PSU for Prusa i3

[Roxy]

So basically you make the PSU think there is a load connected to the 5v rails?

Yes. But a better way to look at it is this: Because it is a switching power supply, it turns its output transistors all the way on. Then all the way off. It feeds this into a capacitor to smooth out the power. This is efficient because it isn't dropping a lot of voltage across the output transistors. But... One of the rubs is you need to have some amount of a load on it. Without the load, it can't regulate the output voltage. And if it can't detect a load, it shuts down.
Maybe that isn't quite true. But that is close to the truth and one of the electrical engineer types can jump in and correct this thought.