Question on 2 Power Sources

Using Arduino General Electronics
1

I came across this article, New Page 1 while searching for connecting to 2 power sources.

I have a few questions:

In the analog world, the higher of the 2 input voltages will be passed to the output, and the lower one will be ignored.

The may sound silly, but is there a reason why the higher voltage will take precedence and the lower ignored? Or is this question the same as asking why does the sun rise everyday?

  1. What if both voltages are exactly the same, which power source will the load source from?

  2. If I were to ignore the con of the voltage drop from the diode, is using diode ORing fundamentally a simpler and better solution than using P channel mosfets?

2
  1. The output voltage will be Max(inputs)-diode drop.

The diode from the smaller supply voltage will have a lower or negative forward voltage over it, and so can't conduct.

  1. It's near impossible to have exactly equally balance due to the positive thermal coefficient of PN diodes. The diode with more current gets hotter and conducts better (more current again).

Having said that the identical voltage consideration comes in. If the internal resistance of the supplies cancels out the diode difference.

  1. Diodes are simple
3

You have been most helpful, thank you CommonRodent.

4
  1. It is simpler. But no matter what, you've got that 0.3v voltage drop (0.7 if not using schottky's). The reason people use P-channel FETs is to get around the voltage drop, which is especially important when running off batteries