Hey, I'm in the process of doing my first build. It's all going fine, except I don't really understand how I should wire my case fans.
I have two 120mm case fans, one (the rear fan, which came with the case) has a 3-pin connector and also a 4-pin molex adapter. The other (front, which I bought separately) only has a molex connector.
Is it ok to have the front fan only connected by molex to the PSU or does it need to be plugged into the motherboard somehow? And should I plug the rear fan into the motherboard via a 3 pin connector or to the PSU via 4-pin molex? Thanks.
You can plug both on a molex no worries,
The 3pin that would go into the mobo socket lets the system see what that fan is doing,adjust its speed if necessary and allows it to report back rpms or failure to you, plug either in, not both
Front fan psu direct is fine, no mobo connection needed
but both on molex would be fine at a push
yes, it is fine to plug the fan into your psu via the molex connector. as for the rear fan, it is up to you. if you just want the fan to run full speed all the time, then plug it into the psu. if you want to control the speed of the fan via software, plug it into the mobo
Most mobos these days offer to control the case fan speed automatically for you, and actually turn on this feature by default. (You can change this in BIOS Setup.) To do this the case fan must be connected to one of the mopo's SYS_FANx pinouts. Check your manual, though. I have seen some that have two such pinouts, but only the first one is actually speed-controlled (the other is just full-speed all the time, as if it had been plugged into a PSU 4-pin Molex anyway).
Case fan speed control by the mobo is usually based on actual temperature measured by a sensor built into the mobo. Some of these systems allow you to adjust some of the control scheme parameters (like what temp causes it to turn on to minimum speed, what temp to reach maximum speed), and some are just fixed. Most such mobo systems that use 3-pin connections also monitor the actual fan speed (through the pulse signal sent from the motor on the yellow wire) and can show it to you for information.
To get all this to work you just need to set it up in BIOS Setup (as I said, often done already as the default configuration), and no other software is needed. Your mobo's CD MAY have some utility that allows you to custom-tune this automatic control, or to monitor and display the fan speed and temperature on-screen during normal use, but you do not have to use this for regular operations.