/diy/ - Do It Yourself

question, I have a DC fan running off a controlled voltage, and I want to add a secondary DC voltage source to it that takes priority over the primary one. So for ex. the fan is running off a 5V from one source, but I can plug in a say 12V connector to the same DC input, and then the 12V input should be the one going to the fan. Basically I want the fan to have two DC input voltages, one that's always on, and a secondary one that is optional.

How would I put this together so neither the fan nor any of the voltage inputs end up frying? can I just use a pulldown resistor on the primary input (so the electricity will just flow towards the least resistance, ie. the secondary source if one is plugged in, the primary source otherwse), or do I need something more complex?