Arduino Uno not working with external power supply

Hi, I am a newbie with Arduino. I am trying to build a Line bot, but it is not working. I read a lot of information on the web but nothing solve the issue. The issue is basic, the board and the robot works fine when it is connected to the USB cable and the computer but as soon as i disconnect the USB from the computer and run with an external power supply the board stop executing the program loaded on it.

I am using a NiCd battery pack of 9.6volt used for RF cars, it is made from 8 AA battery. I can see the motor shield board and the Arduino are powered because their LED are on but the Arduino stop running the program loaded on it. If I use in parallel the External power supply and the USB cable the Arduino works fine and execute the program as expected, but if I unplugged the USB cable the Arduino stop to execute the program and restart to execute the program if I connect the USB back to the computer.

I though the program could have been to complicate but I load a simple flashing LED program and I got the same result, the LED is working only while connected to the USB cable and the computer.

I validate the power supply on the VIN pin and the ground and it seem OK the meter shows 8.7 volts. I validate in board Manager and the right board is selected.

Please any help would be appreciated

Check the voltage on the 5v pin. If you do not have 5v, the regulator may have been damaged.

The power plug has a diode in series so the Vin would be about 0.6v lower than the input. This is what you have.


You are right, I do not have 5v between the 5v pin and the ground, I have 1.8 volts. I will try with a new board to see if it works.

TO don't make the same mistake what could have damage the regulator???

Thanks for your help.

Have a look at the Uno schematic and the Eagle files for both sides of the board. The regulator is pretty much protected if you power up through the 2.1-mm socket, the idea being that it hard to get the polarities wrong. However, if you power up through the GND and Vin and introduce a short somewhere along the line, you will take out the power line protection diode which is on the Vin side only (I know because I’ve done it - the diode is the black component next to the blank socket next to IOREF on a Rev3 Uno board) - a good diode with a DMM in diode test is open circuit one way round, 1 to 2 volts the other way round. It’s not a disaster, you either link it out and be very careful how you connect, or fit a suitable 1N4000 series diode (or higher power 1N54??).
Regulators are generally bomb-proof, but exceeding the maximum input voltage will fry them. Some regulators will fry at 6-V input. I tend to use automotive grade regulators as these are resistant to most abuse that vehicle electrics can throw at them. You need to take extra care when working through the Vin header socket

The OP has 0.6 volts less on the Vin than the input to the socket. This suggests the diode is ok.


HI ! I have same problem with arduino uno and ethernet shield. everything works fine when it is plugged in USB from PC.

When when powered by an external power ( 12V, 1A ) sketch not work.... ( Between 5V and GND I measured 5.6 - 5.8V )

I tried powering it with car battery 12V not work. ( connected to jack )

Try 5V from my samsung charger not work. ( connected to usb )

What can I do ?