As modern x86 PCs grow more powerful so does their hunger for electrical power. Almost every PC converts most of this electrical power into heat, usually in the CPU, graphics adapter, RAM, mainboard, hard disk drives and add-on-cards. The PSU itself generates a significant proportion of this heat.

Up until a few years ago PCs didn't need active ventilation. Then the PSUs needed forced cooling, and still later took up the duty of force-cooling the rest of the PC with the ATX-Standard.

The byproduct of this fact is that the fan(s) need to move increasing amounts air and thus, need to be more powerful. Since they still have to conform to the same area, they will naturally get very noisy.

In fact if one installs a few extra fans in a PC case, the noise levels can reach up to 70 db (decibels). Since fans get a lot louder when the RPMs are increased slightly, you can inverse that and just throttle them down a small amount. (stay on reasonable grounds as coponents can overheat if a vital fan is slowed too much) If done correctly, you can enjoy the (sometimes drastically) reduced noise levels in your PC.

There are many ways of reducing the speed of a fan, but this article will only discuss a few of them.

• PWM
• Resistor (Rheostat)
• Diodes
• Zener
• series of silicon diodes

PWM or pulse width modulation

PWM is a common method of throttling fans (which are basically DC motors), but has a big disadvantage if used to silence fans: if the frequency used is in the range of 20-20000 Hz it will be transmitted via the fan, which is acting quite like a loudspeaker where the motorcoils and the fan blades act as coil and membrane of the speaker.

Resistors

Resistors are the simplest method of reducing fan noise but they add to the heat generated inside a PC Case . They need to be of the appropriate Wattage rating (i.e. higher than the fan). For variable fan control, Potentiometers could be used along with a Transistor such as a MOSFET whose output voltage is controlled by the potentiometer. Instead, it is possible to just use a Rheostat instead.

Diodes

A diode in series with the fan will reduce the voltage being outputted to the fan. You can use a zener diode (select one for the desired voltage drop) or a silicon diode (Produce the required voltage drop by connecting multiple diodes in series. Each diode reduces the voltage by approximately 0.75 volts.)