mostly I am using ESP32's but now I have a question about the classical Arduino Uno R3.
What happens if I would connect a power-supply directly to the +5V-Pin of the Arduino Uno R3 and at the same time a very normal USB-Cable is connected. Which - of course - can be used as power-supply too.
I looked up the Schematic here
But I do not really understand the schematic because there are so many points with this circle and an abbreviation. Pointing to some other point of the schematic.
If there is an extra power-supply (in short exPWR) connected to the Arduino +5V pin
I will call the +5V wire from the USB USB-PWR
there are three cases
exPWR 5,1 V / USB-PWR 4,9 V
exPWR 4,9 V / USB-PWR 5,1 V
exPWR 5,0000 V / USB-PWR 5,0000 V
case three is clear but pure theory. The voltages will never match down to te millivolt.
Do USB-Ports on computers have a protection against to high voltages that might come from the connected device?
If Yes would it be sufficient insert a schottky-diode between exPWR and Uno +5V-pin?
I use a small 9V power supply and feed the Arduino from that. If I only want to use an external 5V supply, I use a boost converter to create 9V from it and feed it into the Vin and the external 5V supply for the rest of the circuit.
Forget about those circles with abbreviations. Just follow the 5V power symbol.
PS
There are users here on the forum who hack USB cables and put a switch in the 5V line.
According to the schematic, the two supplies would be connected together, which is generally not a good idea. USB power is shut off only if there is a high voltage on Vin. If there is nothing on Vin, then USB directly connects to 5V.
This would not be a problem on a Nano clone, which puts USB power through a schottky diode. Any voltage source on Vin, or 5V for that matter, will reverse bias the diode, which automatically shuts off, and protects, USB.
So this apparently was a design shortcoming in the Uno, corrected in later designs.
