PC Fans

Fans for personal computers are governed by specifications from Intel and Noctua:

• Intel's 4-Wire Pulse Width Modulation Controlled Fans spec, revision 1.3 (2005-09)
• Noctua's PWM Specification White Paper (unknown date, but it references 1.3 ibid)

Almost all available fans are either 5V, 12V, or 24V, with 12V dominating personal computers (there is no readily-available 24V source from an ATX power supply). Fans operating at higher voltages will require less current for the same work. Two-pin fans offer neither feedback nor PWM control. Three-pin fans typically offer a tachometer signal on the third wire. Four-pin fans add a PWM control on the fourth wire (some fans without PWM control can still be controlled via varying voltage; voltage ought be kept constant for a PWM fan). PWM is a 5V logical control that does not modify supplied power.

Fans can be run at less than their maximum duty to reduce acoustic noise and consume less power.

Power draw

Intel's spec mandates 12V±5% (11.4–12.6V). A fan in its steady state operating in freestream (V_∞) conditions ought not draw more than 1.5A (this would be a maximum of 18.9W; most fans draw *far* less). During startup, 2.2A of inrush current may be drawn for a period of no more than 1s (27.72W).

Note that the powerful (and bracingly loud) Noctua iPPC-3000 is speced at 3.6W, i.e. 0.3 amps.

PWM

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 (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. The PWM high signal ought be 5V with an absolute maximum of 5.25V. The absolute max current drawn is 5mA. 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 requirement), 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 more-or-less 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.

(an aside about Intel terminology: a "Type A" fan never shuts down, a "Type B" fan shuts down at 0% PWM, and a "Type C" fan shuts down at all PWM duty cycles below some threshold; one would think it sufficient to specify a "maximum PWM for shutdown" and simply specify -1 for "Type A" fans, but who knows?)

Tachometer

Two pulses per revolution on an open collector. Maximum current is 5mA for 5V and 12V fans, and 2mA for 24V fans.