Computer fans are an important component in every computer. Components like the CPU, GPU, and PSU require electricity in order to function and electricity generates heat.
Fans are used to both intake air and to exhaust heated air. Without them, the aforementioned components could experience thermal throttling, or even damage.
While passive components like heat sinks and heat pipes used to be enough, for most computers today, this is rarely the case.
PWM and DC Fans Explained
There are two main types of fans used in computers. They are referred to as PWM (pulse width modulation) and DC (direct current). Both of these fans have their own uses, however it’s important to understand how they function and when to use them.
What is a PWM Fan?
The acronym PWM stems from electrical engineering and stands for pulse width modulation or pulse duration modulation. In essence, PWM allows your motherboard to control the revolutions per minute or RPM of the fan based on perceived temperatures for various components in the computer.
DC Fans are 3-pin/3-wire and have power, ground, and tachometric signals. Where-as PWM fans are 4-pin/4-wire, with the 4th wire being for PWM.
In order to make use of a fan with PWM, your motherboard needs a PWM header as well as software to interpret the digital signal. Most newer motherboards have at least one 4-pin PWM header.
A PWM fan works by essentially switching the fan on and off very quickly. In a low-frequency PWM this switching creates some commutation noise. However, most PWM fans drive the frequency at 22.5 kHz which isn’t audible. The perceived output is then changed by modifying the duty cycle.
A duty cycle measures how long the signal is turned on versus turned off. For example, a 50% duty cycle means the fan is turned on half the time and off the other half. In contrast, a 100% duty cycle means the fan is always on.
PWM fans are useful because they minimize noise output and are more energy efficient than DC fans. Due to how they function, the bearings in a PWM fan will last much longer.
What is a DC Fan?
DC fans or Direct Current fans are computer fans that operate on a fixed voltage value. The common voltage values are 5V, 12V, 24V, and 48V. By manipulating these voltage values, you can effectively alter their speed or RPM.
To illustrate, most DC fans are 12V. However if you send 7V to them, you get roughly 60% of the potential speed. This also means the range of speed isn’t as accurate and any unused power is essentially wasted as heat. Being able to control a DC fans voltage value isn’t very common.
Note: If you lower the voltage enough, you may even stall the fan as it doesn’t have enough energy to spin the rotor. With most 12V fans, the lowest voltage value is 6-7V before it stalls.
As mentioned above, DC fans are typically 3-wire and have a power, ground, and tachometric or “tach” output. The Tach provides a signal as it’s related to speed or revolutions per minute (RPM).
Most DC fans offer no control of their speed (run at 100% speed/voltage at all times) or have a thermostatic (on/off) control switch.
As you might expect, DC fans are typically louder than PWM fans as they spin at maximum RPM, regardless of perceived temperature.
Airflow vs Static Pressure
Both airflow and static pressure are important metrics that are used to evaluate the overall performance of a fan.
Airflow metrics are typically measured in cubic feet per minute or CFM. Where-as pressure is rated as mm H2O. Airflow helps to illustrate how much air that a fan can move while static pressure outlines the pressure of the air being moved.
Air is a very poor conductor of heat. As we noted above, without fans, your PC would experience thermal throttling due to heat build-up. The fan blades create mechanical airflow and help to exhaust warm air and intake cool air.
Static pressure fans are typically used on heatsinks (CPU) and graphics processing unit (GPU) coolers. Essentially areas where airflow might be blocked by an object. These fans will typically have a higher CFM value to compensate.
It’s important to remember that by adjusting a fan’s RPM you also affect air volume, pressure, power usage, and noise levels or “fan affinity laws” or “pump laws.”
A DC fan requires a minimum voltage value in order to spin continuously. Most DC fan manufacturers will outline this value so that you don’t stall your fan. Where-as the same fan using a PWM controller could spin at much lower RPMs (voltage) without stalling the fan.
As you might expect, the major benefit of a PWM is the overall noise output. As we noted above, most switching isn’t audible due to operating out of a frequency range that humans can perceive.
This also means that PWM is more efficient than DC as it only uses the energy it needs in order to cool the system and dissipate heat. Where-as a DC fan will spin at a fixed voltage or RPM regardless of a thermal output.
Your choice between DC or PWM fans is more or less governed by your motherboard. If you don’t have a 4-pin header, you won’t be able to modulate your fan speed with PWM.