Pin Definitions

• Fan connector basics
  Typical cooling fans use 3-pin or 4-pin connectors (common 2510 form factor).
  A 4-pin fan can usually be connected to a 3-pin header with alignment.

• 4-pin fan connector
  

• 3-pin fan connector
  

FG / TACH Signal

TACH / FG is the fan speed feedback signal, usually output as pulses.

The motherboard estimates real-time RPM by counting FG pulses. The pulse count per revolution depends on fan design (for example 2 pulses/rev or 3 pulses/rev).

A common formula is:

• If pulses per revolution = 2, then RPM = FG frequency * 30
• If pulses per revolution = 3, then RPM = FG frequency * 20

DC Speed Control

DC fan speed control is achieved by changing supply voltage.

PWM Speed Control

PWM Duty Cycle

Duty cycle D is the ratio of high-level time t_on to period T.

PWM fan control adjusts speed by changing duty cycle:

• 100% duty: full speed
• 50% duty: roughly medium speed
• 0% duty: stop

PWM Frequency

PWM frequency is the switching frequency of the PWM signal (in Hz).

Common practical ranges:

• Low frequency: 1 kHz to 5 kHz (still usable on some older controllers)
• Typical range: 20 kHz to 30 kHz (common for many motherboards/fans)
• Higher frequency: >30 kHz (used in some designs, with different noise/efficiency trade-offs)

PWM Voltage Level

PWM amplitude is the voltage difference between high and low levels. Common logic levels are 3.3V or 5V, depending on fan/controller design.
Always verify the fan’s electrical requirements in the datasheet before final design.