From: theom+news@chiark.greenend.org.uk   
      
   s|b  wrote:   
   > So I bought a case called 'Unity V2' and it's supposed to be for a RPi4.   
   > There's assembly instructions on their site showing the fan should have   
   > 3 cables (red, black and blue), but the fan has a fourth (green) cable?   
   >   
   >    
   >   
   > The sticker on the fan says 'Pi-Fan model: ODS-232-3007' which should   
   > have 3 cables.   
   >   
   > My question: can I still use this fan for my RPi4? Cut the green wire or   
   > simply not use it?   
      
   Usually 4-wire computer fans use the 4th wire as a speed control input, to   
   be driven by a PWM signal.  If you don't connect the wire it'll run at full   
   speed, which is probably what you want here.  The 3rd wire is a tachometer   
   output so the computer can sense if the fan is spinning at the correct   
   speed.   
      
   Normal PC wiring colours are:   
   1. GND: black   
   2. 12V: yellow   
   3. Tach (sense): green   
   4. PWM (control): blue   
      
   but this is a 5V fan so it looks like they've swapped yellow (12V) for red   
   (5V).   
      
   I'm guessing that the blue pin is actually the speed control PWM, and the   
   green is to report spin speed back to the Pi.  If you don't care about that   
   you don't need it.   
      
   If you're unsure, you can just wire red and black and ignore the other two   
   wires - the fan should spin at full speed and you won't be able to sense the   
   spin speed, but maybe that's ok.   
      
   Theo   
      
   --- SoupGate-DOS v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)