I have designed a board and it can be supplied by USB and/or D-Sub connector. The board gets the 5V from USB or another board via D-Sub connector.
I have placed 2 schottky diodes to prevent reverse voltage between these 2 connectors in case of they are both connected.
This is the circuit:
USB 5V and connector 5V both enter diodes and they get connected as 5V on the cathode side of diodes. My first question is, does this circuit make sense?
I have supplied the board via USB and checked the D-Sub diodes anode voltage. It was same as cathode. Don't the diodes block the voltage? I placed them for this purpose.
I assumed there is a mistake on my PCB and tried the circuit on a breadboard.
This is the first circuit I have tried:
I have read 4.9V on the cathode of diode.
This is the second circuit I have tried.
I have supplied 5V from the cathode of diode to check does it allow reverse voltage. I have read 5V on the probe.
After that I have reversed the diode to see what happens.
I do not understand why it behaves this way. This is the datasheet of the diode.
What I was expecting was when I applied the 5V to the anode, I would see 4.9-4.8V on the cathode. This part is ok. If I apply 5V to the cathode, I should read 0V since diode won't allow any voltage/current to that way. Is there something wrong with my circuit or did I misunderstand the behaviour of the diode?