How fan speed control works

[Tim]

I'm unclear on how fan speed can be controlled by the BIOS, the
motherboard, programs like SpeedFan, etc. Hard-wiring in a
potentiometer somewhere in the power wires to the fans allows
regulation of the voltage to the fan, and so fan speed. But I don't
see how the motherboard, BIOS, or SpeedFan does this.

There is the one line to the mobo from the fan that let the mobo
*monitor* the fan's speed (presumably the RPM?). But the fan power
comes from the power supply directly, and I don't see how that's
changed by the mobo or SpeedFan.

Could someone please explain? Thanks.

 

[Eric]

The faster the fan goes the more current it requires, so the software
can probably monitor the change in the current being drawn and from
that information estimates its rotation speed.

Eric,
PC Buyer Beware!
http://www.pcbuyerbeware.co.uk/

 

[Tweek]

If you have a three pin fan connector that connects to a fan header on the
motherboard, the bios (if it has the capability) controls the fan by
adjusting the voltage to that header. If you have a fan that connects
directly to a four pin molex connector, the bios/motherboard/speedfan cannot
control the speed at all.

 

[larry moe 'n curly]

I'm unclear on how fan speed can be controlled by the BIOS, the
motherboard, programs like SpeedFan, etc. Hard-wiring in a
potentiometer somewhere in the power wires to the fans allows
regulation of the voltage to the fan, and so fan speed. But I don't
see how the motherboard, BIOS, or SpeedFan does this.

There is the one line to the mobo from the fan that let the mobo
*monitor* the fan's speed (presumably the RPM?). But the fan power
comes from the power supply directly, and I don't see how that's
changed by the mobo or SpeedFan.

The mobo can turn the fans on and off, so SpeedFan and similar programs
exploit this ability to pulse the fan on and off rapidly and thereby
vary the average voltage applied to the fan. For example, if the 12VDC
is pulsed on for 5ms and off for 5ms, the average voltage will be 6V,
while if it's pulsed on for 7.5ms and off for 2.5ms, the average will
be 9V. This is "pulse width modulation" and is used by many hardware
fan controllers. A website that explains it better is here:
www.cpemma.co.uk/pwm.html. It also shows lots of other fan
controllers, including this ultra-simple one:
www.cpemma.co.uk/reg.html

Programs like SpeedFan use any third wire on the fan only for
monitoring the RPM.

 

[GEO Me]

...... A website that explains it better is here:
www.cpemma.co.uk/pwm.html. It also shows lots of other fan
controllers, including this ultra-simple one:
www.cpemma.co.uk/reg.html

Good info. Thanks.

Geo