PC Fans

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Pulse width modulation controls the power delivered by dividing time up into quanta, and supplying current only during part of each quantum. The signal is characterized by the quantum length (∝ 1/frequency); the level can then be varied from none to the entirety of the quantum. 100% PWM is equivalent to constant current supply, while 0% PWM is equivalent to no supply.

PC fans are ideally controlled via 25KHz PWM, though the range 21KHz to 28KHz is deemed acceptable (why 21KHz? ~~so that you can't hear it (more precisely,~~ so that you can't hear the commutation noise from switching the drive coils on and off): children hear up to about 20KHz, adults 17). The PWM high signal ought be 5V with an absolute maximum of 5.25V. The absolute max current drawn is 5mA (as an example, the ESP32 specs 40mA on each GPIO pin). The maximum value of logic low is 0.8V. In the absence of a valid signal, the fan ought run at maximum speed. Intel's document puts fairly strict requirements on the PWM-RPM relationship: the fan speed shall be a "continuous and monotonic function of the duty cycle" (a reasonable if almost redundant requirement, see below), and furthermore that the effected speed, as a percentage of maximum speed, should match the PWM duty cycle to within ±10%. If the PWM duty cycle is e.g. 50%, and the fan's max speed is 1900 RPM, conformance requires fan speed of 760–1140 RPM. The response curve must thus be linear in the large.

PC fans are ideally controlled via 25KHz PWM, though the range 21KHz to 28KHz is deemed acceptable (why 21KHz? so that you can't hear the commutation noise from switching the drive coils on and off: children hear up to about 20KHz, adults 17). The PWM high signal ought be 5V with an absolute maximum of 5.25V. The absolute max current drawn is 5mA (as an example, the ESP32 specs 40mA on each GPIO pin). The maximum value of logic low is 0.8V. In the absence of a valid signal, the fan ought run at maximum speed. Intel's document puts fairly strict requirements on the PWM-RPM relationship: the fan speed shall be a "continuous and monotonic function of the duty cycle" (a reasonable if almost redundant requirement, see below), and furthermore that the effected speed, as a percentage of maximum speed, should match the PWM duty cycle to within ±10%. If the PWM duty cycle is e.g. 50%, and the fan's max speed is 1900 RPM, conformance requires fan speed of 760–1140 RPM. The response curve must thus be linear in the large.

Fans have a minimum rotation speed, corresponding to some minimum PWM duty cycle; duty cycles below this minimum result in the the fan running at less than or equal to the minimum speed (perhaps not running at all). The fan will never *start* spinning at less than the minimum rotation speed. The minimum rotation speed must not be more than 30% of the maximum speed.

PC Fans: Difference between revisions

Dank (talk | contribs)

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 Pulse width modulation controls the power delivered by dividing time up into quanta, and supplying current only during part of each quantum. The signal is characterized by the quantum length (∝ 1/frequency); the level can then be varied from none to the entirety of the quantum. 100% PWM is equivalent to constant current supply, while 0% PWM is equivalent to no supply.
-PC fans are ideally controlled via 25KHz PWM, though the range 21KHz to 28KHz is deemed acceptable (why 21KHz? so that you can't hear it (more precisely, so that you can't hear the commutation noise from switching the drive coils on and off): children hear up to about 20KHz, adults 17). The PWM high signal ought be 5V with an absolute maximum of 5.25V. The absolute max current drawn is 5mA (as an example, the ESP32 specs 40mA on each GPIO pin). The maximum value of logic low is 0.8V. In the absence of a valid signal, the fan ought run at maximum speed. Intel's document puts fairly strict requirements on the PWM-RPM relationship: the fan speed shall be a "continuous and monotonic function of the duty cycle" (a reasonable if almost redundant requirement, see below), and furthermore that the effected speed, as a percentage of maximum speed, should match the PWM duty cycle to within ±10%. If the PWM duty cycle is e.g. 50%, and the fan's max speed is 1900 RPM, conformance requires fan speed of 760–1140 RPM. The response curve must thus be linear in the large.
+PC fans are ideally controlled via 25KHz PWM, though the range 21KHz to 28KHz is deemed acceptable (why 21KHz? so that you can't hear the commutation noise from switching the drive coils on and off: children hear up to about 20KHz, adults 17). The PWM high signal ought be 5V with an absolute maximum of 5.25V. The absolute max current drawn is 5mA (as an example, the ESP32 specs 40mA on each GPIO pin). The maximum value of logic low is 0.8V. In the absence of a valid signal, the fan ought run at maximum speed. Intel's document puts fairly strict requirements on the PWM-RPM relationship: the fan speed shall be a "continuous and monotonic function of the duty cycle" (a reasonable if almost redundant requirement, see below), and furthermore that the effected speed, as a percentage of maximum speed, should match the PWM duty cycle to within ±10%. If the PWM duty cycle is e.g. 50%, and the fan's max speed is 1900 RPM, conformance requires fan speed of 760–1140 RPM. The response curve must thus be linear in the large.
 Fans have a minimum rotation speed, corresponding to some minimum PWM duty cycle; duty cycles below this minimum result in the the fan running at less than or equal to the minimum speed (perhaps not running at all). The fan will never *start* spinning at less than the minimum rotation speed. The minimum rotation speed must not be more than 30% of the maximum speed.