Fan
A cooling fan drives airflow through or around a system. The resulting component temperatures depend on that airflow, the heat sources and the thermal path. This entry describes axial fans; centrifugal fans turn the flow outward and have different characteristics.
A motor rotates an impeller. The fan's pressure-flow curve and the enclosure's airflow resistance determine the operating point. Speed control can change that curve. In the Noctua four-pin examples cited here, the fan receives its rated supply separately from the PWM control input, and a tachometer output reports rotation.
In plain terms
A fan is like a helper pushing air through a hallway. A crowded hallway resists the flow, so a big free-air number does not tell you how much air will pass through a heatsink, filter and enclosure.
Why designers use it
- Move air through a cooling path that passive convection alone does not adequately serve.
- Adjust cooling and noise by controlling a compatible fan's speed.
- Monitor fan rotation when a tachometer output is provided.
Best for
- Heatsink and enclosure airflow
- Compatible PWM fan control
- Thermal-system experiments
Key specifications
- Airflow: Pressure-flow curve (Compare it with the resistance of the actual system.)
- Electrical interface: Supply, pinout and control input (Use the exact model documentation.)
- Operating behavior: Noise, speed range and environment (Check the specified test conditions and any stop behavior.)
When not to use it
- When the fan's pressure-flow curve cannot overcome the actual system resistance at a useful flow.
- When the selected fan is not rated for the supply, environment or control interface.
Common mistakes
- Using maximum free-air flow as the installed airflow prediction.
- Assuming every four-pin fan shares the same voltage, pin assignment or zero-duty behavior.
- Powering a fan from a controller pin without checking the pin's current and voltage limits.
- Confusing the low-power PWM control signal with switching the fan's supply rail.
Where you will find it
- Noctua's microcontroller guide describes PWM control and an open-collector RPM output for its compatible fans. Follow the particular fan and microcontroller ratings rather than treating its wiring as universal.
- Sanyo Denki's San Ace C175 technical report compares centrifugal and axial fan characteristics using pressure-flow curves. Its noise and power improvements apply to the stated comparison, not every fan.
A short history
The cited manufacturer guides describe two complementary subjects: fan selection using airflow and pressure, and electrical control of compatible PWM fans. They support these examples but do not establish a universal fan lifetime, blade count or thermal benefit.
Good to know
- Maximum pressure and maximum free-air flow occur at different points on a fan curve; you do not get both at once.
- A compatible four-pin fan can keep its rated supply while a separate PWM input changes its commanded speed.