Fan control is the management of the rotational speed of an electric fan. There are many types of electric fans and many types of fan controls. This article principally describes the control of a computer fan, to provide adequate cooling while lowering noise.
Need for fan control
Most early x86-based computers, up to some of the last 486s, did not need active ventilation. Power supplies eventually needed forced cooling, and fans soon took up the duty of cooling the rest of the PC with the ATX standard. The byproduct of increased heat generation is that the fan(s) need to move increasing amounts air and thus need to be more powerful. Since they must move more air through the same area of space, fans will naturally become more noisy.
In fact, if one installs extra fans in a PC case, the noise levels can reach 70 dB. Since fan noise increases with the fifth power of the fan rotation speed, reducing rotations per minute (RPM) by a small amount potentially means a large reduction in fan noise. This must be done cautiously, as excessive reduction in speed may cause components to overheat and be damaged. If done properly fan noise can be drastically reduced.
The common cooling fans used in computers use standardized connectors with 2 to 4 pins. The first two pins are always used to deliver power to the fan motor, while the rest can be optional, depending on fan design and type:
- Power - +12 V
- Sense - a tachometer that measures the actual speed of the fan as a pulse train, frequency being proportional to speed. With each fan rotation, there are two pulses sent through this pin.
- Control - PWM signal (Pulse-width modulation), gives the ability to adjust the rotation speed on the fly without changing the input voltage delivered to the cooling fan
The color of the wires connected to these pins varies depending on the number of connectors, but the role of each pin is standardized and guaranteed to be the same on any system.
Cooling fans equipped with either 2 or 3 pin connectors are usually designed to accept a wide range of input voltages, which directly affects the rotation speed of the blades.
Types of control
The simplest method of fan control is simply to leave the fan on all the time. This creates quite a bit of noise and increases power requirements, but keeps the system the coolest.
In this style of fan control, the fan is either on or off. A system thermistor checks the temperature inside the chassis, and if it detects a temperature outside of range, it spins the fans up to maximum. When the temperature drops below a threshold again, the fans are turned back off. This control method reduces power requirements during periods of low usage, but when the system is operating at capacity, the fan noise can become a problem again.
Linear voltage regulation
A standard cooling fan is essentially a bladed DC motor. By varying the voltage input across the acceptable range for a fan, the speed of the fan will increase (to added voltage) and decrease (to reduced voltage). A faster fan means more air moved and thus a higher heat exchange rate. There are a few ways to perform this regulation.
Resistors are the simplest method of reducing fan noise, but they add to the heat generated inside the computer case. Since the voltage drop is proportional to the current, the fan may not start. They need to be of the appropriate power rating. For variable fan control, potentiometers could be used along with a transistor such as a MOSFET whose output voltage is controlled by the potentiometer. It is possible to use a rheostat instead.
The voltage a computer fan receives is defined by the difference between the voltage wire (+12v) and the ground wire ( +0v). By connecting one or both wires to a different voltage, the voltage the fan receives will be different than the default 12v the fan was designed for.
A common method of reducing the fan speed is by moving the 5v wire in the classical Molex power connector in the place of the Ground wire going to the fan, thereby delivering +7v (12v minus 5v = 7v) to the fan. However, this is a potentially risky method, as the components inside the computer using +5V power might be exposed to over 5V in case of a short circuit in the fan.
Increasing the voltage over the default 12V can be achieved by connecting the -12v or -5v power line instead of the Ground wire in the fan connector and by connecting the 5V power line in the +12V input of the fan connector. Through this procedure, 10V, 17V and 24V voltages can be achieved, the later potentially damaging to the computer fans (the thin wire in the coils inside the fan may melt when subjected to such high voltage). However, modern power supplies are no longer required to provide a -5v power line to the computer and the power delivered to the -12V power line is very limited (usually less than 1A of power) and less regulated, which gives users the ability to mod a reduced number of fans using this procedure.
The safest method of volt modding is connecting the +5V power line to the +12V input of the fan, which reduces the voltage the fan receives to +5V. Some fans will not work at such low voltage at all, while other fans may run at +5v once they have started rotating at a reasonable speed. With these later fans, users may be able to use a flip switch to direct the +12v wire of the fan to a +5V power line once the fan has started spinning. Sometimes it is also possible to start these fans by manually spinning the fan blades in the correct direction.
Pulse-width modulation (PWM) is a common method of controlling computer fans. A PWM-capable fan is usually connected to a 4-pin connector (pinout: Ground, +12V, sense, control). The sense pin is used to relay the rotation speed of the fan and the control pin is an open-drain or open-collector output, which requires a pull-up to 5V or 3.3V in the fan. Unlike linear voltage regulation, where the fan voltage is proportional to the speed, the fan is driven with a constant supply voltage; the speed control is performed by the fan based on the control signal.
The control signal is a square wave operating at 25 kHz, with the duty cycle determining the fan speed. Typically a fan can be driven between about 30% and 100% of the rated fan speed, using a signal with up to 100% duty cycle. The exact speed behaviour (linear, off until a threshold value, or a minimum speed until a threshold) at low control levels is manufacturer dependent.
Fan speed controllers
Another method, popular with gamers, is the manual fan speed controller. They can be mounted in an expansion slot, a 5.25" or 3.5" drive bay or come built into the computer's case. Using switches or knobs, attached fans can have their speeds adjusted by one of the above methods.
- AOpen motherboards can use "SilentTEK".
- ASUS motherboards can use "Q-Fan" or "Fan Xpert" or "Thermal Radar" 
- MSI motherboards can use "Core Center".
- Universal abit motherboards can use "μGuru".
- Gigabyte motherboards can use "EasyTune 6".
- Intel desktop boards (older socket 478 etc.) use "Active Monitor" and "Desktop Control Centre".
- Intel desktop boards (newer socket 775 etc.) use "Desktop Utilities".
- Dell laptops can use "i8kutils".
- Lenovo ThinkPad notebooks can use the freeware "TPFanControl".
There are also third-party programs that work on a variety of motherboards and allow wide customization of fan behaviour depending on temperature readings from MB/CPU/GPU sensors as well as manual control. One of the most prominent among such programs is "SpeedFan".
- "Overspin Your Fans", http://www.afrotechmods.com/fanmod.htm
- "4-Wire PWM Controlled Fans Specification". 2005-09. Retrieved 2011-07-21.
- ASUS Q-Fan
- Fan Xpert 2
- Dell i8kutils