Power supply from both USB-C and +3V3 or 5V

[eriksl]

I am using a Lolin Wemos ESP32-S3-mini, but I think this counts for a lot of other modules as well.

The module can be powered by USB-C and external pins (3V3 and VBUS, which I assume, is 5V).

My plan is to solder these modules onto the PCB's that they're going to control. My assumption is that if they're fed with 3V3 they're going to do just fine (3V3 will be plenty available on the PCB).

But what if I need to connect the USB-C for diagnostics/debugging at the same time the PCB is running (and supplying power)? I read somewhere one can't do that. I don't think it's an option to come up with some "modified" USB-C cable that has the power lines disconnected. But it's also not possible to power the whole PCB from USB-C.

So, how to (conditionally) isolate the power supplies from both sides? Anyone experience, thoughts? Just plug and pray?

[powerbroker]

So, how to (conditionally) isolate the power supplies from both sides? Anyone experience, thoughts? Just plug and pray?

my friend simply uses solution you don't consider as an option - 5V-cut USB cable.

i usually route a jumper, disconnecting one of power sources - and very often install it and forget about it. in my practice, when everything resides within table, all the sources isolated and their voltage is the same, things just work and nothing happens.



e.g. here you see USB cable plugged-in at the same time with isolated 12V source. board itself generates +5.02V out of incoming 12 with MC34063 step-down, and 5.02V is routed to ESP32 DEVKIT(and to USB as well).

they report damage in such cases with very long wires: e.g. when connecting PC videocard to digital TV with antenna plugged-in.

some people add schottkey diodes, and there are MOSFET-based solutions i know about but never used. if i need isolation here, i would just add usual diode between on-board +5V source(since ESP32 eats 3.3 from LDO regulator, it doesn't matter if the regulator has incoming 5V or 4.2V) and ESP32 DEVKIT.

[eriksl]

Nice board, do you make them yourself? Mine are from PCB factories so I'm cheating

What you describe is what I was thinking of, that in the end it wouldn't matter that much if the voltages match.

The only thing is that I don't have 5V around on the board, only 3V3. That means there wouldn't be competition between my power supply and the USB "bus" power, but between my 3V3 voltage regulator and the one on the Wemos/Lolin board. I am not sure whether that would work.

I have thought about inserting a (power) diode, but I don't think it will help, because: it will either prevent power flowing from my regulator to the Wemos one OR from the Wemos to my regulator, never both. For that to work, indeed, you'd need to use a jumper that disconnects both 3V3 power rails when not fed from USB. For the moment I think that will be the best way, but I am still open to suggestions...