short the capacitors. The capacitors are the large ones shown in step 6.

Step 2: Tools

Depending on the type of computer and type of power supply, you will need the following tools.

screwdriver (to open PC case and remove power supply screws)
wire cutter / stripper
soldering iron, solder and desoldering pump (in case fan needs to be soldered)
vacuum cleaner / compressed air can (to clean out dust)

Step 3: Open PC

First disconnect all cables connected to the PC and open up the case. Usually the case can be opened without tools, but in some cases you will have to unscrew the cover.

Step 4: Disconnect Power Supply Cables

Remove all cables coming from the power supply to the motherboard, hard discs, optical drives, floppy drive, and whatever else you may have. Sometimes there is a connection to the video adapter, and the motherboard may have two connections in different places. Disconnect all of these.

You need not disconnect other cables, but you may have to remove some data cables to get at the power connectors.

Remember where everything was plugged in! Usually there is only one place where each connector fits, but make sure you know how to put the cables back. In some cases (like Serial ATA hard disks), there are two power connectors, but you only use one (using both may damage the drive).

The easiest option here is to take a photograph (if you have a digital camera).

Step 5: Remove Power Supply

Once the cables are disconnected, you are ready to remove the power supply from the PC case.

First remove the screws connecting the power supply to the case at the back of the PC. Once these are out, you can lift out the power supply.

In some cases (like branded PCs), there is no need to unscrew the power supply, you can unplug it by removing the plastic tabs holding it down.

Step 6: Open Power Supply Cover

Now that you have the power supply out, you can remove the cover to get at the fan. Note that opening the cover will void the warranty. The case is usually opened by unscrewing the top. If you are unlucky, the top will be riveted on to prevent tampering, and you will have to drill out the rivets (not covered in this Instructable).

After opening the cover, unscrew the fan as shown. The four screws on the rear hold the fan in place.

Now you can finally figure out what fan to buy.

Note that some power supplies have two fans, one at the rear and one at the bottom. Also, the location of the fan(s) may be different from what I've shown.

Step 7: Get Replacement Fan

Buy or salvage a fan that closely matches the existing one. You have to consider the size, and voltage and current ratings. Further, you have to make sure it is powerful enough to cool the power supply. You need to make sure it will fit inside your power supply, so get one with the same dimensions (or as near as possible; DIYers can usually file away the excess bits ;-))

The size is the dimensions of the fan. My fan was the 80mm (8cm) type, meaning it is 80mm by 80mm. The voltage and current ratings are written on the label. You can usually use a regular PC case fan here. I picked the replacement based on its noise rating as well.

The cooling capacity is decided by the flow rate in CFM which is c ubic f eet of air moved per m inute (can be approximated by RPM) of the fan. Unfortunately, my power supply fan gave neither, so I picked one based on the wind output (felt by hand). The computer parts store had several fans being demonstrated, so I could compare the flow rate. Alternatively, try searching the fan manufacturer's web site for specifications.

You can check this site for reference.

Step 8: Remove Old Fan

After you buy the replacement fan, you can remove the old fan from the power supply.
If the fan is connected via a header on the PCB, you can simply unplug it and plug in the new fan. In my case it was soldered directly. If this is the case, make sure you get the same type of connector for the new fan.

If the fan is soldered, you have two options - cut mid-wire, join and insulate, or, as I decided to do, unsolder from the PCB and solder the new fan directly. This is slightly more trouble but the result looked better.

If you decide to unsolder the fan, first remove the power supply PCB from the case. This is attached via several screws. It is best to move it as little as possible, because of the dozens of wires connected. Moving unnecessarily will probably result in broken wires, which will be extremely hard to find. I moved the PCB only enough to remove and replace the fan.

Another advantage of this is, you are unlikely to touch the high voltage area of the PCB.

Step 9: Connect New Fan

Prepare the new fan wire for connection. First cut the cable to slightly longer than the original. The additional length is to allow for different placement / routing of the wire.

If there are three wires, you only need the red and black ones. The yellow is a sensor wire, not used here.

Solder the wires to the same place as the old fan. Make sure of the polarity. Then screw the fan to the case, once again making sure of the correct orientation (reversing this will mean the air will be blown in the wrong direction).

Step 10: Test Power Supply

Once you have fitted the new fan, it is a good idea to test that the power supply actually works, and the fan rotates properly. I skipped this step myself, so you'll need to check the following site, which describes how to operate the power supply without connecting it to the motherboard.

http://www.overclock.net/faqs/15751-info-can-i-use-two-power.html

Step 11: Reconnect and Power Up

If the power supply works OK, you can put it back into the case and reconnect it to the motherboard. Make sure all the connectors are in the right places. Reconnect all cables to the back.

Once connected, power up the PC. I had a hair-raising moment when the PC refused to boot at first, turned out I had dislodged one side of the CPU heat sink, and it was floating without making contact. Make sure that you don't mess up other connections when replacing the power supply, double check everything before switching on.

Enjoy your new fan!

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This is my first instructable, so please point out any mistakes / deficiencies