A computer's power supply converts electricity received from a wall outlet (120V AC in the U.S.A.) into DC current amounts that are needed by the various components of the system. The back of the power supply has a plug for the cord that goes to the wall outlet. There are 2 different types of power supplies that correspond to 2 different types of motherboards, and hence, case designs.
Most power supplies have a switch on the back that sets the power supply to 115 or 230 volts (for European countries). Setting this switch to 230 in the U.S. won't damage anything, but the PC probably won't boot, or will shut down in the process. Setting this switch to 115 volts in Europe, will fry the power supply and possibly other components in the computer. Make sure the switch is in the correct position if there is one.
Every device in a PC uses power which means that you need to have a power supply with enough wattage to run the system. If you have a 250 Watt power supply for a server with 10 hard drives, there are going to be problems. In fact, it may not boot up all the way. Power supplies for new computers are almost always capable of handling normal loads. If you are going to add a ton of drives or new devices to a system, that is when you might consider upgrading the power supply.
For most current PCs the ATX power supply is the standard. There are slight variations such as the 12v and 12v 2.0, but for the most part power supplies for desktops are pretty standardized, although newer BTX power supplies are a different size and shape than the ATX type. Power supplies offer 12, 5 and 3.3 volt currents to power the various electronics in a computer. This is done via power cables coming out of the power supply. Below are some of the common power connector types.
P1 Power Connector - ATX power supplies use a single 20 or 24 pin (on newer versions) to connect to the system board. Some motherboards require an additional 4, 6, or 8 pin auxiliary power connector. Power connectors are keyed to make sure that the connector is plugged in properly.
SATA Connectors - SATA hard drives use a special 15 pin power connector. This connector supports 3.3, 5, and 12 volt devices. Make sure your power supply has one of these or you can get a molex to SATA adapter if it doesn't.
Molex Connectors - These connectors are used for connecting IDE hard drives, DVD and CD drives, and other devices that require 5 or 12 volts of power. These are keyed to prevent plugging them in upside-down, however, it is possible to do and will cause serious problems/damage to the system.
Mini Connectors - These connectors also supply 5 or 12 volts, but are basically only used to connect floppy drives. These are even easier to plug in upside-down.
Laptops and portables utilize an external power supply and rechargeable battery system. Batteries were typically nickel-cadmium, but newer techologies have introduced nickel metal-hydride and lithium-ion batteries that provide extended life and shorter recharge times. Lithium batteries are also used to power a computer's CMOS ROM.
To remove a power supply from a PC, follow these steps:
Unplug the computer from the wall
Disconnect all of the internal power connections(i.e. CD Rom, Motherboard, hard disk, etc)
Remove the 4 retaining screws
Pull power supply out of the computer
Repeat these steps in opposite order to install a power supply
Power supply problems can be some of the most difficult to diagnose particularly when the problem is intermittent. Often the fuse in a power supply will blow and you may actually hear a pop and/or smell smoke. Oftentimes, if you shake the power supply, you will hear a rattle. This means it is dead and needs to be replaced. You should not open the power supply and replace the fuse or try to fix any other part of it. There are capacitors inside that hold a charge and power supplies are way too inexpensive to risk injury on.
Intermittent problems can display a wide range of symptoms from not booting correctly, to errors, to locking up after a period of use. It is a good idea to keep a spare on hand for troubleshooting.