Hardware Reference
In-Depth Information
Chapter 18. Power Supplies
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 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 the 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 120V (nominal) 60Hz AC (alternating current) or 240V (nominal) 50Hz
AC power into +3.3V, +5V, and +12V 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.
Voltage Rails
The PSU normally supplies +3.3V, +5V, and +12V to the system. These voltages are often called
rails
, referring to the fact that although there are multiple wires carrying a specific voltage, they are
normally tied to a single rail (or tap) in the PSU. Multiple wires are used because, if all of the current
were carried over a single wire, the wire and the terminals, connectors, and even the traces on the
circuit boards would all have to be extremely large and thick to handle the load. Instead, it is cheaper
