Case Fan has both 3 pin and 4 pin connector?

[Ego Banana]

I recently purchased a replacement case fan and instead of coming with either a single 3 pin or 4 pin connector it came with both.

The 3 pin connector on the new fan is connected with a yellow wire and the 4 pin has a black and red wire.

Should I hook both connectors up or should I leave one out?

My current fan is plugged into the mother board through a 3 pin connector.

 

[Paul NZ]

If the mobo has 3 pin use the 3 pin

 

[Dark Lord of Tech]

3 - pin case fans use = voltage, ground, and rpm sensor
4 - pin case fans use = voltage, ground, rpm sensor, and PWM = fan speed control

Pulse-width modulation
http://en.wikipedia.org/wiki/Fan_control

Pulse-width modulation (PWM) is a common method of controlling computer fans. A PWM-capable fan is usually connected to a 4-pin connector (pinout: Ground, +12 V, sense, control). The sense pin is used to relay the rotation speed of the fan and the control pin is an open-drain or open-collector output, which requires a pull-up to 5 V or 3.3 V in the fan. Unlike linear voltage regulation, where the fan voltage is proportional to the speed, the fan is driven with a constant supply voltage; the speed control is performed by the fan based on the control signal.

The control signal is a square wave operating at 25 kHz, with the duty cycle determining the fan speed. Typically a fan can be driven between about 30% and 100% of the rated fan speed, using a signal with up to 100% duty cycle. The exact speed behavior (linear, off until a threshold value, or a minimum speed until a threshold) at low control levels is manufacturer dependent.

Many motherboards feature firmware and software that regulates these fans based on processor and computer case temperatures.

 

[Ego Banana]

The 3 pin connector on the new fan only has a yellow wire and not a ground or red wire. Should I still use it?