sometimes you have a 12V supply to work with, sometimes you have a 24V supply to work with. 24v is common in some comm. equipment IIRC.

This is an easy tutorial showing how you can control the brightness of an LED or adjust the speed of a DC fan using Arduino Board, a small BC547 transistor and a potentiometer. In my example the whole ...

Noctua has today announced its NA-VC1 24V DC to 12V DC step-down voltage converter allowing for users to run standard 12V fans in 24V environments such as 3D printers or industrial and automotive ...