Fans with only red and black wires, but no connector are known as untailed fans. They're usually very inexpensive, but you must "tail" them yourself. You can build a Molex connector---these are the rectangular four wire connectors that hang from the power supply. They are keyed so that they get connected correctly--- that will plug into the "daisy chain" of Molex connectors. Or you can splice the two fan wires to the proper Molex wires.
It's MUCH better to buy or build a jumper connector so you DON'T splice into one of the PS's own set of three or four Molex connectors!
Important things to remember: Shadow OO's mention of ruining a motherboard is a very valid caveat.
While DC Voltages within PC's range from 1.5v to 12v, you gotta remember that there's plenty of Amperage behind those little voltage values. Think car battery. Obviously 700 Amps isn't a concern in PC's, but .5 Amps can kill you!
Assuming that we're talking about standard fans made for PC's, those fans run on 12 volts DC. However, the red wire from the fan does NOT plug into the red wire of a Molex connector. Why? Because Molex connectors have a yellow wire--this is the 12 volt supply. There is a black wire next to the yellow wire from Molex "city." This is the 12v yellow wire circuit's return to ground. The fan's red wire goes to the Molex yellow wire. The fan's black wire goes to the Molex black wire that is next to the yellow wire.
Looks like the Molex red wire is left out of the picture. It is, 'cause this is the PS's 5 volt line. The black wire next to the Molex red wire is the 5 volt circuit's return to ground. It's sometimes said that a ground is a ground is a ground. While this is often true, I like to keep my wires in a shipshape and Bristol fashion.
CPU fans generally have three wires. The standard red and black, plus a yellow or blue wire. If the CPU fan draws more than 2 Watts, it's better to plug it into the Molex chain. The little "3 to 4pin" adapters that have a Molex plug on the non fan end only have two pins. This is because the third yellow or blue wire from the fan can't be plugged into the Molex chain.
In the case of a CPU fan, the third yellow wire is a tachometer sensing lead that goes to one of the three-pin Fan Risers(along with the red and black wires) on the motherboard. AFAIK, if the CPU fan has a third blue wire(instead of a yellow) it's a sensor from the fan that simply relays if the fan is spinning--or not.
Delta 38 cfm CPU fans have a red, black and blue wire. The Delta should NEVER be plugged into one of the mainboard's 3-pin fan risers. In other words, just because some idiot sold you a 6 Watt case fan with a 3-pin connector on it doesn't mean that it's OK to plug it into the mainboard! When in doubt, go Molex.
The little 3-pin plug from the CPU fan is keyed to match up with the 3-pin mainboard fan risers. It's also fairly obvious how to plug this into a "3 to 4-pin" converter(two actual pins in the tiny plug on one end, 4-wire Molex plug on the other end), in order to tap into the Molex chain.
