Yesterday I wrote in a comment that I didn't break much, except a LED and a LED matrix. However, exactly yesterday I think my Uno gave up, since I cannot upload a sketch anymore (timeouts). For the rest the board seems to work still ok.

I have two possible reasons, and would like to know what I can do to prevent this in future. I connected a 415 MHz RF transmitter and receiver to it, which are working.

  1. To get more power I attached an adapter (12V), TOGETHER with the USB ... I thought always adapter power would be used, but it seems it is not a good idea to have both connected. Is this true?
  2. The Uno was inside a case (with holes near the output pins). I know this causes some extra heat, but I would guess that adding just an RF receiver/transmitter would not cause the Uno to heat up more than allowed.

Which of these reasons is the possible cause ... or are there any other reasons I didn't think off. Everything worked fine until I plugged in the 12V adapter and it still worked for some time (until I tried to upload a new sketch).


2 Answers 2


It is a mistaken belief that powering an Arduino from an external power source increases the power available. It doesn't. All it does is increase the amount of heat produced.

Unless you are powering your Arduino from an unpowered hub you will have around 500mA available from the USB power. The 5V regulator, which converts the incoming 12V to 5V, can supply at best between 800mA and 1A on a good day. However with 12V the amount of heat generated will cause thermal shutdown at those kind of currents due to lack of proper heatsinking. So you can't get anywhere near 800mA out of it - instead you are limited to around the same as (or even less than) the USB power source can provide.

All that excess heat (and at 12V you are throwing away about 58% of your power as heat) in an enclosed case will have caused nasty things to happen - at the very least your regulator should have gone into thermal shutdown. Worst case scenario your voltage regulator failed (especially if it's a cheap version) which would end with one of two scenarios:

  1. No power getting through to the rest of the board
  2. More than 5V getting through to the rest of the board

If it's (1) it'd be obvious - nothing works. No LEDs light, etc. However if it's (2) then more nasty things will ensue. One common one is that the ATMega16U2 chip (the USB interface chip) if it's a genuine Arduino will be fried. You may find it's getting excessively hot itself now. Of course, if it's a clone then the CH340G will no doubt have given up what little ghost it had.

  • So if I'm understanding right, 12V is normally not a good choice unless the power is really 'used' externally (by LEDs or power hungry parts)? I read that using more power could improve the RF range (which it did, but not that much). Also if I understand right, it's not a good idea to power an Arduino with an adapter in an enclosed cage? I think that's not very comfortable. Too bad probably the Mega chip itself is probably ruined, that would be the only useful part being salvaged, since it was an original DIP-socketed chip. Commented Jul 20, 2017 at 10:53
  • 1
    Your bog standard Arduino Uno / Mega is not really good with external power and enclosed spaces, unless you only draw small currents. RF tends to be quite power hungry, especially when transmitting, which will drive up your current use and thus your heat. You are better bypassing the on-board regulator with a 5V buck regulator (e.g., a model vehicle "UBEC") which produces far less heat and can provide about 10x the current.
    – Majenko
    Commented Jul 20, 2017 at 10:56
  • Thanks for that info .. however, it disappoints me, I don't hope I need multiple power sources for my Arduino/RF when using RF. I was hoping I could handle everything with either a USB cable OR 12V adapter. How should I bypass it? By powering the buck regulator completely out of the Arduino with it's own power supply? I never used a buck or UBEC regulator. Commented Jul 20, 2017 at 11:01
  • 1
    Why not just a 5V USB power supply (phone charger) plugged in to USB?
    – Majenko
    Commented Jul 20, 2017 at 11:02
  • Hmm maybe that's better, I would expect they sell typical Arduino adapters with 5V ... going to search for them, that seems like a good way.Btw, I don't have to use buck regulators when I use a 5V adapter? Commented Jul 20, 2017 at 11:08

You should be OK as long as you use a 9 to 12 volt supply since there is an FET that disconnects the 5 volt USB power from the rest of the Arduino circuitry if VIN is greater than approximately 6.6 volts.

But... if your external voltage is less than 6.6 volts you may have a problem since both supplies will be powering the Arduino circuitry and they probably will not be providing exactly the same voltage. If you get below the dropout voltage of the voltage regulator then you may have a bigger problem since the USB voltage will be backfeeding the voltage regulator.

There is a diode between the PWRIN connector and VIN so if you feed the Arduino from that connector the voltage will have to be greater than approximately 7.3 volts to activate the FET and anything less than 7.3 volts may cause the problems mentioned above.


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