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Additional Power Signals

Some power supply form factors define additional power signals beyond the standard voltage outputs and the power good and soft power signals. Most of these are signals that can be implemented optionally by the power supply manufacturers, "optional" meaning that they are not required by the form factor for the power supply to meet the specification. In practice, this means that they are left off of most power supplies, especially less expensive models, to save cost. However, they are present on some supplies, so it's useful to understand what they are..

Note: For these functions to do you any good, you need a corresponding capability in other components of your system, most importantly your motherboard. Normally this means a matching socket for any optional motherboard connectors used by these power supplies to supply the additional signals. See here for more details.

The following additional signals are specified for ATX/NLX systems:

• +3.3 V Sense: This signal is used to detect the voltage level of the +3.3 V signal being provided to the motherboard. This allows the power supply to "fine tune" the +3.3 V output in the event of excessive voltage drop between the supply and the components that use +3.3 V. This is more needed for +3.3 V than the other signals probably because CPUs use +3.3 V. (Note that the ATX specification makes this signal "pseudo-optional"; it is by default included in the main ATX connector but can be replaced by a wire in the auxiliary ATX connector. See here for more details.)
• FanC: This is a fan control signal, which allows the motherboard (and hence the system as a whole) to control the speed of the power supply fan. If implemented, when the voltage on this signal is less than 1 volt, the fan is turned off. As the voltage is increased the fan spins faster, and when it is over 10.5 V, the fan is run at full speed. This can be used to shut the fan off if the system is put into a sleep mode, or to allow the fan's speed to be increased or decreased based on the temperature of the system (saving power and reducing unnecessary noise.)
• FanM: A companion to FanC, this is the fan monitor signal, which allows the motherboard to keep track of the current speed of the power supply fan. Sort of a "power supply fan tachometer" for fans designed to implement it. This could be used to provide a warning to the user if the main cooling fan in the power supply failed.
• 1394V and 1394R: This pair of signals provides a separate, unregulated voltage circuit for powering IEEE-1394 ("FireWire") peripherals. It is not used by the motherboard.

The SFX form factor defines just one optional signal, called "Fan ON/OFF", which is essentially the same as the "FanC" ATX signal described above.

WTX, reflecting its status as a high-end form factor, includes several additional signals. These include the +3.3 V Sense, FanC and FanM signals described as above for ATX/NLX, as well as these:

• Sleep: Puts the power supply into sleep mode. This is used for power savings, to power down parts of the power supply. It is used in conjunction with the Power On signal.
• +3.3 V_AUX: This is a standby +3.3 V signal just like the +5 V Standby signal defined for standard ATX Soft Power.
• +5 V Sense: Just like the +3.3 V Sense signal, but for +5 V. The WTX form factor also provides special, dedicated grounds (called returns) for its sense lines.

Next: Parts of the Power Supply

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