There are many things that we take for granted in our everyday lives. For example, we know that computers always run at 100% unless told otherwise. We take for granted the fact that our phones always work, and that the lights will turn on in our homes without fail. But what about the machines that we use to do our jobs? Are they always running at 100%? Or are there times when they may not be? There is no definitive answer to this question, as it depends on a number of factors. For example, if a machine is used for heavy manual labor, then it may not be able to run at 100%. However, if a machine is used for tasks such as data entry or accounting, then it should be able to run at a high rate. In general, however, most machines can usually run at a high rate unless told otherwise. This means that you should never have to worry about your computer not being able to run because it’s been damaged or its battery has gone out.

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:

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

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…

The Answer

SuperUser contributor Journeyman Geek offers the following insight:

Fellow contributor Hyperslug highlights how the exceptions play out:

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.

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.

  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.

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.