Single 12v railed PSU question about electrical "noise"

[HKPolice]

From Jonny's Seasonic M12 review:

Single 12V rails work well for high end machines with video cards with GPU's that can easily overload your typical rail, but you do miss the advantages of split 12V rails, such as separation of "noise" between rails and the protection provided by the separation so a potential short on one rail doesn't harm a device loation on another.

So is it possible for "noise" from one device to affect all others connected to the same 12v rail?

[jonnyGURU]

Sure. In some cases. Often there is a cap used in the OCP stage. But typically noise from devices like HDD and fan motors, CCFL's, etc. only affects other devices on the same line (one line w/ several Molexes, for example.)

[cypherpunks]

So is it possible for "noise" from one device to affect all others connected to the same 12v rail?

Yes, but the question above implies a misunderstanding: it's just as possible for the "noise" (to be precise, the fluctuations in current draw) from one device to affect (cause a fluctuation in the supply voltage to) all others connected to any 12V rail. The rails are separated only by current limit circuits which, likle fuses, are just special pieces of short wire. They have negligible noise-filtering effect. (The long wires from the PSU to each load have far more effect.)

All this is due to the fact that any real PSU has non-zero output impedance, so a current draw fluctuation causes a voltage fluctuation. That's just Ohm's law. (An ideal voltage source would have zero output impedance, but reality can only approximate that.)

Usually the biggest effects are fluctuations generated by the PSU itself, and then the effect of a load (device requiring power) on itself. Unless you have an exceptionally noisy load, and another exceptionally sensitive one, sharing the PSU doesn't have a huge effect.

(A second common misunderstanding is that more separation of the power supply rails would be desirable. But splitting up the outputs implies splitting up the output filter capacitors, which would instantly double the effect of a load on itself. And that would usually be worse than the savings in interference from other loads.)