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I have been tasked with retrofitting some small fans to an old car, to help move heated air into the footwells. Being a computer-type, I immediately bought some standard computer cooling fans.

The fans are DC, and the only rating information is 8-12V.

It was my hope that a basic potential divider with knobbed potentiometer would provide a voltage-based fan control:

schematic

simulate this circuit – Schematic created using CircuitLab

I only ask because any search for "Fan Control" leads to pulse-width-modulation. Is it possible to control fan speed using this basic circuit above?

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    \$\begingroup\$ That's pretty wasteful, your resistors will waste at least 12/3 = 4 amps! and produce at least 4x12 = 48W of heat. You'll need pretty big high-wattage resistors in order not to start a fire in the footwell. Unless you are using the resistors to heat up the footwell you should perhaps be looking for PWM control. Your potentiometer won't provide any control unless you connect the wiper to one end. \$\endgroup\$ – RedGrittyBrick Aug 7 '13 at 10:24
  • \$\begingroup\$ Rgr on the miswiring of the potentiometer. Is it not possible to scale up the resistances to still provide the 8-12 range from a 12V source while wasting less energy? \$\endgroup\$ – lynks Aug 7 '13 at 10:29
  • \$\begingroup\$ Note I have edited the schematic, and removed the second resistor. My understanding of this goes back to school days, so I may have made things worse. \$\endgroup\$ – lynks Aug 7 '13 at 10:31
  • \$\begingroup\$ I'm afraid that is worse, a 1 Ohm potentiometer will produce 144 Watts of heat at 12 V. It may be hard to find a vendor who sells a 1 Ohm 150W potentiometer. I'd look for PWM solutions. \$\endgroup\$ – RedGrittyBrick Aug 7 '13 at 10:37
  • \$\begingroup\$ Thanks for your advice, maybe I'll stick with a nice simple off switch. \$\endgroup\$ – lynks Aug 7 '13 at 10:38
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PWM is used because it wastes much less heat than "linear" methods like that shown in your schematic. A TO-92 voltage regulator and a DIL8 microcontroller is all you need to read the voltage from a 10kohm pot and then generate a PWM signal to put through a power MOSFET on the low side of the fans. You could even substitute a 555 and associated passives, but the MCU will give you better control.

schematic

simulate this circuit – Schematic created using CircuitLab

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