S.M.P.S (Switch Mode Power Supply)

The case is almost universally sold with the power supply installed and included in the price. You can build a PC on a workbench without a case (technicians often do this when testing parts), but it takes up a lot of space, interferes with the radio, and is awfully hard to pick up and move in one trip. The function of the case is to house all the parts that make up your PC, provide ventilation for the heat they generate, and protect the local environment from radio frequency interference.

All electrical devices that produce radio frequency emissions are required by law to be certified by the Federal Communications Commission (FCC) as no interfering with assigned broadcast frequencies. Computers produce a lot of radio frequency "noise" in the FM radio band and higher, but at very low power levels. Normally, if a computer in your home interferes with a radio or television, moving it to another room or even just changing its position by a couple feet will fix the problem.

Computer parts are sold as being FCC Class A or B approved. Class A is for business use, the Class B rating meets more stringent limits for residential use. Assembling a collection of approved parts is no guarantee that the completed computer would pass an FCC test suite for one rating or the other, but as a home hobbyist, you aren't required to have your computer tested. However, if you decide you love building PCs so much that you want to go into business selling thousands of them, you'll want to buy partially assembled or packaged systems that come with an FCC approval sticker.

Power supplies are equipped with a 115V/230V switch, so they can be set to 230 volts for Europe and most other regions of the world that don't use the U.S. standard 115 VAC distribution system. Just a few years ago, this 115 or 230 volts was wired directly to the switch on the front of the PC, like the switch on a lamp or a toaster oven. However, in all new PCs, the high voltage never leaves the power supply. The switch on the front panel is really just a logic switch that closes a circuit on the motherboard, which tells the power supply to come on at full power. The power supply is always providing a trickle of current to the motherboard to enable this "wake up" logic, whether the signal is generated by the power switch or by incoming traffic to the modem or network card.

ATX Power Supply

The second function of the power supply is to generate a cooling airflow for both itself and the other parts in the case. This fan in the power supply is thermion source of noise coming from most PCs. The manufacturers of the newest high-speed components often recommend that you include additional fans in the case of increase the cooling airflow. The most common location of a single additional fan is at the bottom of the front of the case, to draw in air. A second fan can be added under the power supply at the back of the PC to exhaust more hot air. The goal is always to increase the airflow through the case, not just to blow hot air in a circle, so don't install several fans to draw air into the case and none to exhaust it, or vice versa.