power supply - What is the usage of the negative voltages on a PC motherboard?

What is the usage of theses negative voltages? Are they there only for backward compatibility?

In nowadays PC power supplies, we have:

• +12V


• +5V


• +3.3V

but also:

But the current rating of the negative rails are much smaller than the positive ones.

If we were back to the 80' where op-amps were always powered symmetrically at +12V -12V: Okay.. But nowadays, almost everything you may find on a motherboard is digital logic only powered by positive voltages.

Except for the RS232, which is an almost obsolete bus, I don't see any reason for having negative rails distributed by the power supply.

Because it's very high volume, I suppose that cost drives everything here. Thus, why each PSU has to deliver those voltages if they are barely used ? (the very low current rating of the negative rails of PSUs let me suppose this).

Wouldn't it be less expensive to let every hardware provider to add their own embedded SMPS when a negative voltage is required?

Answer

PCs are stuffed with requirements which relate to backwards compatibility - and -Ve rails are part of that. I'm not sure about -5V, but there's a -12V line on the original PCI bus, so if you want to provide proper PCI sockets, then you need a -12V rail, even if the last person making a PCI card which needed -12V died in 2002.

Then if you want to design a standard power connector pin-out which can be used by people building motherboards with PCI connectors on it, then it needs a -12V rail, or else the motherboard manufacturer needs to start adding power supplies to his motherboard. So now you have a -12V rail on your power connector even after people have stopped fitting PCI connectors.

Some of these things are remarkably difficult to get away from — the 'legacy free' PC with no PS/2-style keyboard/mouse connections was being talked about as imminent 15 years ago, but desktop machines still tend to have those connectors.

It just turns out to be cheaper/easier to keep supporting all the old cruft than it does to drop it and clean-up the design. Or perhaps it doesn't, and PCs have sunk under the accumulated weight of all this baggage and people have moved on to other form-factors...