Thread: -12V and -5V rails explained:

I've seen some postings on this group about use of various rails on an ATX power supply, so I thought that this might be useful...

12V rail: Used by 3.5" Hardrives and 5.25" CDROMs motors, high-end video cards and often used to step down V(core) for the CPU power --a P4 CPU consumes 2-5A on this rail. VIA C3/C7 processors do not use this rail for V(core). As a matter of fact, the 12V rail us used only for MOSFET gate drive, about 50mA. If you have a system fan, then add another 100mA for the 12V rail.

5V rail: Used by 3.5" (logic), 2.5" drives (motor), M/B and I/O logic, video, USB, pretty much everything. VIA C3/C7 processors use the 5V rail for V(core), must reserve 2A for processor alone. Attention: older PIII and PII systems use this rail for V(core).

5VSB: Used by the 'sleep', monitoring circuitry and sometimes by 'always-ON" USB devices (depending on your BIOS settings), 1.5-2A should be more than enough.

3.3V rail: Core logic, memory and some monitoring circuitry. Usually 2-6A.

-12V rail: Used *only* by serial devices. If -12V rail is not present, your motherboard will still fire-up. Some modern serial COM ports have an internal -12V chargepump so you might not need -12V native. Cheap motherboard manufactueres will still use -12V serial for COM chipsets, VIA included -- with the exception of EPIA PD. Depending on baud rate, current consumptin on the -12V rail should be anywhere from 5-50mA per port.

-5V rail: Used only by legacy ISA devices, your motherboard will fire-up without the need of a -5V rail. The -5V was removed from the last two generations of ATX power supply standards.

If in doubt of the -12V and -5V rail needs, just cut these two wires from your ATX cable harness and then test. Only 12, 5 and 3.3 are really needed along with the PS_ON and PWR_GD signaling.

Hope this helps,
Andrei