slow my fan

By erwin1978 ยท 12 replies
Oct 2, 2002
  1. I want to slow down my 3 wire fan. Which wire do I add the resistor to?
  2. Vehementi

    Vehementi TechSpot Paladin Posts: 2,704

    An easy solution would be to get a baybus :D

    You can slow down/speed up about 5 fans with one...
  3. Th3M1ghtyD8

    Th3M1ghtyD8 TechSpot Paladin Posts: 664

    its either the red or black not the other one (that is for rpm monitoring) :D
  4. Vehementi

    Vehementi TechSpot Paladin Posts: 2,704

    Both the red and black. Yellow is always for RPM monitoring...

    Baybus linkeh
  5. Mictlantecuhtli

    Mictlantecuhtli TS Evangelist Posts: 4,345   +11

    I have green, yellow and black wires in my fan. Green is for RPM monitoring. I slowed the fan down by inserting yellow wire to yellow pin in molex connector (the hdd/cdr power connector), ie. 12V, and black to red pin, ie. 5V. This gives 7V to the fan.
  6. Elcarion

    Elcarion TechSpot Paladin Posts: 169

    The resistor goes on the red wire. You might try a 25 ohm 3W Potentiometer available from Radio Shack. This will give you variable fan speed for $3.99 plus tax...much cheaper than a BayBus if you're only quieting 1 fan. Be sure that your fan is under 3W at 12v; otherwise, you'll need to go with a higher capacity Pot.

    Watts = Volts x Amps

    3W = 12v x .25A
    (You need a fan .25A or less @ 12V)
  7. erwin1978

    erwin1978 TS Maniac Topic Starter Posts: 290

    If I don't then what happens?
  8. SNGX1275

    SNGX1275 TS Forces Special Posts: 10,742   +421

  9. erwin1978

    erwin1978 TS Maniac Topic Starter Posts: 290

    the only rheostats available at any of the Radioshacks at my area are the big long ones(the volume control). Yuck!
  10. Justin

    Justin TS Rookie Posts: 942

    If there is more current being dissapted by the resistor that it is design to withstand then it will eventually burn out and open, leaving you at some random time with a fan that stops working. Not good, yes?

    You can place the resistor on either positive or ground, it does not matter because it is series (between source and fan). No matter where you add the resistor, current will go down, and therefore the voltage across the fan will likewise go down.
  11. Elcarion

    Elcarion TechSpot Paladin Posts: 169

    That's probably about all you will find easily. You should be able to find a place to mount it where it looks nice. You could also calculate what voltage you want the fan to run at for a given CFM/RPM and try to find a resistor(s) to drop the voltage. You'll probably find that the resistors are bigger than the rheostat!
    You could use the rheostat to determine what resistance gives you the results you're looking for and buy the appropriate resistors. The difficulty in calculating the resistor directly is that as the RPMs of the fan slow with lower voltage so do the amps that the fan draws.
  12. StormBringer

    StormBringer TS Maniac Posts: 2,244

    Radio Shack sucks, if you want quality parts, try Mouser or
    Digi-Key. They both have websites, and NTE is also a good source for quality components. I rarely use Radio Shack for anything unless it is something I need right now.
  13. StormBringer

    StormBringer TS Maniac Posts: 2,244

    Something important that many people overlook when controlling fan speed is that the CFM to RPM ratio is not a linear curve. It is a logarithmic equation. What this means is that halving the speed doesn't half the CFM, it would be way less than half, more like 20-30%. An example would be a 60CFM fan, we cut the speed in half, it is now 18CFM. This is a rough estimate because I can't remember the exact equation to use to figure the CFM/RPM ratio
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