Conputer Power Supplies

[dpaengineer]

Looking for some info here fella's.

What is the -12v for on a pc power supply?

[Guest]

From what I can gather, accessory cards. What exactly constitutes an accessory card is another matter.

Negative DC Voltages

If you look at a specification sheet for a typical PC power supply, you can see that the supply generates not only +3.3v, +5v, and +12v, but also –5v and –12v. The positive voltages seemingly power everything in the system (logic and motors), so what are the negative voltages used for? The answer is, not much! Some of the power supply designs, such as the small form factor (SFX) design, no longer include the –5v output for that reason. The only reason it has remained in most power supply designs is that –5v is required on the Industry Standard Architecture (ISA) bus for full backward-compatibility.

Although –5v and –12v are supplied to the motherboard via the power supply connectors, the motherboard normally uses only the +3.3v, +5v, and +12v. The –5v is simply routed to the ISA bus on pin B5 so any ISA cards can use it. Today, though, not many do. However, as an example, the analog data separator circuits found in older floppy controllers do use –5v.

The motherboard logic normally doesn't use –12v either; however, it might be used in some board designs for serial port or LAN circuits.

NOTE

The load placed on the –12v output by an integrated LAN adapter is very small. For example, the integrated 10/100 Ethernet adapter in the Intel D815EEAL motherboard uses only 10mA of +12v and 10mA of –12v (0.01 amps each) to operate.

Although older serial port circuits used +/–12v outputs, today most run on only +3.3v or +5v.

Taken from here.

[dpaengineer]

Nice 1 Lurch. Knew somebody would know something