Making 3 pin fan run at 7V?

[pulse8]

I know there's a web site that says how to do this. Anyone know where I can find it? I've tried searching, but can't seem to find it.

David

 

[WarCon]

If your talking about motherboard fan headers. The only way to mod these is cutting lans and resoldering I would think. Because one line is +12, one line is ground and one line is sensor for fan speed. If you hear of a different way make sure you post it please.

 

[pulse8]

Well, I've seen a website with a diagram. I just can't remember where I saw it for the life of me.

David

 

[WarCon]

Well just used brain and thought of easier way. If you stuck a resistor inline with the positive wire. If my math for power doesn't fail me P=VA so a 2 watt fan has an amperage reading of 2=12A or A=1/6 or A=.167amps. And A = V/R or .167=12/r 1/r = .167/12 or r=12/.167 or r=71.9 ohms. Now for a ratio of 7/12 or voltage you want over voltage available. So the equation would be 71.9*7/12 equals resistance of inline resistor. Or the value for resistor is 42 ohm resistor ~. Now bear in mind that the fans resistance is inductive so its rating is subjective to other variables, but thats a place to start for a value. Or you could put a little pot inline for adjustment, it needs to be able to handle 2 watts of power or as much as your fan can handle.

Heck I haven't done that kinda math for a long time, how sad. :frown:

 

[Ig]

Simplest way is to rewire a 3-4pin adapter and run it off the psu. Just move the ground wire to the +5v line. You'll lose rpm monitoring but it will run at 7v. I'm not sure if you need to wire both the ground and rpm wire from the fan to the motherboard header to re-enable rpm monitoring. So experiment and you can have both. Or you can make a fan controller to lower the voltage. Variable Speed Fan Controller - OCMod.com Just add a wire connecting the rpm wire at each end of the circuit if you want to keep rpm monitoring.

 

[WarCon]

I like that article, I may make some of those. They are definately better than my idea as they use a voltage regular that is made for that kinda power. A 2 watt pot is probably pretty big hehe.

 

[Tonec]

Here's something for those of us who are lazy
http://nerdsbyte.safeshopper.com/19/300.htm?198