Power Supply Router implementation
Something like this.
Of course, I am assuming that all of your supplies are the same voltage; all 12V, or 5V, or something.  And this obviously won't work for AC.  The way to select precedence, is to make your primary supply the highest voltage (not by much), and then step down a tenth or quarter volt for every other supply.  Take into account the diode forward drop.







simulate this circuit &ndash; Schematic created using CircuitLab