ATX12V vs. ATX Power Supplies

What's the difference between the two specification standards?

ATX and ATX12V are specification standards for desktop computers. When purchasing a power supply for a desktop PC, make sure it is at least ATX12V v2.01 compliant or higher. These power supplies will still function with older ATX systems using the 20-pin main power connector, provided there is sufficient space.

Information in this article applies broadly to different types of power supplies. Make sure a power supply is compatible with your computer before making a purchase.

ATX 12V vs ATX

What Are ATX and ATX12V?

Desktop specification standards were created to define the various dimensions, layouts, and electrical requirements so that parts can be exchanged between vendors and systems. The original Advanced Technology (AT) design was developed in the early PC years for IBM compatible systems. As power requirements and layouts changed, the industry developed a new definition called Advanced Technology Extended (ATX) in 1995. A new standard known as ATX12V was defined in 2000, and an update called ATX12V 2.4 has been in use since 2013.

ATX vs. ATX12V Power Supplies

All computer systems require a power supply capable of converting electrical power from high voltage wall outlets to the low voltage currents used by the system components. Part of the purpose of having specification standards is ensuring backward compatibility, so power supplies that are ATX12V compliant will usually work with ATX systems, but older ATX components do not work with newer machines. The differences between the current ATX12V 2.4 standards and the previous ATX standards for power supplies include:

Overall Findings

ATX
  • Uses a 20-pin main power connector.

  • 6-pin aux power connector required.

  • Serial ATA power connectors required.

ATX12V
  • Uses a 24-pin main power connector.

  • 6-pin aux power connector not required.

  • Use of dual 12V rails if greater than 20A.

Your motherboard will be the biggest decider on which power supply to use. Make sure you check to see if you have a 20-pin or 24-pin connector on the motherboard. If you're not sure, always check with the manufacturer of your motherboard for proper power needs.

Connectors Are Key

ATX
  • 20-pin standard.

  • Cannot be used on newer motherboards.

ATX12V
  • Can have a 20-pin or 24-pin configuration.

  • Connectors could have separate 4-pin molex. (20-pin only)

  • Is backwards compatible with ATX.

24-Pin Main Power Connector

The most notable change from ATX to ATX12V is the number of connector pins. PCI Express has a 75-watt power requirement, which was not possible with the older 20-pin connector, so four additional pins were added to supply the additional power through 12V rails.

Serial ATA Connectors

Even through Serial ATA connectors can be found on many ATX v1.3 power supplies, they are not a requirement for the standard. The rapid adoption of SATA drives forced the ATX12V standard to increase the minimum number of connectors on power supplies. Older ATX v1.3 units typically only provide two while newer units have four or more.

The pin layout on ATX12V is keyed such that the 24-pin power connector can actually be used on older ATX motherboards with the 20-pin connector. The caveat is that the four extra pins will reside off to the side of the power connector on the motherboard, so be sure there is enough clearance for the extra pins if you plan on using an ATX12V unit with an older ATX motherboard.

Rails Aren't Just for Trains

ATX
  • Operates on +3.3V or +5V.

  • Typically uses a single rail.

  • Not as power efficient.

ATXV12
  • Operates on +12V,

  • Can have multiple rails.

  • Additional rails give better power efficiency.

Dual 12V Rails

As the power demands for processors, drives, and fans keep growing, the amount of power supplied over the 12V rails from the power supply has also grown. In order to address this, the ATX12V standard now requires the 12V rail to be split into two separate rails for any power supply that produces extremely high amperage. Some high wattage power supplies even have three independent 12V rails for increased stability.

ATX power supplies normally only use a single rail at 3.3V or 5V. Some increased power demands required some additional power. You may find some ATX power supplies with an additional rail to provide that additional power. However, as ATX power supplies cannot provide enough power for modern PC's, you may find it challenging to locate an ATX power supply.

Power Efficiency

When the electrical current is converted from the wall outlet voltage to the lower voltage levels needed for the computer components, there is bound to be some waste that gets transferred into heat. So, even though a power supply may provide 500W of power, it is actually pulling more current from the wall.

The power efficiency rating determines how much power is pulled from the wall compared to the output to the computer. While some older ATX power supplies have efficiency ratings lower than 70%, the ATX12V standard requires a minimum efficiency rating of 80%, and there are power supplies with much higher ratings.

Using a higher efficiency power supply for your PC can lower your power bill.

Final Verdict: ATX12V Is the Power Standard

Older ATX power supplies simply do not have the power, efficiency, or availability of the ATX12V. As manufacturers have all but stopped production, if you have an older motherboard, you'll have to rely on the ATX12V's backward compatibility to keep older PC's running.