Micro ATX Power Supply

A Micro ATX power supply is based on the ATX ("Advanced Technology Extended") form factor for personal computers.

The "ATX" standard replaced the existing "AT" form factor previously used by the IBM AT personal computer.

This article discusses the Micro ATX power supply, rather than the larger "ATX" or "Mini-ATX," or the smaller "Flex-ATX." "Micro ATX" has external dimensions 5x3x4 inches (height X Width X depth), and uses three mounting screws.

Major Features

This power supply has three main output voltages (+3.3V, +5V, and +12V) and two low outputs (-12V and +5V-standby). Originally, a -5V output was specified but has been made obsolete.

The standby power output permits software to power up the computer. This is the "Wake-on-LAN" or "Wake-on-ring."

Many details have changed from the introduction of the ATX form factor in 1995 to the present. Most changes were required to deal with increased power demands for +12V by CPU chips and video cards.

Something Old, Something New

When ATX was introduced, most of a power supply's wattage went through the +3.3V and +5V rails. The Pentium IV chip was first to seriously draw on +12V. Even if the connectors match, it may be useless or damaging to mix older and newer components.

The Importance of Low Technology

A power supply would seem to be a low-tech item in a personal computer. While that is true when compared to a processor or video card, the power supply is required to meet several stringent demands.

Both the voltage and wattage must remain within specified parameters. Otherwise, the computer will fail or its components will fry.

Compare Some Units

Just for comparison purposes, here are a few options for a Micro ATX power supply recently offered by one online retailer.

The "SilenX IXTREMA Pro IXM-2014 200W Micro ATX Computer Power Supply, 115V/230V Switching" supply is the least costly on offer. It is rated at 200W (watts).

The "KDM 300W TFX 12V Computer Power Supply MTFX9300, Supports SATA, w/ 12V Internal Fan" supply is in the middle of the price range. It is rated at 300W.

The "KDM Nickel M6400/7400 v2.2 400W Micro ATX P4 12V Power Supply with Dual Fans" is another mid-price unit but provides 400W (rather than 300W).

The "KDM Nickel Coating 480W Switching Power Supply v2.2 M7480DM2.2 (Micro)," also a mid-price unit, is rated at 480W.

The retailer offered a limited one year warranty for all these models.

Price, Performance and Reliability

What factors drive the price?

The first factor is performance, as measured by the maximum power rating in watts. A high-end gaming computer with a top-of-the-line graphics card and multiple-core CPU will use a lot of power.

The quality of the power supply's components is also important, and high quality leads to reliability. A design decision as simple as using heavier gauge wire will reduce the operating temperature (for a given power output), and lead to a longer operating life.

One study found that both performance and reliability were strongly correlated with the weight of a power supply. Heavier supplies used more components, thicker wires, and larger heat sinks.

Heat is the silent killer of power supplies. Reliability depends on generating less heat, and removing heat effectively (with fans).

How to Choose

The first factor is: does the computer need the Micro ATX power supply, or a different one?

Secondly, are the dates compatible? Check that both the computer and power supply are from the same vintage. If not, check the specifications carefully.

Next, be sure to look for a unit that can supply the necessary power, with some spare capacity.

Finally, within the same power range, a more expensive Micro ATX power supply should run cooler and be more reliable.