PC Fans

Fans for personal computers are roughly 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

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 *logical control*, and 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 incredibly 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.

Tachometer

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