Computer fan control

Fan control is the management of the rotational speed of an electric fan. In computers, various types of computer fans are used to provide adequate cooling, and different fan control mechanisms balance their cooling capacities and noise they generate.

Need for fan control

As modern PCs grow more powerful so do their requirements for electrical power. Computers convert one part of this electrical power into heat generated by all major components.

Processors in most early x86-based computers, up to some of the early 486s, did not need active ventilation. Power supplies needed forced cooling, and power supply fans also circulated cooling air through 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 become more noisy.

Fans installed in a PC case can produce noise levels of up to 70 dB. Since fan noise increases with the fifth power of the fan rotation speed,[1] 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.

Fan connectors

The common cooling fans used in computers use standardized connectors with two to four 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:

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 two- or three-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

Thermostatic

Main article: Bang–bang control

In this style of fan control, the fan is either on or off. Temperature inside the chassis is checked, and if an outside-of-range temperature is detected, fans are set to their maximum speed. 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, as described below.

Resistors

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.

Diodes

A diode in series with the fan will reduce the voltage being output to the fan. A silicon diode provides a relatively constant voltage drop of about 0.7 V per diode; data sheets for a specific diode specify its voltage drop, for example the 1N4001 silicon diode's voltage drop varies from approximately 0.7 to 0.9 V as the current varies from 0.01 to 1 A. [2] The power rating should be noted and some diodes may require cooling to operate at their rated current. It should also be noted that the voltage drop across the diode will fall with temperature, causing the fan to speed up.

Like other series regulators, the diode will dissipate power equal to its voltage drop times the current passing through it.

Volt modding

The voltage a computer fan receives is defined by the difference between the voltage wire (+12 V) and the ground wire (+0 V). By connecting one or both wires to a different voltage, the voltage the fan receives will be different from the default 12 V the fan was designed for.

A common method of reducing the fan speed is by moving the 5 V wire in the classical Molex power connector in the place of the Ground wire going to the fan, thereby delivering +7 V (12 V − 5 V = 7 V) to the fan. However, this is a potentially risky method, as the components inside the computer using +5 V power might be exposed to over 5 V in case of a short circuit in the fan.

Increasing the voltage[3] over the default 12 V can be achieved by connecting the −12 V or −5 V power line instead of the ground wire in the fan connector, and by connecting the 5 V power line in the +12 V input of the fan connector. Through this procedure, 10, 17 and 24 V voltages can be achieved, the latter 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 −5 V power line to the computer and the power delivered to the −12 V power line is very limited (usually less than 1 A 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 +5 V power line to the +12 V input of the fan, which reduces the voltage the fan receives to +5 V. Some fans will not work at such low voltage at all, while other fans may run at +5 V 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 +12 V wire of the fan to a +5 V 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.

Integrated or discrete linear regulators

Common voltage regulator ICs like the popular LM78xx series are sometimes used to provide variable or constant voltage to fans. When thermally bonded to the computer's chassis, one of these ICs can provide up to 1 A of current at a voltage of 6, 8, 9 or 10 V for the LM7806, LM7808, LM7809 and LM7810, respectively.[4] Adjustable versions like the popular LM317 also exist; when combined with a potentiometer, these adjustable regulators allow the user to vary the fan speed of several fans at currents far in excess of what a standard potentiometer could handle.[5]

For higher currents, discrete linear regulators are relatively simple to construct using a power transistor or MOSFET and a small signal transistor or a Zener diode as a voltage reference. While discrete regulators require additional components (a minimum of two transistors, three resistors and a small capacitor), they allow for arbitrarily high currents, allowing for the regulation of additional fans and accessories.

As with other linear regulators, the waste heat that is produced will be roughly P = (Vin - Vout) Iout.[6]

Pulse-width modulation

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, +12 V, 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 5 V or 3.3 V 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 behavior (linear, off until a threshold value, or a minimum speed until a threshold) at low control levels is manufacturer dependent.[7]

Many motherboards feature firmware and software that regulates these fans based on processor and computer case temperatures.

Fan speed controllers

A fan controller with LEDs to indicate fan status and potentiometer and switch to control fan speed.

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.

Software

The method by which the software physically controls the fan is usually PWM (see above). Many companies now provide software to control fan speeds on their motherboards under Microsoft Windows.

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 the motherboard, CPU, and GPU sensors, as well as allowing manual control. One of the prominent examples of such programs is SpeedFan.[15]

Computers running Linux can use lm_sensors.[16]

See also

References

External links

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