How can I check the voltage of a fan?
If you just want to run the fan at max speed:


You can first test for 5V and then 9V. Visually observe the fan and if it is not running fast then go for 12V.
Do not exceed the 12V limit as most PC fans are rated for max 12V.


Since, there is no label/datasheet it is quite hard to precisely tell at what voltage the fan will achieve the max speed, thus only thing that can be done is trial &amp; error.