DIY Fan speed reduction?

Discussion in 'Electronics & Electrics' started by tt_j65, Sep 11, 2006.

  1. tt_j65

    tt_j65 Member

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    Is it possible that I just wire a resistor in series to the 12v line for the fan to reduce the fan rpm? Its for a case fan
     
  2. Stygian Shane

    Stygian Shane Member

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    There's an easier way - instead of the fan running off the black and yellow wires in the molex plug (12v) relocate the fan's ground connection to the red (5v line) wire.

    Easy.
     
  3. jomo

    jomo Member

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    Or to the Yellow and Red cable and run off 7vlts (unless you have a shitty psu that isnt very stable)
     
  4. killer65210

    killer65210 Member

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    This SPCR article covers all that you want to do.
     
  5. OP
    OP
    tt_j65

    tt_j65 Member

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    thanks all! This is what i was looking for :)
     
  6. nux

    nux Member

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    Yep, using a resistor isnt a very good idea.

    Best to either use 7v or 5v, or make up a fan regulator that can step down the voltage (or use PWM).
     
  7. Stygian Shane

    Stygian Shane Member

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    Which is what I said.... and it won't need a good PSU... or even a STABLE PSU.
     
  8. OP
    OP
    tt_j65

    tt_j65 Member

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    The site that was quoted before mentioned that drawing power from the 12v and the 5v to have 7v isn't too good for the PSU. Whys that anyway? (Go as technical as you want :Pirate: )
     
  9. dakiller

    dakiller (Oscillating & Impeding)

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    Current should flow out of the +ve rail and into ground, and if you run it normally that is what happens

    But when you substitute ground for the 5V rail then the current has to flow out of the 12v rain and into the 5v rail as the return current, but current does not like to flow the other way in computer psu's so that is bad

    But when we normally use the psu to power a computer then there will be a significant amount of current flow out of the 5v psu and so the current that wants to flow back into the 5v rail basically is canceled out by this significantly larger current so overall we have a net current in the right direction = happy psu
     
  10. mAJORD

    mAJORD Member

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    I use resistors to slow my fans as I run them extremely slow, just to keep a small amount of air moving.

    I run a 1000uf capacitor in parralel with the resistor to get the fan started (manatory if you're slowing them as much as I am)
     
  11. OP
    OP
    tt_j65

    tt_j65 Member

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    I plan to run two fans in parallel to the 7v mod.

    Code:
    @ 12v:
    Fan 1:
    I = 0.14A
    R = 85.7 ohm
    
    Fan 2:
    I = 0.09A
    R = 133.3 ohm
    Which in parallel would draw around:
    Code:
    @ 12v - I = 0.23A
    @ 7v  - I = 0.13A
    I hope that wouldn't be too stressing on the PSU :p
    However the PSU is currently showing the 12v rail to be ~11.58v - signs of dying? I'll get a multimeter on it soon...

    EDIT: checked the voltages on the multimeter, 12v rail was around ~12.2v so i guess its ok :p
     
    Last edited: Sep 12, 2006
  12. Mensuri

    Mensuri Member

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    MAte - you know you can get a four channel fan controller for 29.95 dont you - its the Rheobus - i think pccasegear and coolpc have them
     
  13. OP
    OP
    tt_j65

    tt_j65 Member

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    I can get a two channel fan controller for 10 bux but wheres the fun in that when your doing a case mod and not doing it diy :p
     
  14. Mensuri

    Mensuri Member

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    Good point, let us know how it turns out
     

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