[Lmandrake]

A brushless fan will not spin in the opposite direction if you reverse polarity. If you bypass the diode, you will likely just fry it.

I have yet to see a DC operated computer grade fan that is not a brushless design. These fans basically depend on four components to spin - 1.) A hall effect sensor 2.) a ring magnet glued to the inside of the hub, 3.) Two coils (electromagnets) and 4.) Two switching transistors.

The ring magnet is just a flexible strip magnet glued inside. There are two coils on a circuit board inside the hub. When current is passed through one coil and the other is switched off, the magnetic field created by the coil attracts one pole of the ring magnet and fan hub starts to rotate. Once the fan starts to move, the hall effect sensor - which is basically a magnet operated switch - turns one coil on and the other off (through the switching transistors) and continues to alternate turning one coil on and the other off as one pole of the magnet passes by.

Removing the diode and reversing polarity won't work because the hall sensor and the transistors can only operate when the polarity is correct.

Brush motors all eventually suffer performance problems as brushes wear out, get dirty, etc. Plus dirty brushes create very "noisy" arcs.

The brushless fan is a very elegant little design that accomplishes the same thing as a brush motor with a minimum of moving parts and much more reliability.