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...
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.
Like the other guys said, pretty much yes, but I can think of 3 exceptions:
PSU's with specially marked Fan-Only PSU connectors (Antec TruePower 550W) can adjust the speed of the fan:
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.
Full speed ahead!
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.
