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I have a CPU fan that I'd like to wire to a 12v PSU.

I have tried following the advice here and here, and connected the black outside wire on the fan to my PSU -V, and then connected the next, red wire on the fan to my PSU +V. Yellow and green wires are left disconnected; I want the fan to run at full speed. The fan powers up just fine in this configuration, but spins in the wrong direction (it blows air down).

I'm using this little project as a learning tool to understand basic electrical theory. My simplistic guess was that I had the wires backwards, so I switched the red and black fan wires (black to +V and red to -V). When I turned the system back on the fan didn't start up at all. I tried cutting and restripping the wires and then switched them back and forth a bit to make sure I didn't just have a bad connection, but it works every time in configuration 1 and never in configuration 2. I'm a little confused as to why.

Why is this fan spinning backwards, and how do I wire this CPU fan so that it spins in the right direction?

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    \$\begingroup\$ The fan in the link should be blowing air down (i.e. into the heatsink fins). \$\endgroup\$ – Andrew Morton Apr 17 '17 at 17:46
  • \$\begingroup\$ @AndrewMorton Hmmm, that would certainly explain things :P. I assumed it should be blowing up; sucking air from below the fins and expelling it upwards out of the case. Do most CPU fans blow down? I admit I've never really paid attention. \$\endgroup\$ – Nicholas Apr 17 '17 at 17:48
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    \$\begingroup\$ It is desirable to blow air onto the motherboard because some components around the CPU, in particular those making the voltage regulators, work better with a bit of cooling. \$\endgroup\$ – Andrew Morton Apr 17 '17 at 17:55
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CPU fans can't run in the wrong direction; the diodes inside make it impossible. If you would like to reverse the direction of airflow, you can remove the and flip it around. A little modification to the mounting may be required. Also the fan blades are designed to be very efficient going in one direction and not the other

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  • \$\begingroup\$ Thank you; it looks like my problem was an incorrect assumption about the intended functionality of the fan. Good suggestion to change the fan direction mechanically; I'll try that. \$\endgroup\$ – Nicholas Apr 17 '17 at 18:01
  • \$\begingroup\$ If you look at the blades, youll notice their slightly cupped. This is similar to boat propellera in that they last second ramp the water backwards..a sharp increase in pitch at the trailing edge...now if you took the plade off the motor and flipped it around..well it wouldnt fit as the blade sits around the guts of thw matter..but if you could...the cupping would be..well .anti cupped...and it do the aquatic version of cavitate badly..it would still work but at a guess of about 40 percent effecinetcy..in the propeller world their are such things as a universal prop.. \$\endgroup\$ – Chris Akira Takemoto Apr 17 '17 at 19:52
  • \$\begingroup\$ Thank you; you're right. I think I've decided to redesign to make use of a fan/heatsink that blows side to side instead of trying to find one that forces air upwards; apparently fan direction is not something that is mentioned in most product descriptions. \$\endgroup\$ – Nicholas Apr 18 '17 at 16:59
  • \$\begingroup\$ minor quibble: it's not just that there's a diode built in: the fan is driven by a brushless motor, which requires sensors to sense the position of the fan, and a driver chip to switch the different coils on and off at the right time. So there's not a diode artificially preventing it from working backwards; it's just not capable of working backwards. \$\endgroup\$ – dn3s May 31 '17 at 2:22

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