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Topic: Help me understand why I fried my UNO? (Read 379 times) previous topic - next topic


Hi all,

I'll be unafraid to reveal my only very basic understanding of electronics and ask if someone here wouldn't mind explaining what I did.

I have a basic low-side pushbutton circuit taken from that 'typical switch wiring' diagram. I wanted to understand better what INPUT_PULLUP is really doing, so I took a variable resistor, set it to 30K with my multimeter, and wired it as shown in the attached schematic. I have read the internal resistors (INPUT_PULLUP) are 30K.

Running my UNO off of USB power / 5V, I saw that the switch worked properly, just as it did with INPUT_PULLUP.

Then running it off of 12V, it didn't. And I shortly realized that I did some damage because I could no longer upload new sketches. And none of the pins were giving me 5V anymore.

I think I fried it. Why did it work fine off of 5V? Should I have used a 10K or 4.7K resistor? Any idea if I could repair the board or is it trash?

Thank you.


Dec 08, 2018, 04:53 am Last Edit: Dec 08, 2018, 04:54 am by Grumpy_Mike
Where is the other end of that switch going? It should be wired to ground.

How did you power the Arduino at 12V? It should have been through the power jack or the Vin pin.

One way to fry an Arduino is to connect 12 V to any input.


Whups yeah, corrected schematic below. Other end is wired to ground.

I powered it at 12V via the barrel jack from a battery.


In which case you must have done something else because what you describe you did will not harm the Arduino.


Dec 08, 2018, 06:57 am Last Edit: Dec 08, 2018, 07:00 am by DrAzzy
That should all be fine. Was the 12v supply an ac supplth or anything? Old supply with unregulated output (how to tell - old ones often are heavier than youd expect for the power rating, and with no load, output significantly more than rated voltage)

I see a disturbing number of posts where people do nothing wrong, but kill the 16u2 when they connect to external power....
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Dec 08, 2018, 02:11 pm Last Edit: Dec 08, 2018, 02:19 pm by ReverseEMF
I fried a few el-cheapo Pro Minis that I got from eBay, by merely powering them with 12V [to the Vin].  A proper ProMini can handle 12V, and in fact sports a MIC5205 regulator that can take an input up to 16V [but, in the case of powering an Arduino Uno et al, not at the full current range specified]. 

So, no offense, but perhaps your "Arduino" is some cheap knockoff with a sub-standard regulator?

BTW: I was still able to run those ProMinis with 5V [on the 5V input].

Fix it?  Perhaps.  If it's the regulator that blew, and if you can unsolder/solder surface mount [though, as a beginner, perhaps not ;) -- but then, perhaps a nice educational project?  What have you got to lose?  The price of a regulator chip, maybe?], you could replace the thing [or any other components that fried].

Silly question: are you sure your battery is still supplying sufficient voltage?  Probably needs at least 6V [too lazy to look it up].  Also, if you connect the USB cable, does the Arduino show signs of life?
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Thanks for the replies.

Yeah, I verified that the power source is still good. It's definitely a cheap board-- no pretending there.

I get a green power light and sometimes the amber data one too connecting via either way. It just doesn't accept any new programming.

After inspecting further, I found a clue. The negative side of the 12V wiring between the battery and the board is slightly melted, exposing copper. I'm guessing that touched something, though I can't imagine it would have been something hot with more than 12V.

It's good to know that the way I added an external pullup resistor seems correct.

Still the regulator seems like a reasonable place to start. You're right, not much to lose at this point, I'll maybe research where that is located and try to fix. I'd need a new regulator component, correct?


though I can't imagine it would have been something hot with more than 12V.
If you short something out then your copper track can get hot enough to melt things. It doesn't take anything special in the way of voltage to do this, any voltage can drive too much current if the power supply is up to it.


Yea, must be a short. Touching pins 11-13 creates some interesting behavior with the 'L' LED. When I touch them, the light fades out. Otherwise, it stays on. Seems a little like those are toasted.


Based on this:
Then running it off of 12V, it didn't. And I shortly realized that I did some damage because I could no longer upload new sketches. And none of the pins were giving me 5V anymore.
I am guessing you connected your pullup resistor to +12V.
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Based on this:
I am guessing you connected your pullup resistor to +12V.
I used a potentiometer as a resistor (https://www.digikey.com/catalog/en/partgroup/trimpot-3296-series/5818) and soldered it to 5V and the input pin with a couple of jumper wires. I am thinking I inadvertently touched the very much exposed leads of that to 12V, as I do not remember being careful to hold it up an away from the board. Very good chance it was resting on it.

Definitely seems like the most likely cause.

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