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[ The PC Guide | Systems and Components Reference Guide | Power | The Power Supply | Power Supply Form Factors ]

ATX (NLX) Form Factor

At the time of its introduction by Intel in 1995, the ATX form factor was the most significant change in system design since the invention of the PC over a decade earlier. Although it took several years to "catch on", the ATX form factor and its variants are now the standard in a large segment of the marketplace. In addition, the NLX motherboard and case form factor--designed to replace LPX--intentionally use the same power supply because Intel wanted to avoid having another power supply form factor on the market. Therefore, the ATX form factor is sometimes called the "ATX/NLX" form factor.

On the outside, the ATX power supply appears virtually identical to an LPX power supply in terms of its dimensions and component placement. The biggest visible difference between the two is that the power pass-through outlet for the monitor has been removed (primarily because modern monitors always come with their own power cord these days, so the pass-through hasn't been commonly used for some time.)

Diagram of the side and rear views of an ATX/NLX form factor power
supply, with approximate dimensions. Note that the dimensions are
essentially unchanged from those of the LPX form factor. The biggest
change is the removal of the monitor power pass-through connector
from the standard. By the time ATX was introduced these were rarely
used by modern PC monitors in any event.

Original image PC Power & Cooling, Inc.
Image used with permission.

The inside of the ATX form factor, however, is an entirely different story. The ATX power supply design differs from the previous market standards, the Baby AT and LPX form factors, in several important ways:

  • True Standard: The ATX form factor is a standard, as opposed to the "de facto standards" of prior form factors. You can find detailed specifications about ATX and other newer form factors at the Platform Development Support Web Site. Included there is a document specific to the ATX power supply.
  • +3.3 V Power: ATX systems were the first to include +3.3 V power directly, avoiding the need for voltage regulators to provide it on the motherboard.
  • Soft Power: ATX systems were the ones where the +5 Standby and Power On signals were introduced. These signals are used along with a change to the way the power switch works, as part of the "Soft Power" feature that enables features such as allowing the operating system to turn off the PC.
  • Additional Signals: ATX defines several additional signals used for fan control, IEEE 1394 compatibility, and more.
  • Changed Motherboard Connectors: Breaking with 15 years of tradition created by the PC/XT, AT, Baby AT and LPX form factors, Intel specified new motherboard connectors for the ATX form factor. This was in part due to the additional signals used by the ATX power supply and motherboards. For compatibility, some motherboards include both the new and old style of connector. Read more about the motherboard connectors here.
  • Modified Fan Direction and Placement: One of the goals of the original ATX specification was to change the way the power supply fan worked. At around the time ATX was introduced, cooling fans were becoming the standard for the newer, faster CPUs on the market. Instead of exhausting air out the back of the case as had always been the norm, Intel wanted to use this exhaust air to cool the processor directly, saving the cost of a cooling fan. Therefore, the ATX specification calls for the fan to run in the opposite direction and be placed near the CPU's location on the motherboard, to blow on it for cooling. The other advantage of this method is that it keeps the system cleaner, since air entering the case all comes from one place, and can be filtered if necessary.
    Unfortunately, while a good idea, this hasn't worked out quite the way Intel hoped. The primary problem is that newer CPUs continue to generate more and more heat as they get faster, and a regular power supply fan doesn't have enough flow to cool them properly. This problem is compounded by the fact that the air blowing on the CPU is warmed by the components in the power supply itself, so it is several degrees above ambient temperature before it ever gets near the CPU. Thus, newer versions of the ATX specification make the fan direction optional. The newest ATX power supplies have gone back to the old style of placing the fan on the back of the power supply and exhausting air to the outside.

Rear view of an ATX power supply, showing its
motherboard and drive connectors. This power supply
blows air out of the system; the vents on the side
are for drawing air from the inside of the system case.

Image Enlight Corporation
Image used with permission.

Since it has become the industry standard, ATX power supplies are found everywhere. Ostensibly designed to work with ATX cases and ATX (and Mini-ATX) motherboards, ATX power supplies are also used in NLX systems, as mentioned above. They can also be used for microATX motherboards in microATX cases if the case is large enough, because the ATX and SFX main motherboard connectors are essentially the same.

Next: SFX Form Factor

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