Here’s the story: Today, a little buddy asked me a question: “I connected a fan to the processor on my board. I bought a two-wire fan, and I’m worried it might burn out, so I thought I’d add a resistor to limit the current—just like with an LED. I even added a 1K pull-up resistor. But after connecting the resistor, the fan stopped spinning!” Hmm... As for this issue, I think it’s mainly due to a lack of proper understanding of fans. Although cooling fans are used all the time in our daily work, it seems that many people don’t really have a clear grasp of them. What are these “two-wire fans,” “three-wire fans,” “four-wire fans,” and “six-wire fans”? How can there be so many different types? Honestly, I just can’t figure out what you’re talking about! So today, let’s learn together about the basics of fans.
For many people, their first experience with fans was the electric fan found in their homes. This type of fan operates on 220V AC power and is therefore called an AC fan—though it’s not within the scope of today’s discussion.
Hardware engineers often work with cooling fans for computers and cooling fans for various types of circuit boards. Today, we’ll focus primarily on these fans.
First, we need to understand whether we actually need a fan. In the current era we’re living in, the underlying trend is still toward electrification and transformation. Even though electricity serves as our primary power source, it has become indispensable in everyday life and consumption. However, in the process of using electricity, we’ve run into a problem: as anyone who’s studied physics knows, electric current has a thermal effect—meaning that during transmission, some of the electrical energy is converted into heat. When boiling water, this might not bother us much; but when using our smartphones or computers, we start to find it rather annoying. As soon as we launch even moderately demanding applications or games, our devices begin to heat up. That’s precisely why heat sinks were invented. A heat sink is essentially a metal block. Metals remain stable even at high temperatures, and they conduct heat very efficiently, allowing them to quickly carry away excess heat. As a result, many devices come equipped with metal heat sinks that draw heat away from the device and dissipate it into the surrounding air. You can think of it this way: without requiring any additional energy input, the heat is naturally transferred away—a process known as passive cooling.
However, different devices generate varying amounts of heat. As CPUs become increasingly powerful, they consume more and more electrical energy—and consequently produce ever more heat. Passive cooling alone can no longer keep up, so a fan is added to the heatsink to increase air circulation and remove heat more quickly. This is called active cooling.
The commonly used cooling fan is a two-wire DC fan. This type is quite easy to understand: one wire connects to the positive terminal of the power supply, and the other connects to the negative terminal—once connected, the fan starts working. In essence, a fan is simply an electric motor equipped with blades. We’ve all learned about the principles of electric motors before, so let’s briefly review them.
According to the definition on Baidu Baike, an electric motor is a device that converts electrical energy into mechanical energy. It operates by using a current-carrying coil to generate a rotating magnetic field, which interacts with the rotor to produce electromagnetic torque and cause rotational motion.
It might sound a bit hard to grasp at first, but everyone knows that magnets have polarity: like poles repel each other, while opposite poles attract. If you place a conductor in a magnetic field and pass an electric current through it, the current itself generates a magnetic field. Since the direction of this induced magnetic field is aligned with that of the fixed magnetic field, the two fields will exert forces on each other. As one of the magnetic fields—or the coil—is fixed, the other will stop moving under the influence of these forces, thus causing rotation.
To enhance heat dissipation performance, we tend to choose fans with the highest possible power rating. However, the device’s power consumption isn’t always at its maximum; sometimes, we even find the fan running too loudly. Therefore, it becomes necessary to control the fan’s rotational speed. The simplest way to achieve this is by connecting a resistor in series. Taking this small fan as an example—its rated power is 5V at 0.2A—the equivalent resistance when it’s running is 5/0.2 = 25 ohms. When you connect an external resistor in series, you need to perform a simple voltage-divider calculation to determine how much voltage is actually left for the fan. For instance, if you connect a 1k-ohm resistor in series, the voltage across the fan will drop to just 0.12V. Under these circumstances, the fan simply won’t be able to operate—it’ll behave like a plain wire when stationary, consuming electrical energy unnecessarily. If the power consumption becomes excessively high, it could even burn out the motor. Moreover, this method isn’t convenient for real-time adjustment of the resistance; thus, it’s suitable only for applications where the resistance remains fixed. Once the fan speed has been reduced using this approach, it’s difficult to change the speed again.
To enable real-time adjustment of the fan speed, we’ve introduced PWM control. Typically, the control is applied to either GND or VCC, and the control circuit looks like this:
Of course, the VCC terminal can also be controlled; the selected component must be a PMOS device.
One problem with this approach is that there’s no way to check whether the fan is actually spinning. Even if the fan is broken, you wouldn’t know it. While this method might be acceptable in some ordinary settings, when used with equipment like computer cases—where the interior isn’t visible from the outside—continuing to run the system after the fan has failed can lead to CPU overheating and damage.
This led to the development of 3-wire fans, which have a built-in FG feedback mechanism inside. The third pin is an FG signal—the speed feedback—through which the system can detect whether the fan is actually spinning. Many motherboards use this type of connection for their system fans.
With this three-wire fan connection, the ability to control the fan speed depends on the motherboard. The motherboard’s connector must support speed regulation for the fan to actually adjust its speed. If the motherboard doesn’t offer this feature, the fan will simply run at full speed. The advantage is that you can easily determine whether the fan is operating normally.
Later on, the fan was upgraded—its internal design now includes FG feedback and PWM speed control functions. As a result, the external control circuit has become much simpler, and the speed-control function has been moved to the fourth pin.
A typical example is the CPU fan. Traditionally, CPU fans have four pins arranged as follows:
Some dedicated servers and other specialized hosts may feature fans with 6 wires or other wire configurations. These are considered dedicated fans, and you should refer to the corresponding device instructions when using them.
The above are introductions to several commonly used cooling fan connectors. When designing, you need to select the appropriate fan connector based on system requirements.