Hi, could someone please help me understand what's wrong with my project? (Sorry if I put the topic in wrong category, not sure which would be the right one, so I picked general). So I have Arduino Mega board, powered with an external power source (step-down converter from 12 volt to 5 volt) via VI…

The OP has already been told, feed at least 7V to Vin. If they do that, no problem. The issue becomes powering significant auxiliary circuits, as we know the 5V regulator onboard is limited. To do that, the OP should use his 5V source to power the auxiliaries, connecting it's negative to the nega…

Your word description is imprecise, a schematic would be much better; if I read your text correctly, you're feeding 5V into Vin, which is way too low for Vin; when you connect USB, the power comes from USB, so the board works correctly. Feed Vin with at least 7V.

[image] yurii_orlov: external power source (step-down converter from 12 volt to 5 volt) via VIN pin Vin requires 7V or more.

5V should be fed into the 5 V pin. Not in vin. As the regulator needs at least 8V.

And camsysca did beat me :rofl: I guess that OP will get the point :wink:

[image] build_1971: 5V should be fed into the 5 V pin. Not a good idea From the official Arduino documentation on the product page: "Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. We don't advise it."

:facepalm: thanks guys :smiley:

So, what is your suggestion... It does not help op if we tell him what not to do...

[image] build_1971: It does not help op if we tell him what not to do... It does when someone tells the OP to do something that may damage their board.

So, the solution: Leave existing 5v power supply to power everything else besides Arduino (tft display and RA8875 controller) Add another power supply specifically for Arduino, which outputs correct voltage to power Arduino's VIN (7-12 volts according to doc, can't believe I missed that, lol) Did…

Wrong analogRead value with external power

Projects General Guidance

oldcurmudgeon June 20, 2025, 2:43pm 16

Here is a post I did related to the UNO. It will give you some idea of the voltage effect.

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Wrong analogRead value with external power

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