Power Supply isolating loads

[dinofx]

I was wondering if anyone know off hand how a computer PSU would isolate different loads so that a problem/short on one load would not damage the other. For example, the motherboard is supplied with a +5V line. The accessories (hard drive, DVD, etc.) also have a +5V line. How would a surge (+12V for example) on one line be prevented from going onto the other.

If there's nothing in the PSU, Is it the motherboard's responsibility to guard itself from this surge?

 

[Optikon]

I was wondering if anyone know off hand how a computer PSU would isolate different loads so that a problem/short on one load would not damage the other. For example, the motherboard is supplied with a +5V line. The accessories (hard drive, DVD, etc.) also have a +5V line. How would a surge (+12V for example) on one line be prevented from going onto the other.

If there's nothing in the PSU, Is it the motherboard's responsibility to guard itself from this surge?

I suppose it depends on how bad the "surge" is... The power supply is designed to have each output rail somewhat immune to load changes on others. It is captured as a cross-regulation spec. Typically, the supplies will regulate off of the rail that has the highest load - no doubt it's the +5V.

If the surge you are thinking about is mains related, the power supply is designed to deal with it and prevent this from causing harm to its output circuits. But everything breaks at a limit, if the surge is bad enough to get through the power supply (faulty protection or inadequate protection), then there will be all kinds of avenues for the outputs to become corrupted. The motherboard design will likely not provide additional overload protection for these types of extreme transient events (other than what is built into IC's etc..)

 

[dinofx]

I'm not really talking in terms of rails. I don't think there is ever more than one 5-volt rail. I've just talking about the many wires coming out of the PSU. One set of wires goes to the motherboard. Another set has several MOLEX connectors on it. Both "branches" have +5V.

The surge I am referring to is shorting a wire carrying +12V with a wire carrying +5V. How far can this surge go? Obviously, it can travel from one molex to the others on the same branch. Can it enter the PSU and then travel to the motherboard? Could the power supply isolate the branches without having dual 5-volt rails? e.g., diodes?