Editing Computer fan control (section)

=== {{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.