Will this computer case fan power a lightbulb?
All the DC computer fans that I have seen are brushless .They use a pushpull CT winding with the CT connected to B+ and each side going to B- via a NPN BJT .If you get into the fan like others have said you can lift the two sides of the winding and place two diodes ,one in each side ,going to B-.If you put the fan back together and blow hard onto the blades you will see a voltage on a DVM .I used BYV1040 diodes on my experiment .Any low drop diode would produce the same result .If the fan spins fast enough it will easily power a LED.