Why does my fan stall or stop at low PWM duty?

Every DC fan has a minimum duty floor — commonly 10–20% but model-specific — below which it cannot sustain rotation: it stalls, stops, or runs rough with unstable tach. Below the floor the motor cannot generate enough torque at the commanded drive to overcome friction and cogging. Design the control range above the floor, verify the floor on a sample at your minimum rail voltage, and decide the low-speed policy explicitly.

What happens below the floor

The rotor accelerates partway and stalls, or runs intermittently: tach output stutters, RD may assert on some models, current wanders, and the fan can emit a low-frequency growl. Repeated stall-restart cycling stresses the drive electronics and produces exactly the 'fan randomly stops at night' field report. The floor also rises with conditions: lower rail voltage, colder lubricant, and mechanical load from dust or filter pressure all push it upward.

Designing the low-speed policy

Decide explicitly what 'off' means for the product: true zero (power the fan down rather than PWM-ing it at the floor — cleaner electrically), or a minimum idle speed above the floor (keeps airflow and the tach alive for monitoring). If you need running-but-slow, request a model with a low minimum-duty spec or a fan with speed-skip firmware — custom floor values are a standard request in MEGA Tech's DC fan range.

• Never park the fan at a duty just above the measured floor — leave margin.
• At product start, command full duty first, then ramp down; cold starts below the floor can fail to spin up at all.
• Log time-near-floor in telemetry: a floor creeping upward is an early wear indicator.