I have a system that is powered by 5V and 9V, I have two supplies one is from a solar panel with an output of 9V dc and another one is a USB with an output of 5V. During the day the system is using the solar panel output after discharge how can automatically switch from solar panel to USB supply? Suggestion Please
It can be very very simple as using diodes in series with each power supply. The problem when using diodes in such a way is the power loss. So you can make this better by using OR-ing diodes with smaller power loss: http://www.ti.com/power-management/oring-and-smart-diodes/overview.html.
All this is true if both power outputs connect to the same power input, and provided the solar power can be permitted to go down to less than 5V on its output. If for some reason you can't, you should design a sensing circuit that would decide to switch power supplies when the solar panel output is below a certain level.