Arduino boards are made for a centre-positive barrel plug. Check the polarity symbol on the adapter before you plug it in.
What the message means
Adapters come with either polarity. The wrong one can stop the board from powering or damage parts. Too high a voltage makes the regulator hot.
Fix it in this order
- Look for the symbol on the adapter with a dot in the centre meaning positive. Use a multimeter if the symbol is missing.
- Check the voltage range recommended for your board, and stay inside it.
- Use an adapter with enough current for the board and what you power from it.
- If the regulator gets too hot, lower the voltage or power the board from a buck converter.
- Never connect USB and an adapter from different sources without checking the board design.
Why it happens
Most Arduino barrel jacks expect a centre-positive plug, where the middle pin is positive and the outer sleeve is ground. Adapters are made in both polarities, and the plugs look identical, so reversed voltage can reach the board without any visible sign. Reverse polarity can damage the input protection and the regulator, and an incorrect voltage can leave the board dead or cause overheating. Checking the marking or measuring with a meter before connecting prevents that damage.
The regulator drops extra input voltage as heat, so an adapter with too high a voltage makes it hot and can shut the board down. Current matters too. An adapter that cannot supply enough current sags under motors and servos, which causes resets that look like code bugs. Stay within the board’s recommended voltage range, and power motors from a separate supply that shares a common ground.
If you see this, do this
| You see | Do this |
|---|---|
| Board dead on a new adapter | Check polarity and voltage. |
| Regulator very hot | Input voltage too high. |
Confirm the fix
- Set a multimeter to DC volts and confirm the adapter output is at the rated voltage, with centre pin positive, before plugging it in.
- With the adapter connected, measure the board’s 5 V pin and confirm it reads close to 5 V.
- Note the regulator temperature after a few minutes of running, and if it is too hot to touch, lower the input voltage.
Common mistake: Assuming any barrel plug that fits will work. Polarity and voltage are both specific, and a wrong adapter can damage the board even though the plug seats firmly.
Related
Uno 5 V limit, buck converter.
Last reviewed: November 2026. These are the usual causes and the order that finds them fastest. Test on your own board; clones and cables differ.