One thing to note: In order to guarantee that the fan starts with a series
resistor, a large electrolytic cap (e.g., 500 uF) should be placed across
the resistor or diode(s). Then, power will initially be applied at
full voltage long enough to get it going.

A typical 12 V 3-1/2" fan will run just above stall speed with a 200 ohm
resistor in series to 12 V. I've built regulated temperature controllers
by using the speed range from this to full speed in a feedback loop.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
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