Why do computers often spin up the CPU fan to high speed momentarily when powered on


Many computers spin up the CPU fan to high speed the moment they are powered on, even before POST, before settling down to normal operating speed.

While this behavior seems normal for the most part, why do computers do this, and what purpose does it serve?

(Note that on at least one computer I've worked with, including an old Sony VAIO desktop, the fan doesn't spin up until after about one second from the time the system is powered on. The fan revs up for a moment, then settles down.)

Best Answer

Because the power is turned on to the fan, before the BIOS loads any real time controllers that will base the speed of the fan on the temperature of the processor. This also keeps the processor from getting excessively hot if you were to try the alternative... which would be to keep the fan off until those controllers were loaded and basing the fan speed on processor temp. More of a safeguard than anything. The processor is starting to work the moment you turn the computer on, but the BIOS still needs time to load.

Here is another alternative. What if the BIOS didn't load for some reason... a stick of Ram gone bad, for example. Would you really want your processor sitting there with no fan, waiting for the BIOS to turn on the fan after it loaded the appropriate controllers? I know I wouldn't.