ATX Design Changes
As one might expect with an ever changing technology, component specifications and modifications represent an ongoing development process.
The operation of the microcomputer power supply, while not dictated by INTEL, is nonetheless heavily influenced by the largest player in the motherboard
chipset market. INTEL publishes a design guide specification for power supplies to ensure they will be compatible with the most recent chipset offerings and
regularly revises this guide as PC power needs change. Revisions may occur as frequently as twice a year and power supply manufacturers must
likewise modify their designs to remain current.
While manufacturers will advertise their power supplies as meeting ATX12V 2.2 compliance standards -- this may or may not always be the case as
some suppliers may be in partial, but not full compliance. INTEL does state that not all ATX12V supplies must conform exactly to their specification.
The major changes that have been recently implemented are summarized below:
The industry standard color coding remains the same with two additions:
- Remove guidance and reference to the -5 VDC rail
- Increase the +12 VDC output capability
- Increase minimum measured efficiency
- Replace the 2x10 main power connector with a 2x12 connector to support the 75 watt PCI Express requirements
- Assign a separate current limit for the 12V2 rail and specify a 2x2 connector
- Add remote sensing to the +3.3 VDC output to compensate for excessive cable drops
If your supply is equipped with the higher current rated 12V2 rail, the color coding for this
will be yellow with a black stripe and will terminate in a 2x2 keyed MOLEX connector.
The second addition is of more importance to individuals converting their power supplies. The +3.3 VDC remote sensing wire (brown or orange) is connected directly to the
orange +3.3 VDC at pin 11 of the power connector (pin 13 if a 2x12). The sense wire will usually be of a lighter gauge (22 AWG) than the power wires (16 or 18 AWG)
and its purpose is to monitor the voltage at the connector in order to provide feedback for voltage compensation by the supply.
If your PS has two wires attached to the same pin on the motherboard power connector, orange +3.3 VDC and a brown sense wire on Pin 11 for example, then
these two wires should be joined when you make your conversion. Some power supplies may also have sense wires running to the +5 VDC and +12 VDC connector pins.
If there are multiple sense wires, then they will usually be of the same color as the primary supply wire, but of a lighter wire gauge. These sense wires will also terminate in
a different location on the PS printed circuit board than the heavier supply lines. As with the +3.3 sense wire, these additional wires should also be connected to the
corresponding supply lines. Failure to complete these connections may result in your power supply not latching into a Power_On mode when switched on.