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I would like to power my Raspberry Pi 3B using an Arduino Uno.

I have been able to do this by connecting the 5v on the Arduino to the 5 V on the Pi and connecting the GND to GND, and the Pi turns on. But I get a constant under voltage sign.

How can I get the Arduino to output the right amount of power (5 V and 2.5 A)?

  • Neither Arduino nor RPi can produce electricity, so "powering one using the other" isn't representative of what's happening. You're powering both using whatever power supply you have, and the evidence is that it's not enough. – Dmitry Grigoryev Jan 21 at 13:52
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You can't.

The on-board regulator of the Uno, the NCP1117, is only able to supply up to 1A before starting to dropout voltage (hence your undervoltage warning).

Even worse, because the NCP1117 lacks a proper heatsink, it will probably start dropping out voltage before reaching 1A, due to its internal thermal shutdown protection.

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If you are using PC USB, your total current cap is <500mA. Arduino + Pi + accesories is exceed this cap.

Use an adapter for Arduino. Ardiuno will suck up some current and rest can flow to Pi or whatever else is there.

Arduino has a linear voltage regulator. Try not to use >5v adapters. Voltage regulator will generate more heat as more current pass.

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If you reverse which board gets the power adapter you should be able successfully power them both from a single 5V 2.5A power supply.

Connect the power adapter to the Raspberry Pi3 and power the Arduino from the Pi's 5V pin. The Uno should only draw about 200mA @ 5V from the Pi (depending on everything else you have connected).

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