Whether on laptops, desktops, or even game consoles, we’ve all heard the whir of the system fans in the background. How exactly do all these little cooling fans know when to spin up, down, and otherwise moderate the amount of air they’re moving?

Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-driven grouping of Q&A web sites.

The Question

SuperUser reader darkAsPitch wonders:

What happens if I do not plug it into the motherboard and it does not have a switch?

I have seen fans before with an “off/low/high” switch that allows you to manually set their speed.

I have seen fans that plug into the motherboard and are controlled by the OS.

I am curious… what does a fan do by default when plugged into a PSU? Does it run at 100% unless otherwise told to go slower? This is what I would expect…

So how do they work? Full bore unless otherwise commanded?

The Answer

SuperUser contributor Journeyman Geek offers the following insight:

Well, kind of. Fan speed control is done by varying voltage or through Pulse Width Modulation (using a constant voltage and cutting it up into pulses to vary speed). If you’re plugged into a motherboard, that’s what it does. If it has 3 wires, the third wire tells the motherboard what’s the speed the fan is running as. If you have a 4th wire, fan speed control is done via PWM rather than voltage control.

You can plug in a 2 wire fan to a 3 or 4 wire connector (though you lose speed feed back), and a 3 wire to a 4 wire connector (where you lose PWM control). You can also use a fan controller to control speeds.

Else, assuming the power source is unaware it’s a fan (as it would be connected through molex for example), and is at a fixed voltage, yes, it goes at full speed all the time.

Fellow contributor Hyperslug highlights how the exceptions play out:

Like the other guys said, pretty much yes, but I can think of 3 exceptions:

1. A few fans have a built in thermistor that will adjust speed depending on temperature.

2. PSU’s with specially marked Fan-Only PSU connectors (Antec TruePower 550W) can adjust the speed of the fan:

3. If you plug it into the PSU molex using the 12/5 volt rails (7 volt) or 12/7 rails (5 volt) it’ll be run slower, but at this point you’re modding.

And there you have it: outside of external control by some mechanism (be it the motherboard’s input, a case thermometer, or an intelligent PSU), fans will run at the highest speed the power source will allow.

Have something to add to the explanation? Sound off in the the comments. Want to read more answers from other tech-savvy Stack Exchange users? Check out the full discussion thread here.

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Jason Fitzpatrick is the Senior Smart Home Editor at How-To Geek. He has over a decade of experience in publishing and has authored thousands of articles at How-To Geek, Review Geek, LifeSavvy, and Lifehacker. Jason served as Lifehacker's Weekend Editor before he joined How-To Geek.
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