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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...

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up vote 14 down vote accepted

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 its a fan (as it would be connected through molex for example), and is at a fixed voltage, yes, it goes are full speed all the time.

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Perfect, just the answer I was looking for. – darkAsPitch May 26 '11 at 12:54

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:

    enter image description here

  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.
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That's pretty much it - if the fan just has a power connector for one of the plugs on the PSU it will run at full speed all the time unless you fit a controller unit.

enter image description here

Full speed ahead!

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There are some fans, especially the ones that advertise as "quiet" or "silent" that have interchangeable resistors which decrease the voltage from the PSU molex connector. One of these fans I believe is Noctua. Although expensive, they make high quality fans.

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