Editing Computer fan control (section)

== Types of control ==
=== Thermostatic ===
{{Main|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 noise issues and 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 a [[DC motor]] with blades attached. 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 ====
[[Resistor]]s in series with a fan's power pin 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, [[potentiometer]]s 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&nbsp;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&nbsp;V as the current varies from 0.01 to 1&nbsp;A.<ref>{{Cite web |url=http://www.diodes.com/datasheets/ds28002.pdf |title=Archived copy |access-date=2015-02-26 |archive-date=2012-09-04 |archive-url=https://web.archive.org/web/20120904002853/http://www.diodes.com/datasheets/ds28002.pdf |url-status=dead }}</ref> The [[Electric power|power]] rating should be noted and some diodes may require cooling to operate at their rated current. 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.

==== Voltage modification ("volt modding") ====
The voltage a computer cooling fan receives is defined by the difference between the voltage wire (+12&nbsp;V) and the ground wire (+0&nbsp;V). By connecting one or both wires to a different voltage, the voltage the fan receives will be different from the default 12&nbsp;V the fan was designed for.

Increasing the voltage<ref>{{Cite web |url=http://afrotechmods.com/fanmod.htm |title=Overspin Your Fans |access-date=2017-04-05 |archive-date=2002-06-09 |archive-url=http://web.archive.bibalex.org/web/20020609204344/http://afrotechmods.com/fanmod.htm |url-status=dead }}</ref> over the default 12&nbsp;V can be achieved by e.g. connecting the −12&nbsp;V or −5&nbsp;V power line instead of the ground wire in the fan connector, and by connecting the 5&nbsp;V power line in the +12&nbsp;V input of the fan connector. Through this procedure, 10, 17 and 24&nbsp;V voltages can be achieved, with voltages exceeding 12&nbsp;V being potentially damaging to the computer fans rated at 12&nbsp;V. However, the combination of modern power supplies no longer being required to provide a −5&nbsp;V power line and the limited power delivery capability of the −12&nbsp;V line (usually less than 1&nbsp;A of current) reduces the total capacity for volt modded fans in modern systems.

Connecting the +5&nbsp;V power line to the +12&nbsp;V input of the fan reduces the voltage the fan receives to +5&nbsp;V. Some fans will not work at such low voltage at all, while some other fans may run at +5&nbsp;V once they have started rotating at a reasonable speed.{{citation needed|date=March 2021}}

Another method of reducing the fan speed<ref>{{Cite web |url=http://www.silentpcreview.com/article6-page1.html |title=Get 12V, 7V or 5V for your Fans |access-date=2016-09-03 |archive-date=2008-09-18 |archive-url=https://web.archive.org/web/20080918053545/http://www.silentpcreview.com/article6-page1.html |url-status=dead }}</ref> is by moving the 5&nbsp;V wire in the classical [[Molex connector|Molex power connector]] in the place of the Ground wire going to the fan, thereby delivering +7&nbsp;V (12&nbsp;V − 5&nbsp;V = 7&nbsp;V) to the fan. However, this is a potentially risky method, because +5&nbsp;V PSU line is intended to source current only, not sink it, so the PSU is likely to get damaged in case of load on 5&nbsp;V PSU line being below the load generated by 7&nbsp;V fans (e.g. when PC enters idle/sleep state). Also, the components inside the computer using +5&nbsp;V power might be exposed to over 5&nbsp;V in case of a short circuit in the fan.

==== Integrated or discrete linear regulators ====
[[File:Samsung NC10 - motherboard - SMSC EMC2102-93150.jpg|thumb|SMSC EMC2102 rotational-speed-based fan controller with hardware thermal  shutdown]]
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&nbsp;A of current at a voltage of 6, 8, 9 or 10&nbsp;V for the LM7806, LM7808, LM7809 and LM7810, respectively.<ref>{{cite web|url=https://www.fairchildsemi.com/products/power-management/voltage-regulators/positive-voltage-linear-regulators/LM7808.html|title=LM7808|work=fairchildsemi.com|access-date=2014-08-13|archive-url=https://web.archive.org/web/20150402112751/https://www.fairchildsemi.com/products/power-management/voltage-regulators/positive-voltage-linear-regulators/LM7808.html|archive-date=2015-04-02|url-status=dead}}</ref> 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.<ref>{{cite web|url=http://www.ti.com/product/lm317|title=LM317 - Single Channel LDO - Linear Regulator (LDO) - Description & parametrics|work=ti.com}}</ref>

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 {{math|1=''P'' {{=}} (''V''<sub>in</sub> - ''V''<sub>out</sub>) ''I''<sub>out</sub>}}.<ref>{{Cite web |url=https://www.exar.com/common/content/document.ashx?id=1225 |title=Thermal Considerations for Linear Regulators  |date=November 28, 2006 |access-date=2015-02-26 |archive-url=https://web.archive.org/web/20150402105439/https://www.exar.com/common/content/document.ashx?id=1225 |archive-date=2015-04-02 |url-status=dead }}</ref>

=== {{anchor|PWM-FAN}}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&nbsp;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&nbsp;V or 3.3&nbsp;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&nbsp;kHz, with the [[duty cycle]] determining the fan speed. 25&nbsp;kHz is used to raise the sound of the signal above the range of human hearing; use of a lower frequency could produce an audible hum or whine. 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 at low control levels (linear, off until a threshold value, or a minimum speed until a threshold) is manufacturer dependent.<ref name=4wirepwm>{{cite web|url=http://www.formfactors.org/developer/specs/4_Wire_PWM_Spec.pdf|title=4-Wire PWM Controlled Fans Specification|access-date=2011-07-21|date=September 2005|archive-url=https://web.archive.org/web/20110726062453/http://www.formfactors.org/developer/specs/4_Wire_PWM_Spec.pdf|archive-date=2011-07-26|url-status=dead}}</ref>

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

=== Fan speed controllers ===
[[Image:Custom Fan Controller.jpg|thumb|A fan controller with LEDs indicating fan status and potentiometers and switches to control fan speeds]]

Another method, popular with PC hardware enthusiasts, is the manual fan speed controller. They can be mounted in an expansion slot or a 5.25" or 3.5" [[drive bay]] or come built into a computer's case. Using switches or knobs, attached fans can have their speeds adjusted by one of the above methods.