HowTo Silence the Loud PC Fan

During my Home Server building process, I have catch problem with very loud PC power supply fan. After some tries, I have found a solution how to decrease the fan noise to completely acceptable values.

My solution is targeted on power supply fan but will works for any other DC fan unit.

I have to notice here, that this solution is not for everyone. You must have base knowledge from electrical engineering and you need some electrical engineering tools. If you do not have good electrical engineering skills, and you can try this solution, ask somebody with those skills for help.

Additionally by this modification you broke warranty of your PC power supply and you can damage your computer or power supply if you provide modification incorrectly or your PC consuming lot of power.

I do not take any responsibility for any damage or injury which you can get by this PC modification or during your PC modification process. All is on your own risk.

Still interested in? Ok, we can continue.

The main reasons of high level of sound generated by fans are bad quality of bearings and high revs. The bearings cannot be influenced easily by us, because they are deeply integrated to the fan. Of course, you can buy fans with good roller bearings. I have seen several advertisements for this kind of fan. But I have not experience with this and from some posts I can deduce that use this fan doesn’t help so much.

The second and more significant reason of the fan noise is high fan RPM. So, if we can make our fan soundless, we have to decrease the fan RPM. But we have to be careful with this approach, because we can overheat our system and brings a significant damage to it.

Fans are not integrated to computers gratuitously. They are responsible to get out all heating produced by computer components from computer case and keep it healthy. Of course in many cases computer vendors use cheapest working solutions: small cheap fans with high RPM. It is working solution but loud. When we can make system quieter, we have to decrease RPM of the fan, but we still have to hold appropriate RPM to get all unwanted heating out. Our luck is that vendors highly overdraw fan RPMs so decrease it can be relatively safe.

If you are going to decrease the RPM of your computer fan, calculate briefly its real power consumption and compare it with maximal power values. It gives to you easy hint if you can use fan decreasing speed approach or not. Finally check if a part of your computer doesn’t go to be overheating after RPM reduction.

For example in my case, when my Home Server power consumption is around 35W and the fan is constructed to give 250W, I have really huge space for experiments. My first try had to be disabling the fan completely. But after an hour, my power supply started to be really hot. From this experiment, I have discovered that some cooling by fan is really necessary.

Ok, now we have knowledge how to reduce the fan sound level. Let’s take look on realization.

The easiest way how to decrease the fan RPM is to reduce voltage for the fan. And the well known method how to do this is to use resistor connected to the fan power wire. This approach really works, but you have to find balanced resistance value, when fan is still working and noise level is reduced to acceptable value.

You can provide small experiment. Use a potentiometer instead of the fixed resistor and then set the potentiometer to 0 resistance value. Switch on the power supply for the fan and start to increase the potentiometer resistance and see how the speed and noise of the fan is reduced. You will probably find a point where fan stops working. Now the logic says: if you can reduce the RPM to minimal possible value then use resistor with resistance value little bit less then value where fan stops working, right?

Disable fan power supply, decrease the potentiometer resistance value little bit and turn on power supply again. With high probability the fan will stay stopped. Why? In previous test, the fan was fully functional with this resistance!

The answer is: The fan needs more power to run-up then for regular rafter power on. You can discover that you have to decrease resistance significantly to grants that fan will be running after power on. This doesn’t sound good. To slow down the fan as possible we need to use high resistance values. But we have to use lower resistance to grant the fan start. Can be this problem solved?

Yes, it can. To grant fan start with higher resistance, we have to “kick it” by a power pulse. It grants correct run-up. And a capacitor is exactly right thing to do that. During the fan power on process the capacitor generates pulse to spin the fan.

Then on this picture, you can see my final solution:

schema

To provide this modification in your power supply, you have to disassemble it and solder resistor with capacitor inside the power supply.

Do not provide this modification when your power supply is plugged in!! In the power supply is really HIGH VOLTAGE which can cause serious injury or death.
Cut the fan red wire and solder parallel connection of resistor and capacitor to newly created ends of the red wire. Take care to connect electrolytic capacitor properly. Contact with + sign have to be connected to power source and contact with – sign to the fan. When you finish soldering new parts in the power supply unit, take care to insulate new soldered joints properly. During final power supply unit assembly take care to place new parts to don’t interfere with fan or other power supply parts.

On those pictures you can see my power supply modifications:

PWSPicture RCDetail

Enjoy your silent fan!

Posted by:

Zdenek Polach

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