What is PWM and how does it relate to PC fans?

If at any point over the last 10 years you’ve been on the hunt for a new set of case fans for your next build, you’ve probably seen this initialism, but what does PWM mean?

It’s a good question and given that we make a broad range of PC fans, we thought we’d take a moment to help you understand what PWM is all about and what it means to you as a PC enthusiast. So, let’s take a look.

PWM stands for Pulse Width Modulation. When it comes to most PC fans, they are powered by a constant load of 12v. The voltage never changes, however, by turning the power off and on, we can modulate (adjust) the speed of the fan. The frequency at which the power is turned on and off is way too quick for humans to tell the difference, so we don’t see the fan going on and off. We just see it smoothly ramping up and down. Of course, a fan has momentum, too, so it’s not like the fan is literally stopping and starting hundreds of times per second either.

The amount of time on compared to the amount of time off is known as “Duty Cycle”. If then PWM signal tells a fan to turn off as often as it tells it to turn off, that would be a 50% Duty Cycle. We’ll use one of our iCUE LINK QX120 fans as an example.

A QX120 fan can run at a maximum of 2,400 RPM and a minimum of 400 RPM. Using PWM to crank the fan all the way up to maximum speed, we’d set the Duty Cycle to 100%. But we don’t want the fan to be at max speed all the time.

So, as an example, if we set the fan with a duty cycle of 50%—remembering that this means the fan is turned off 50% of the time, and receiving power for the remaining 50%—we end up with our QX120 spinning at 1,200 RPM. 1,200 RPM is 50% of 2400 RPM, so this makes sense.

However, we mentioned earlier that the QX120 can’t run slower than 400 RPM. So surely setting it to 50% should be 1,400 RPM? After all, the midpoint between 400 and 2,400 is 1,400. So, what gives? Well, you wouldn’t want to run a PWM fan below a certain speed, and we’ll explain why.

It’s a good question. Why don’t we allow you to run your QX fan lower than 400 RPM? Why not 250 RPM for example? Perhaps your computer is so well ventilated and so well cooled that 250 RPM would be sufficient? 250 RPM would be a Duty Cycle of around 10%.

Remembering that PWM fan controls speed by essentially turning the power off and on at different frequencies depending on the desired speed. Well, there is a certain Duty Cycle threshold (somewhere around 20–30%), below which the frequency would actually become audible.

That’s right, you’d be able to hear the power being pulsed to the fan, because our example of 250 RPM is far below that threshold. Of course, 250 RPM wouldn’t result in you being able to hear distinct noises each time is fan is turned on and off, it’s not that slow, but it is slow enough that you’d hear a constant whining noise coming from the fan.

This is because, at this speed the power is pulsed at such a low frequency, that we enter the range of frequencies that humans can hear—around 20Hz to 20kHz. Anything outside this range is not perceived as sound.

This is why PWM fans have a minimum speed that you can’t go below. It would create an unacceptable noise. However, don’t think that this is lost performance that is being hidden from you. Fans these days are quiet enough that going below the minimum rated speed (400 RPM in our example), wouldn’t result in a fan noise perceptibly quieter than the rated minimum. Human hearing isn’t that sensitive.

So, we hope you enjoyed the brief lesson about PWM fans and how they work. If you found yourself on this page as part of your search for some new PWM fans, we strongly recommend our iCUE LINK QX fans. They come in 120mm and 140mm, and in black and white colorways. If these don’t tickle your fancy, then take a look at our main retail site for more PWM fans.