Want to slow down CPU Fan

[lasa168]

I want to slow down my 12V CPU fan. I want to quiet down the machine. Which is the best way? Can I put a resister in line to drop the voltage? If yes, what kind of resistor. Or is there a better way. Thank.

[OuTpaTienT]

You could just hook the fan up to the 5v line instead of the 12v line. The 12v is the yellow wire & the 5v is red. Problem is, 5v usually isn't enough juice to run a 12v fan and it won't get it spinning. But also it might work fine.

If you find it needs a little more than 5v you can do this: connect the 12v (yellow) wire to the fans positive lead and the 5V (red) to the fans gound. What this will do is allow 8v to travel through the fan. Ought to get it spinning just fine and it'll be much more quiet.

[lasa168]

I am a little confused. I have a retail Itel fan that has three wires coming out to the motherboard. One black, yellow and red. How do I arrange these to get the 8 volts. Thanks.

[lasa168]

I am assuming that it is a 12 volt fan. Is it another voltage since it has three wires coming from the fan?

[OuTpaTienT]

I know this is a tad confusing at first glance, but really it's simple once you understand. Ok, let's start here, with just some facts:

FROM POWER SUPPLY
From the power supply you should have 3 colors of wires that go out to all the plugs (ignoring the big one that goes to the motherboard).
These are:
12v(+) - yellow wire
5v(+) - red wire
ground(-) - black wires

Usually a pair of ground wires w/one 12v & one 5v to each plug. These are the plugs that go to your various devices (hard drives, etc.) and you should have one or two of these plugs unused.

WIRES FROM THE FAN
Although the wires from the fan might be the same colors as the ones we know from the PS, they are NOT the same.
Fan wires are:
12v(+) - red wire
ground(-) - black wire
RPM monitor - yellow wire

The fan's yellow wire, which is the one that reports the motors RPMs to any monitoring software is most often not necessary at all. Personally I just snip it. Or you may keep it in tact and see if you can still utilize it later...but for now let's just completely ignore it.

You only need two wires to make a fan go: the positive(+) and the ground(-).
The positive is expecting 12v.

So if you were to connect
- the fan's red wire(+) to one of the power supply's yellow wires(+12v), and
- the fan's black wire(-) to the power supply's black wire(-),
-- then the fan will operate completely normal.

However if you were to connect
- the fan's red wire(+) to one of the power supply's red wires(+5v), and
- the fan's black wire(-) to the power supply's black wire(-),
-- then the fan will operate at a much slower speed and be much quieter.
(of course you realize the fan also will not be nearly as effective at cooling.)

That will probably work for you & if that's what you want, then you're done. But 5v might not power your particular fan enough to even get it spinning, or you might feel that you need slightly better cooling from the fan (but just not the full 12v). In this case you can do this:

Connect the fan as follows:
- the fan's red wire(+) to one of the power supply's yellow wires(+12v), and
- the fan's black wire(-) to one of the power supply's red wires(+5),
- the positive 5v from the red wire will negate 5v from the yellow wire leaving 7v to travel through the circut.
-- The fan will operate at a slower (& quieter) than normal but it will be faster (& slightly louder) than if it works at 5v.


---
duh, I meant 7v not 8v. Arrrgh, math.

[This message has been edited by OuTpaTienT (edited 12-27-2000).]

[lasa168]

O.K. I get the picture now. I really appreciate the help OutPatient. Thanks.

[big_block_buick]

hey outpatient,that's pretty far out,i have never worked with any other than 12v,so? because you are dealing with separate voltages you dont damage the fan?.but when or where does this ground the fan to complete the circuit?.this is a very intriguing piece of knowledge

[djurom]

In standard connection you only need to connect black wire to ground to complete the circuit. In this case the circuit is completed, just connected to 5V wire.

[This message has been edited by djurom (edited 12-28-2000).]

[lasa168]

Worked great. Connected positive fan wire to 12v wire and negative to 5v wire and now my fan is running at a quiet 7v. Outpatient, you rock. Thanks.

[OuTpaTienT]

Yeah, yeah, I know. http://sysopt.earthweb.com/forum/wink.gif j/k!

Good deal. Glad to help. After all, that's why we all hang out here right?...to share knowledge & help each other out.

