我也不懂，只好靠google了...

Power Supply @ Everything2.com

"Output Voltages and their Usage
These have remained mostly unchanged throughout the history of the PC, with the exception of adding some important ones and dropping some less important ones. Here's the basic set:

-12v : Seldom used, intended for some serial ports, less than 1 Amp
-5v : Also obsolete, used for ISA
0v : Ground
+3.3v : Introduced by Intel to reduce power consumption in newer CPUs and some components
+5v : On newer systems, just runs motherboard, used to run almost all components
+12v : Runs drive motors and fans "

原來 -5v 輸出是給ISA卡用的....