SpeedFan is a freeware program that monitors voltages, fan speeds and temperatures in computers with hardware monitor chips.
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SpeedFan can even access S.M.A.R.T. info for those hard disks that support this feature and show hard disk temperatures too, if supported. SpeedFan can even change the FSB on some hardware (but this should be considered a bonus feature). At the lowest level, SpeedFan is an hardware monitor software that can access temperature sensors, but its main feature is that it can control fan speeds (depending on the capabilities of your sensor chip and your hardware) according to the temperatures inside your pc, thus reducing noise and power consumption. Several sensors, like Winbond's and the AS99127F support fan speed changing, as well as others from Maxim, Myson, Analog Devices, National Semiconductor and ITE, but the hardware manufacturer must have connected the relevant pins to some additional, yet trivial, circuitry. This means that if you have, say, a Winbond W83782D on a BP6 then you're ok, but not every motherboard with such an hardware monitor chip will be able to change fan speeds. Learn more.