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ATX12V vs. ATX Power Supplies

A Look at the Differences in Power Specifications

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ATX12V Power Connector

ATX12V Power Connector

Mark Kyrnin

Introduction

Over the years, the base components of computer systems have dramatically changed. In order to standardize the design of the system, specifications standards were developed for desktop computers that define the various dimensions, layouts and electrical requirements so that parts could be easily changed between vendors and systems. Since all computer system require electrical power that is converted from high voltage wall outlets to the low voltage currents used by the components, power supplies have very clear specifications.

AT, ATX, ATX12V?

Desktop design specifications have been given a variety of names of the years. The original Advanced Technology or AT design was developed in the early PC years with the IBM compatible systems. As the power requirements and layouts changed, the industry developed a new definition called Advanced Technology Extended or ATX. This specification has been used for many years. In fact it has undergone a large number of revisions through the years to deal with various power changes. Now a new format has been developed over the years called ATX12V. This standard is officially known as ATX v2.0 and above.

The primary differences with the latest ATX v2.3 and ATX v1.3 are:

  • Use of a 24-pin Main Power Connector over 20-pin Connector for PCI Express Support
  • 6-Pin Aux Power Connector Not Required
  • Use of Dual 12V Rails if Greater than 20A
  • Serial ATA Power Connectors Required
  • Minimum Power Conversion Efficiencies

24-Pin Main Power

This is the most notable change for the ATX12V standard. PCI Express requires a 75 watt power requirement that was not capable with the older 20-pin connector. To handle this, 4 additional pins were added to the connector to supply the addition power through 12V rails. Now the pin layout 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 4 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.

Dual 12V Rails

As the power demands of the processors, drives and fans keeps growing on the system, the amount of power supplied over the 12V rails from the power supply has also grown. At higher amperage levels though, the ability of the power supply to generate a stable voltage was more difficult. In order to address this, the standard now requires any power supply that produces extremely high amperatge for the 12V rail to be split into two separate 12V rails to increase stability. Some high wattage power supplies even have three independent 12V rails for increased stability.

Serial ATA Connectors

Even through Serial ATA connectors could be found on many ATX v1.3 power supplies, they were not a requirement. With the rapid adoption of SATA drives, the need for the connectors on all new power supplies forced the standard to require a minimum number of connectors on the power supplies. Older ATX v1.3 units typically only provided two while newer ATX v2.0+ units supply four or more.

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 is transferred into heat. So, even though the power supply may provide 500W of power, it is actually pulling more current from the wall than this. The power efficiency rating determines how much power is pulled from the wall compared to the output to the computer. The newer standards require a minimum efficiency rating of 80% but there are many which much higher ratings.

Conclusions

When buying a power supply, it is important to buy one that meets all of the power specifications for the computer system. In general, the ATX standards are developed to be backwards compatible with older system. As a result, when shopping for a power supply, it is best to purchase one that is at least ATX v2.01 compliant or higher. These power supplies will still function with older ATX systems using the 20-pin main power connector if there is sufficient space.

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