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What’s a 4-Pin PWM Header and How Does It Work?

Key Takeaways

  • Learn the basics of what a 4-pin PWM header is.

  • Understand how a 4-pin PWM fan works.

  • Find out what you need to design a 4-pin PWM fan controller.

4-Pin PWM for a DC Fan

Some cooling fans are driven by 4-pin PWMs.

If you have ever wondered what a 4-pin PWM header is doing on a motherboard, you are not alone. The 4-pin PWM header is unfamiliar to some, but its advantages make it worth knowing about. In this article, we will uncover the benefits of the 4-pin PWM header and discuss what you need to design a fan controller.

What Is a 4-Pin PWM Header?

A 4-pin PWM header connects to a PC fan’s connector. To understand the reason for its existence, it’ll be best to revisit what PWM is all about. 

Pulse Width Modulation

PWM stands for pulse width modulation, a technique that changes the duty cycle of a periodic digital pulse. 

PWM is commonly used to turn digital value into analog voltage by sending pulses with a duty cycle that results in the desired analog voltage. PWM is also seen in applications where information is encoded into a carrier pulse, by altering the interval of the active high state. You’ll find PWM in controlling devices like servo motors, dimmers, buzzers, or as a telecommunication encoder. 

So, why does a 4-pin PWM header exist on a motherboard?

The reason is quite simple: The 4-pin PWM header is used to control the cooling fan that’s attached to the enclosure. Most modern motherboards are likely to be equipped with such a header, which is connected to a PWM-supported BLDC fan. In the next section, we’ll explore how this works.

How Does a 4-Pin PWM Fan Work?

PWM can be used as a form of voltage control, and that’s exactly what it does for a PWM fan. The fan rotates according to the voltage supplied through the PWM control pin. It is also a way of providing better and smarter control for thermal management of the motherboard.

CPUs and GPUs are notorious for producing heat. However, they do not always operate at their limits. A cooling fan that rotates at full speed is known to be noisy. Therefore, PWM is used to control the fan’s speed according to the usage of the CPU or GPU. 

PWM-signal is used to control the speed of the cooling fan.

A 4-pin PWM fan is driven by pulses with varying duty cycles.

This brings us to the next question. Why is there a need for a 4-pin PWM connection when a 3-pin header will do?

A 4-pin PWM fan does more than the 3-pin’s power, ground, and signal connections to the motherboard—it also has a fourth wire that provides feedback from a built-in tachometer sensor to the motherboard. This feedback is useful for the motherboard to have, as it tells the motherboard if the fan is running at the required speed. It would be dangerous to have a malfunctioning fan with the motherboard temperature rising unnoticed, as overheating can damage internal components.

Designing a 4-Pin PWM Fan Controller

4-Pin PWM fan control with a microcontroller

A microcontroller with a PWM module can be used for PWM fan control.

Count yourself lucky if you’re tasked to design a 4-pin PWM fan controller. It’s actually quite simple because you can find microcontrollers that have built-in PWM modules. Built-in PWM modules spare you the hassle of writing code to emulate a software PWM, which can be tedious and challenging.

Most 4-pin PWM fans require a signal with 25 kHz frequency, which should be within the capacity of most low-end microcontrollers. As for controlling the fan’s speed, it’s a matter of writing the desired value into the PWM register. You’ll also need an interrupt-triggered input pin so that the tachometer feedback can be detected to adjust the fan speed accordingly. 

For PCB designers, PWM isn’t exceptionally challenging, but it is still prudent to keep analog signals away from the PWM trace. You don’t want to risk noise coupling from the PWM signal to adjacent analog traces.

If you’re using quality PCB design software, you’re able to set clear design rules when working on a PCB layout. Allegro PCB designer will come in handy when designing a 4-pin PWM fan controller. You can also use InspectAR to interactively assess and improve your PCB design process using augmented reality. Inspecting, debugging, reworking, and assembling PCBs has never been simpler or more accurate.

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