Interfacing a Fan Tachometer Without Confusing PWM and Speed Pulses

A four wire fan’s PWM control input and tachometer output serve different purposes. The control duty cycle requests behavior; the tachometer reports rotation according to the fan’s output format. Counting the control waveform or assuming one pulse per revolution can produce a believable but incorrect speed reading.

Identify the exact signal specification

Record fan supply, connector pinout, tachometer output type, permitted pullup, and pulses per revolution. The Noctua PWM specification white paper describes open collector tachometer outputs with two cycles per revolution for the fans covered by that document. Other fans must be checked against their own specifications.

Keep the tachometer pullup voltage compatible with both the fan output and controller input. The motor supply voltage does not automatically define the correct logic pullup voltage.

Calculate the speed conversion

Speed in revolutions per minute equals pulse frequency multiplied by 60 and divided by pulses per revolution. With two pulses per revolution, a hypothetical 100 Hz tachometer signal represents 3000 RPM.

Determine whether the counter counts full cycles or both edges. Counting rising and falling edges doubles the event rate and requires a corresponding conversion. Confirm the method with a known waveform before relying on a displayed RPM value.

Review stopped and low speed states

Define a timeout based on the minimum expected operating speed and observation method. A fixed short timeout can report a slow but healthy fan as failed. At the other extreme, a long average can delay detection of a stopped rotor.

Check the documented behavior at zero or low PWM command. Some fan models stop while others continue at a minimum speed. A tachometer held high or low when stopped is not necessarily an electrical fault; interpret it together with the requested operating state.

Test startup and control changes

Capture supply, PWM input, and tachometer output during startup, low speed operation, and duty changes. Verify that the PWM frequency and electrical drive meet the selected fan’s requirements. Keep power continuously applied when the interface expects speed control on the dedicated PWM pin.

Save fan model, pullup, pulse conversion, timeout, and startup allowance. Test the system’s fault response using a method appropriate to the fan rather than assuming an absent pulse always means a disconnected cable. A useful interface distinguishes requested stop, startup, slow rotation, and unexpected loss of rotation, so its status reflects the actual operating condition. Check whether the timer reports a stale speed value after pulses cease, and clear it according to the defined timeout.