In article ,
"Arny Krueger" wrote:

Series resistors have been largely
superceded by variable regulators (often PWM in the case of motherboard fan
regulators) in 12 volt fan applications because of the problems with
stalling.

Yep. But it's not even necessary to use a real regulator. I've used a
zener diode or a chain of ordinary diodes to provide a fixed voltage
drop. This offers much better regulation than a resistor, preventing
startup and stall problems so long as you keep to the fan's rated
voltage range, and is much cheaper than a real regulator.

PWM, I've heard of it causing problems (rough operation or
chattering) with some brushless fans. Seems plausible to me since
brushless motors must use some kind of timed circuit to energize and
deenergize coils in a sequence, and a chopped power supply could easily
interfere with the proper operation of such a circuit. Wouldn't
surprise me if it's used a lot in motherboards anyways, since PWM is
cheaper than a programmable regulator, might still work on many fans,
and most PC motherboards are all about cheap.

--
Tim