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My apologies in advance if this has already been asked

I have a MCU (esp8266 or esp32) which will primarily live off mains power with transformer 110VAC -> DC 3.3V

Occasionally USB is plugged into the device for programming and diagnostic. Expect a CP210x, FTDI or similar connected to the esp.

I'd prefer not to 'cut' the USB +5V power line to the device, or in other words, I'd like both power sources to be able to coexist. What would you recommend?

Additional consideration: some of the boards considered (like Wemos D1) already have the CP210x hard wired to power the device

UPDATE:

The design will have a 5V -> 3.3V regulator onhand for powering from USB.

I am interested in powering the device from USB for the programming and diagnostic portion. I'd prefer not to have to unplug the mains -> 3.3V supply at that time, and would also like the option of powering the device solely from USB occasionally.

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    \$\begingroup\$ My apologies in advance if this has already been asked It does not work like that here, you are aware that this might have been asked before. So what is your excuse for not using this site's search function? Also: draw a schematic of the situation. \$\endgroup\$ – Bimpelrekkie Oct 4 '18 at 9:55
  • \$\begingroup\$ You can't plug 5V onto the 3.3V rail, but if you're using 3.3V to power the MCU, how do you plan to power it from 5V? My view when given this set up would be to not connect the 5V on the USB side, as that's easier. Why do you want them both? Will there be times you will power it from 5V? \$\endgroup\$ – Puffafish Oct 4 '18 at 10:26
  • \$\begingroup\$ @Bimpelrekkie you've presented a loaded statement. In fact, I did use the search function. No suitable results presented themselves. So after about 10 minutes of searching I posted, but had a nagging feeling it had been asked before. Check your hostility. Schematic - that's a good idea \$\endgroup\$ – Malachi Oct 4 '18 at 19:06
  • \$\begingroup\$ @Puffafish ah, I neglected to mention many of the boards used have a 5V -> 3.3V regulator on them. Updating the use case, thank you \$\endgroup\$ – Malachi Oct 4 '18 at 19:07
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    \$\begingroup\$ So you did search (excellent!) and could not find a similar question. Then why apologize for asking? Next time, write "I could not find a similar question" that makes clear you made the effort. Then if there is already a similar question someone will place a link in the comments. \$\endgroup\$ – Bimpelrekkie Oct 4 '18 at 19:11
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Powering your device directly from a 3.3V wall adapter is not the most optimal choice - It's better to power from >5V, then use a POL regulator to get 3.3V. BUt I digress...

Usually it's just fine to parallel two 3.3V rails. There would be no significant effect, especially given the fact that you only use USB occasionally together with Wall. So the 5V from USB would go through your regulator and merge with the AC adapter output. That's totally fine in most cases. On the USB side there is nothing so no worries about current backing or domain issues as well.

You see, if you had used 5V you also would have had more maneuver space for a diode or two.

Optionally, you could implement a power prioritizer or powerpath controller to handle switching.

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