When interfacing my Arduino Uno with a relay I was facing reliability issues, and while investigating it with a multimeter, I found something strange.

I noticed that in the 5V power pin (with which I was powering my circuit) is producing only 3.3V when using the power adapter. However, when connected via the USB-A port, the 5V pin was producing the expected 5V voltage.

I tried powering it in many ways - computer-to-USB-A, computer-to-barrel-jack, phone-charger-to-USB-A, phone-charger-to-barrel-jack and an external 5V DC power supply. In all cases, the above observation held true - powering the board through the barrel jack made the 5V power pins produce only 3.3V.

Question: Why is this happening? Does that mean my board is faulty? Is there any way to fix this?

  • What voltage are you putting into the barrel jack? Is it 5V?
    – Nick Gammon
    Mar 10, 2018 at 5:02
  • Yes, I'm using a 5V barrel jack power input - in one trial from a phone charger, and in another trial from a computer's USB output. I measured it with a multimeter and checked that it is indeed 5V.
    – jrharshath
    Mar 10, 2018 at 18:50

2 Answers 2


and an external 5V DC power supply

OK, assuming you are putting 5V into the barrel jack this is exactly what you expect to happen.

From the Arduino Uno specifications you are supposed to supply a recommended 7V to 12V. Your supply of 5V is too low.

The barrel jack first goes through a M7 protection diode which would have about a 0.7V forward voltage drop, and then through a NCP1117ST50T3G voltage regulator which has a dropout voltage of (typically) 0.95V at 100 mA.

Thus you have:

5V input - 0.7V - 0.95V = 3.35V

That is what you are finding.


  • If you have a proper, regulated, 5V DC supply you can put that directly onto the 5V pin of the Arduino. Better get the wires around the right way because you are bypassing the protection diode!

  • Get a DC supply in the range 7V to 12V and put that into the barrel jack.

  • Another simple idea is to just get a wall-wart that is used for charging phones via USB and run a USB cable into the Arduino USB port.

Empirical test

I put 5V into the barrel jack of my Uno and read 3.33V on the 5V pin, which is pretty-much what you got. I measured a 0.77V drop over the M7 diode.

  • 1
    Thanks for explaining this. Indeed my barrel jack input is 5V, and I forgot that the input needs to be 7V to 12V. I'll change the barrel jack input to a 7V input. But I'd like to learn more... could you please talk more about how I can built a "properly regulated 5V DC power supply" from 4 AA batteries? Some starting links would help - I'll dig into it more.
    – jrharshath
    Mar 10, 2018 at 18:48
  • Tempted as I am to amend my answer to respond to this, I think this is a whole new (and quite interesting) question. If you would be kind enough to make a new question out of it then I'm sure you'll get some interesting responses. Possibly one of them would be that this is purely an electronics question, and not an Arduino one, but we can only hope that people don't do that. :)
    – Nick Gammon
    Mar 10, 2018 at 20:58
  • For example, make a question along the lines of: how I can built a properly regulated 5V DC power supply for my Arduino Uno from batteries? Answers might then discuss one battery, two batteries, four batteries, a 12V car battery, a LiPo batttery, etc.
    – Nick Gammon
    Mar 11, 2018 at 1:00

While Nick's answer is 100% (technically) correct. Functionally, it's a much easier story (given you feed 5V into the barrel jack).

Check the Arduino's product page.

Microcontroller ATmega328P
Operating Voltage   5V
Input Voltage (recommended) 7-12V

So yes, while the Arduino runs on 5V (and does so over USB). The recommended input voltage is 7-12V. You're simply not supplying a high enough voltage.

The difference, as per Nick's answer, is the protection circuit and regulator snooping off a few volts (which is bypassed when using USB).

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