My Arduino UNO board is powered with external 5V SMPS through the DC input. On measuring the 5V pin on the Arduino board pinout, I find the voltage to be around 3.7V and the L13 onboard LED glows quite dimly. If I connect an additional USB cable over this DC voltage, the pinout reads 5V and the the L13 led glows bright. Is my Arduino cooked or I doing something wrong?

  • \$\begingroup\$ Did you measure the 5V output of SMPS with and without connecting to UNO. \$\endgroup\$
    – Mitu Raj
    Nov 8, 2020 at 12:54
  • \$\begingroup\$ Smps voltage output reads 5v \$\endgroup\$
    – seccpur
    Nov 8, 2020 at 13:15
  • 2
    \$\begingroup\$ VCCin has to be more than 7V if I remember well. Always start from datasheet :) \$\endgroup\$
    – Mitu Raj
    Nov 8, 2020 at 13:19

2 Answers 2


You are doing something wrong.

The DC input on your UNO needs more than 5V. There is a voltage regulator between the DC input jack and the 5V line that you are measuring. That regulator requires more than 5V in so that it can provide 5V out.

When you plug in the USB cable, the 5V from the USB cable goes directly to 5V line of the UNO. The regulator is then disconnected from the 5V line.

You need a power supply with more than 5V in order to use the DC input jack.

The Arduino UNO page say you need at least 7V, though no more than 12V in to the DC input jack.

The Arduino UNO uses anNCP1117 regulator. The 5V output version of the NCP1117 has a minimum input requirement of 6.5V.


The DC barrel connection is connected to a 5V voltage regulator, and is therefore intended to powered from a higher voltage (recommended 7-12V, see the Tech Specs here ) due to the regulator's voltage drop.

If you're supplying 5V, it could be connected via the USB port or directly to the 5V pin.


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