& fyi, (I'm sure I don't need to tell you this, but I'll mention it for the sake of any others that might take on a project like yours)...be sure to keep a close watch on your system's temperature for the next few days. Obviously with the fans not working as hard they will also be less effective at cooling the system.

Alrighty then, enjoy your peace & quiet. http://sysopt.earthweb.com/forum/smile.gifuT

[BFlurie]

Shouldn't matter much if your case is open or box is sitting in a cool location, but remember, you're not getting as much airflow now.

[wyvrn]

Too add to the discussion:

You can also pick up a potentiometer at a local Radio Shack and put it inline with the power wires. This would act as a volume control and give you complete control over how fast your fan spins and now much noise it puts out. They can be mounted in a free 3.5" or 5.25" bay, just drill a hole in the bay cover. I do not remember which pot resistance works best (I am shaky on my circuit classes from 2 years ago), but they are inexpensive and you can just try a few different ones out.

[Biff]

I would say 10 ohms would be the way to go, or at least thats what I used. I set my fans up in pairs and have rheostat for each pair here is my setup http://members.home.net/biffland/cooling.htm

[OuTpaTienT]

Well, I didn't just make this up munesh. The tip is from a well respected web site. I'll see if I can dig up the link.

[OuTpaTienT]

Actually munesh, there are dozens of sites describing this trick as well as how to overclock the fans to run at 17v.

I guess if this is a "bad" thing to do, then you have a lot of people to set straight.

Here's just a few of the MANY sites I found:
http://www.hardocp.com/articles/kuperman/ocp_fan_voltage_tweak.htm

http://overclocking.telefragged.com/articles/psutricks-1.html

http://members.core.com/~jcorwith/ultimate/

http://www.g256.com/guides/fanok/fanok.shtml

[tepo]

Or do what I did:

Few switches, one to case fan, 96 mm 12V, in pos.1 = 100% (12V)
pos.2 = ~50% (7V) The way OuTpaTienT descriebed
pos.3 = off

one to CPU fan, 60 mm 12V:

pos.1 = 100%
pos.2 = ~50%

The idea was to quiet the torbo down a little bit, wich worked fine, when I left my 'puter alone, I could turn the case fan off and cpu fan to 50%, with the result it was really quiet. (Well, that was before SETI@HOME, it's a nogo nowadays, gotta keep the sucker cool also!)

BTW, I also have LCD temp meters, one to cpu and one to case inside temp.

Some tech-engineer could of course build this with some logiccircuit, or temp controlled, wich is under development here at "Tepo's 'puter's". (Haven't got it reliable yet, it's in a test bench in a 486)

Good luck !

Tepo

PS: When will there be a 0 db 'puter ?!

[munesh]

i dont think i would want to generate 7 volts using the method described above considering how a switching power supply works. i dont know whether what i will describe below apllies practically but i think theoritically it should. and to all you electro-heads out there please cmiiaw.
a typical computer power supply is a switching power supply that uses the 5 volts output as a feedback for duty cycle regulation. puttting the fan in the position described above would mean filling the 5 volts capacitors from the 12 volts through the fan. if the 5v is being lightly loaded than this filling from the 12v could easily raise it to >5v and then the switching control logic would decrease the duty cycle in order to try to adjust the 5v output, but this would automatically decrease the 12v output and since the 12v was helping in powering the 5v output a decrease in the 12v would help the control logic in trying to decrease the raised 5v. this happens until the 5v output is stable at 5v at which by then the 12v would go down to <12v and so might cause system troubles...
i am still thinking over this but in the mean time i wouldnt do it for myself.

[munesh]

I have been thinking over it again and also visited the pages outpatient mentioned, and the following is my conclusion so far: ideally, using a PC power supply to "make" 7v is not allowed since doing this would mean filling the 5v capacitor from an external source (i.e 12v). But if the current flow that is doing this is small, then it couldnt do much in "fighting" against the 5v loads. A typical Pentium 1 fan has a resistance of 160 Ohms. This means that it would only allow 0.044 Amps to flow through it @ 7V. This is very small compared to what the 5v has to provide (i think its more than 10 Amps in a typical CPU). Therefore putting the fan in the position outpatient described SHOULD NOT BE CAUSING ANY PROBLEMS. But when big currents are required (which are still below the ratings), this method might not work.