The 3-pin fan connector is the legacy standard for CPU and chassis fan headers on ATX motherboards, predating the 4-pin PWM scheme formalised by Intel in 2004. It provides a ground return, a +12 V supply rail, and a tachometer sense line that allows the motherboard to monitor fan speed. Fan speed control in this era is achieved by varying the supply voltage rather than a separate PWM signal, so the fan runs at full speed whenever the full 12 V is applied. The connector uses a 2.54 mm pitch housing with a polarity rib to prevent reverse insertion.
| Pin | Name | Color |
|---|---|---|
| 1 | GND | black |
| 2 | +12VDC or +5VDC | red |
| 3 | Tachometric Signal | yellow |
Signals
- GND — ground return for both the fan motor and the tachometer circuit.
- +12VDC or +5VDC — main power supply to the fan motor; most CPU fans operate at 12 V, while some smaller chassis fans are rated for 5 V. Voltage-based speed control is applied on this pin.
- Tachometric Signal — open-collector tachometer output from the fan, generating two pulses per revolution; the motherboard must supply a pull-up and monitors this signal to detect stall or under-speed conditions.
Notes
- Not all fans or motherboards control or power the fan assemblies the same way.
- Fans may be either 5 Volt DC or 12 Volt DC fans.
- Wire colours follow the conventional assignment (black/red/yellow) but may vary between manufacturers.
- Chassis and CPU fans may use either 3-pin or 4-pin power connectors. 3-pin connectors are usually used for the smaller chassis fans with lower power consumption. 4-pin connectors are usually used by CPU fans with higher power consumption. Fans and on-board fan headers are backwards compatible. Proper fan connector placement is shown in the figures below:
- Fan has a 3-pin power connector; desktop board has a 4-pin fan header:
Note: when using a 3-pin power connector with a 4-pin fan header, the fan will always be on; there is no fan control. - Fan has a 4-pin power connector; desktop board has a 3-pin fan header:
- Fan has a 3-pin power connector; desktop board has a 4-pin fan header: