4N25 Schematic check

[audioguru2]

MP,
If a back-EMF protection diode is built into a fan then it would be in parallel, not in series. So if you connected it backwards then the diode would be forward-conducting across the power supply which may blow-up the diode and/or the power supply.

Amps,
Maybe you misunderstood my explanation of the reason for having a protection diode across a fan or relay coil: without a protection diode then a high voltage will occur across the fan and its driver, not the power supply, when the fan is turned-off.

 

[MP1]

Now where in my post did you read me to mean the diode was in series?
Yes, if you have a fan with this protection, it is parallel, but no it will not blow anything up. Your power supply can handle this. The diode protection and current limiting in the power supply will pull the current down to nothing.

If you buy a replacement cooling fan for your PC from radio shack, they are not going to expect you to know to wire in a diode.

MP