PWM Fan Control

Pulse-width modulation sends the fan a fixed 12 V supply plus a separate control signal that switches on and off around 25,000 times per second. The fan’s own circuitry reads how long the signal stays high in each cycle, the duty cycle, and adjusts its speed to match. Because the motor always receives full voltage, a PWM fan can spin reliably at very low speeds and can stop entirely at 0% if its design allows. The fourth pin on the connector carries this control signal; the third pin carries the tachometer pulse that reports RPM back to the motherboard.

Three-pin fans have no control input, so the only way to slow them is to lower the supply voltage. Motherboard headers in DC mode do exactly that, typically from 12 V down to about 5 or 7 V, below which the motor stalls. That is why three-pin fans rarely run below 30 to 40 percent and cannot idle at zero. DC control still works well for exhaust fans that never need to be silent, but for a quiet build four-pin fans on PWM headers are the better tool.

• A 4-pin fan on a 4-pin header set to PWM mode: full control range, the ideal case.
• A 4-pin fan on a header in DC mode: works, but the fan behaves like a 3-pin fan and loses its low-speed range.
• A 3-pin fan on a 4-pin header set to PWM: the fan runs at full speed and ignores every command. Switch the header to DC in the BIOS.
• Fans on a splitter: all share one control signal; only one fan’s tachometer wire should be connected so the RPM reading is valid.

Most BIOS setups offer an Auto mode that detects the fan type at boot. It usually works, but if a fan refuses to slow down, setting the mode manually is the fix.

A typical board has one or two CPU headers and three to six case headers labeled CHA_FAN, SYS_FAN or similar. Each can supply about one amp, enough for two or three normal fans on a splitter. For more fans, a powered PWM hub takes a SATA connector for power and copies the PWM signal from a single header to every port. Fan Control sees the hub as the one header it is plugged into; all fans on it move together, which is exactly what you want for a matched set of intakes.

Case fans move heat out of the enclosure, so they should follow whatever component is hottest rather than a single sensor. In Fan Control that means a Mix curve:

1. Create a Linear curve on the CPU package temperature and another on the GPU core temperature.
2. Optionally add a third curve on the hottest SSD or the motherboard sensor.
3. Create a Mix curve with the Max function and add those curves to it.
4. Assign the Mix curve to every case fan control, intake and exhaust.
5. Give the case fans a slow step-down so they coast down after a gaming session instead of dropping abruptly.

Try the interactive demo card on the home page to see how a Mix curve blends inputs. The CPU and GPU guides cover the component-specific curves that feed into it.

• CPU package: the primary input for front intakes on an air-cooled build.
• GPU core: the main driver for bottom intakes and rear exhaust in gaming systems.
• NVMe SSD temperature: helpful if a drive sits under the GPU and throttles during large transfers.
• Motherboard or VRM sensor: a useful third input on small-form-factor builds where everything shares the same air.

More speed is not always more cooling. A case with slightly more intake than exhaust stays cleaner and keeps warm air from being pulled in through gaps; two 140 mm intakes at 800 RPM often outperform one 120 mm fan at 1,500 RPM while being far quieter. Use calibration in Fan Control to learn each fan’s stall point, set the minimum just above it, and let the curve stay flat there until temperatures actually climb.

• How to control PC fan speed in Windows
• Fan Control for Windows 11
• Download Fan Control