stderr wrote:Some SBC type boards purport to be capable of being run on various power sources and that they pick the available source that is the highest at that point. In theory, that could mean battery back up support and support for using two power supplies is basically built into the board.
Those SBCs usually have a PMIC that monitors available supplies and switches accordingly.
Regarding running PSUs in parallel: It depends on the type of reguulator that is used. If it can only source and not sink (meaning pull current when the outside voltage is higher than the PSU nominal voltage, even if the PSU is not supplied by mains), then there's no problem. The PSU with the higher voltage will source the current. If the PSUs are close together, they'll share the load/current. I assume
most wallwarts implement this type of output stage.
If on the other hand, the PSU is source/sink, then you're screwed, because the PSU with the lower nominal voltage will try to bring down the voltage of the other one.
If you want to do it properly, look into active diodes. This is basically a FET+Controller which closes with current flowing in one direction, and opens with current flowing in the other direction.
Anecdotally: I had my Cubox-i4pro running with a powered hub connected (HDD on Hub). This worked quite well for some time (read months), untill the PSU of the powered hub gave out spectacularly (meaning emission of toxic fumes and the mains fuse blowing). Seems the cubox has no separation between the USB5V and the 5V input barrel connector, and the PSU of the hub supplied the hub, HDD and the cubox, which probably was too much.