Types of DC Fans: 2-Wire, 3-Wire, and 4-Wire

Systems that need temperature control will often default to using a DC fan to drive airflow across hot components. As is often the case in electronics, there are different types of components to choose from with different capabilities. DC fans are no different and are available in 2-wire, 3-wire, and 4-wire versions. Before you select a DC fan for your system, 

Types of DC Fans

2-wire, 3-wire, and 4-wire fans have distinct pinouts that are used to interface with control and sensing circuits for regulating power delivery to the fan. Controller ICs for fan usage are designed to operate with these specific pinouts as outlined in the table below. In some cases, as is described later, a 4-wire fan can be used in a 3-wire mode if speed sensing (tachometer line) is not needed for regulation.

Some fans will have a rotor state wire instead of a tachometer wire, which will indicate whether the fan rotor is moving or stationary. When speed control is desired to a specific RPM value, 

2-Wire DC Fan

The simplest type of DC fan is a 2-wire fan. By adjusting the DC voltage given to the fan via the PWR wire, the fan will rotate at a specific speed, and the fan circuit is completed via the GND wire.

2-wire fans are actually not driven by adjusting the DC voltage into the fan. Instead, they are driven with a modulated voltage, where a PWM signal is used to modulate a switching FET. The average current driven into the fan will determine the average power delivery, and this will determine the rotational speed of the fan. The average current into the fan could be measured with a current-sense resistor, and this value will be related to the fan’s rotational speed.

The current measurement is useful as it can be used to determine the speed of the fan. However, the relationship between the fan’s speed and power delivered to the fan must be known. This is not always detailed in a datasheet but it can be determined through testing, and the PWM duty cycle can be set using values from a look-up table. An alternative approach is 3-wire fans, which include speed sensing that can be used in a controller circuit.

3-Wire DC Fan

A 3-wire DC fan is driven in the same way as a 2-wire DC fan, but the fan includes a tachometer line that provides the fan’s rotational speed measurement. An external gate drive controller or some logic could then be used to set the PWM frequency based on the fan’s speed measurement.

Tachometer wires do not continuously output a speed signal because the PWM driving signal is essentially turning the fan on and off repeatedly. The tachometer can only output an accurate measurement while the fan is on for multiple PWM cycles. Therefore, the fan will need to be periodically driven with stable DC for short times in order to provide a continuous tachometer output for speed measurement.

4-Wire DC Fan

4-wire fans have the most connections but they are the easiest to set up in terms of driving and sensing circuitry. 4-wire fans accept a PWM driving signal that sets the fan speed, and they provide a tachometer output that can be used to measure the fan speed or rotor state (locked vs. unlocked). Most fan controller ICs will use a 4-wire fan with a feedback loop to sense the speed of the fan and adjust the PWM duty cycle used to drive the fan, which maintains the required speed. The tachometer line functions as described above.

4-Wire Fan in 3-Wire Mode

4-wire DC fans can be used in 3-wire mode as long as the design omits speed sensing with the tachometer wire. 4-wire fans have the driving mechanism built into the body of the fan, so the PWM drive line will be required. However, the tachometer line can be eliminated if speed regulation is not needed or desirable.

Typically the speed control section of the drive and control circuit would require sensing the tachometer line. In a 3-wire usage of a 4-wire DC fan, the 2-wire method of sensing power delivery can be used by placing a series current-sense resistor on the power line if desired.

As was the case above, if you place a current-sense resistor and measure the current, you can determine the average power in the fan. Then if you know the relationship between power delivery and speed, you can adjust the power delivered to reach the desired speed.

Whenever you want to build power delivery and control circuits for different types of DC fans, make sure you simulate your designs with the complete set of tools in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.

Subscribe to our newsletter for the latest updates. If you’re looking to learn more about how Cadence has the solution for you, talk to our team of experts.