@Majenko says in the leading (19 Upvote) answer here not to use VIN and USB socket at the same time.

You must never ever connect VIN to a power source and connect the USB socket. That can destroy the USB port in your computer. There is zero back-powering protection on [LoLin v3 board] board.

I need to do diagnostics on my Wemos (LOLIN) D1 R2 V2.1.0 while it's connected via 9-12V barrel jack and the Vin pin connected to an L9110H H-bridge as an output (bridge input).

Does the Wemos (LOLIN) D1 R2 V2.1.0 (ESP8266) work like this as well? Would I be able to run diagnosics with Serial through my USB socket while running this setup?

  • So that means, that you only have one power source: the USB port, right?
    – chrisl
    May 18, 2020 at 9:16
  • Power source is from the barrel jack (9-12V) @chrisl
    – Joshua
    May 19, 2020 at 13:13

1 Answer 1


The D1 R2 uses the same power supply arrangement as the Arduino Uno (albeit with a better switching regulator for the 5V instead of the Arduino's basic linear regulator).

So it should be safe to use both the USB and the barrel jack at the same time.

The NodeMCU doesn't include a 5V regulator and doesn't have the power switching circuitry of the Uno / D1 R2, and thus is a completely different arrangement.

  • I have a "Uno-like" Wemos D1 R2 (almost said R2 D2 then! :) ) which also powers a NRF24 board connected to it. When I power it from the barrel connector the Wemos Wifi works but the NRF24 board won't communicate. When I power via USB everything works fine. Perhaps the onboard switching regulator you speak of does not have enough grunt to power both but a USB source does?
    – Volksman
    Mar 2, 2021 at 7:22

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