How-To control fan noise by varying fan speed. – Matt Sigman
I recently built a 1.4Ghz Athlon XP based system that I intended to overclock substantially. I purchased a Golden Gate (all copper) with a 26CFM fan. I hoped that this would provide sufficient cooling but still be relatively quiet.
When I started it up the first time, to my disappointment, it sounded like somebody was mowing their lawn in there. Not good! I tried to cover up the sound by placing the computer under the desk, to no avail. So I decided to install a simple potentiometer (also called a variable resistor or rheostat) to limit the speed (and therefore noise) of the CPU fan. It is a simple project and I am extremely pleased at the results. Here is how I did it…
First I needed to calculate what value potentiometer I need to purchase, so I used Ohm’s law which states:
I knew that the fan was running at 12v and drawing about 300 milliamps, so by plugging the numbers in (12 / .3) I got 40 ohms. That is the current resistance the fan is experiencing. This now gives us a base to start with. Referring back to Ohm’s law, you can see that doubling the resistance halves the current (and therefore speed) of the fan.
My fan was running at about 5700 RPMs and I thought that I should set 2500-3000 RPMs as my target speed. If we used a 50 ohm resistor for example, the highest resistance (which yields the lowest speed) would be 40 + 50 = 90 ohms of total resistance.
We can determine that the current at 90 ohms (The maximum resistance possible) would be 12 / 90 or 113mA. 113mA is only 38% (113mA / 300mA) as much current, so at that setting the fan would be spinning at about 2166 RPM (5700 * .38). And guess what? That speed is perfect because it covers the speed range I wanted but doesn’t allow the fan to be completely stopped. That means my new range of adjustability is about 2100 RPM to 5700 RPM, which would allow excellent flexibility and includes my goal of 2500-3000 RPM.
I went out to my local electronics store and bought the 50 ohm linear taper potentiometer as well as a knob (to make it look more professional.)
IMPORTANT: You must purchase one rated at 3 Watts minimum.
Five watts is the best bet, to be safe. Any less and you will fry it, causing it to stop conducting current and thereby stopping your CPU fan entirely. Any potentiometer that is in the 30-70 ohm range would work well too. Don’t go higher than 100 ohms though, because that won’t provide accurate control and it would be very easy to cut off all power, causing your fan to stop.
There are three tabs on the potentiometer. For our purposes, we will only be using two of them. I spliced into the middle red wire (+12v) from the CPU fan and connected it to one terminal on the potentiometer. I then connected another wire from the center tab of the potentiometer into the wire leading to the motherboard. Here’s a diagram to help clarify things:
Note: It doesn’t matter if you choose reverse the connection, because
this isn’t a diode so it doesn’t care which direction the current is flowing.
Now that the potentiometer has been connected, it needs a nice mounting spot. I chose to mount mine in an easily accessible spot, on the front of the case directly below the power button.
First I removed the front panel to expose the sheet metal up front. Then I drilled a 5/16″ mounting hole (specified on the potentiometer packaging) into the sheet metal, making sure I wasn’t going to hit anything inside the case!
Next, I put the plastic bezel back on the case and stuck a pencil through the hole and marked on the plastic bezel where I should drill a hole. I took off the panel one last time, then I drilled the hole in the appropriate spot.
Next, I inserted the potentiometer (already connected) and screwed it down. I put the plastic bezel back on, made sure everything fit properly, then I put the knob on the end of the potentiometer and screwed it on. Viola! Done at last!
Test everything out and make sure that it works properly. I installed Motherboard Monitor and set the fan alarm to go off at 1500RPM (in case something fails, I don’t want to have a fried processor!)
Here is what the final product looked like:
The noise difference is incredible! There is no more high-pitched noise and it’s probably only half as loud. Interestingly enough, I only noticed a slight increase in temperature. Before the modification the CPU was hovering around 38*C idle, now it’s usually around 42*C. Not too bad…definitely a worthwhile trade in my book, and all for under $10.
If you have any questions, feel free to email me.
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