Is the rail a processor draws power from determined by the motherboard?

[PingSpike]

So, I have this old 145watt mATX power supply. I have a biostar mATX motherboard, nforce2 with integrated video. Unlike most boards of this era, it lacks that extra 4 pin power supply connection. Quite a few google searches seemed to have a lot of people indicating that the cpu drew from the 5v rail on this board, since it was unique in not using/needing the extra connector. No idea if those guys were full of it or not though.

The PSU worked fine for a simple pentium 3 system. But it only has like 4.5amps on the 12v rail. It seemed to have plenty on the 5v. I guess I'll just have to give it a shot.

 

[Aluvus]

Yes. On boards with a dedicated 12 V connector, power for the CPU will come from the 12 V rail. On others it will come from the 5 V rail.

You could probably buy a good-quality modern power supply with higher 5 V (and other) ratings for cheap.

 

[mpilchfamily]

The problem you get there is most modern PSUs have trouble when they are cross loaded. Meaning wehn they have a higher 3.3v/5v load then whats on the 12V rails. You loose allot of efficiency and there is a good bit of Vdrop.