Fans
Many PCs have components that generate more heat than can be removed by passive cooling. A fan improves air flow and so helps to dissipate heat. Fans are used for the power supply and chassis exhaust points. The fan system will be designed to draw cool air from vents in the front of the case over the motherboard and expel warmed air from the back of the case.
Typically, the speed of the fan is varied according to the temperature and sensors are used to detect whether a fan has failed.
Smaller fans may be used to improve the performance of the heatsink on the CPU, GPUs, and even hard disks.
A fan is an active cooling device. It requires power to run. Power is supplied to a GPU or case fan by connecting its power connector to an appropriate header on the motherboard (make sure you plug the CPU fan into the header marked "CPU Fan" to ensure that the BIOS can run the fan at an appropriate speed). Power connectors and headers for fans are 3-pin or 4-pin.
- 3-pin models control fan speed by varying the voltage.
- 4-pin models control fan speed by switching the voltage on and off (using a Pulse Width Module [PWM] signal carried by the fourth wire). This gives better control over fan speed.
Fans with a 3-pin connector can usually be used with 4-pin headers but the system may not be able to very the fan speed ( or may need special configuration to be able to do so ). A fan with a 4-pin connector will usually work with a 3-pin header but will not be able to use PWM.