Tom's Hardware and Que Publishing are partnering up to give you four chapters from Scott Mueller's Upgrading And Repairing PCs, 20th Edition. This fifth installment is the beginning of the forth chapter we're making available from Scott's book, which covers Power Supply Fundamentals. Don't forget to check out the previous chapters published on Tom's Hardware, Computer History 101: The Development Of The PC, Hard Drives 101: Magnetic Storage, LAN 101: Networking Basics, and LAN 102: Network Hardware And Assembly. In the days to come, we'll also present a comprehensive look at Power Supply usage factors.

The Power Supply

The power supply is not only one of the most important parts in a PC, it is unfortunately one of the most overlooked. Although most enthusiasts who build their own systems understand its importance, the mainstream PC buyer generally does not. Some that do pay any mind seem concerned only with how many watts of power it is rated to put out (even though no practical way exists to verify those ratings), without regard to whether the power being produced is clean and stable or whether it is full of noise, spikes, and surges.

I have always placed great emphasis on selecting a power supply for my systems. I consider the power supply the foundation of the system and am willing to spend a little extra to get a more robust and reliable unit. The power supply is critical because it supplies electrical power to every other component in the system. In my experience, the power supply is also one of the most failure-prone components in any computer system. Over the years I have replaced more power supplies in PCs than any other part. A malfunctioning power supply not only can cause other components in the system to malfunction, but it also can damage the other components in your computer by delivering improper or erratic voltages. Because of its importance to proper and reliable system operation, you should understand both the function and limitations of a power supply, as well as its potential problems and their solutions.

Primary Function and Operation

The basic function of the power supply is to convert the electrical power available at the wall socket to that which the computer circuitry can use. The power supply in a conventional desktop system is designed to convert either 120 V (nominal) 60 Hz AC (alternating current) or 240 V (nominal) 50 Hz AC power into +3.3 V, +5 V, and +12 V DC (direct current) power. Some power supplies require you to switch between the two input ranges, whereas others auto-switch.

Technically, the power supply in most PCs is described as a constant voltage switching power supply unit (PSU), which is defined as follows:

• Constant voltage means the power supply puts out the same voltage to the computer’s internal components, no matter the voltage of AC current running it or the capacity (wattage) of the power supply.
• Switching refers to the design and power regulation technique that most suppliers use. Compared to other types of power supplies, this design provides an efficient and inexpensive power source and generates a minimum amount of heat. It also maintains a small size and
  low price.