Overheated Arduino Uno R3

Hey guys, I've been playing around with PICs for a few years now and I recently decided to try out an Arduino. After a few weeks I've already blown my first one :frowning: haha. I'm just wondering if I could get some insight into what I did wrong so I don't do it again with the new one I just ordered. Please bear with me as this may be long, but I want to give as much info as possible.

Ok, so I've got an Arduino Uno R3 and a GSM/GPS shield (from DFRobots). The shield requires an external power source of 7-12V and ~1A to run properly.

I had been running the Arduino off a 9V 1A AC wall adaptor. While testing out this shield, I would plug in the external power first, then connect it to the USB. I had read that the Arduino would automatically use the higher power supply (I think) and this seemed to work fine.

However, sometimes the GSM module would work fine for hours, and other times it would keep losing signal and disconnecting from the network. I was told to try another power supply as mine may not be supplying enough current. As I didn't have another AC adaptor, I decided to use one of the molex connectors from my PC's power supply.

I made up a cable which plugged into the +12V and 0V pins of the molex plug and then tested the connector's polarity with a multimeter. It was correct and reading 11.98V. So I plugged it into my Arduino which powered on like normal and then I plugged in the USB cable which also worked as normal.

After having it plugged in for about 10 minutes though, I realised the voltage regulator on the shield (but not on the Arduino) was getting pretty hot, so I disconnected it all.

I figured I'd try a lower voltage, so I plugged my cable into the +12V and +5V pins on the molex connector to get +7V. I checked with the multimeter again and was getting 6.89V at the correct polarity.

I plugged this into the Arduino and it turned on fine and so did the shield. But I then plugged in the USB cable and immediately got a "one of the USB devices attached to this computer has malfunctioned" error on my PC. There was no comm port for my Arduino and I could not longer connect to it at all.

It was plugged in like this for about 2 minutes and then I started to smell the familiar smell of burning components. I quickly unplugged both the USB and power cables and noticed that the USB cable was burning hot. So was the usb port on my PC and the USB plug on my Arduino. The external power cable was fine though.

I let it all cool down for quite a while before plugging the Arduino back into a different USB port with a different USB cable and without the shield or external power. The board powered up and started running the code which was last on it, but the comm port no longer showed up at all.

I'm pretty sure I've blown the ATMega16U2 chip as I can't even get it to enter DFU mode properly. I've tried it on another computer and it has the same problem, it gets power, but the computer acts like there's nothing connected, even in device manager.

Now the only thing that changed in the 2 minutes from it working to it burning up was the external power supply was changed from 12V to 7V. Is connecting to the +5V and +12V the wrong way to do this? I checked the voltage before connecting it to the board and it was almost 7V, so I assume this is fine. Also, the board worked fine with this 7V external power supply until I plugged the USB cable into my PC. Why did the Arduino draw so much current from the USB port? And how did it get so hot when I'm sure I read it had an over current protector fuse to stop exactly this from happening?

Sorry for the long post, but I hope someone can help me work out what went wrong.


The 0V black wires inside of the computer is connected to the usb bus. That will be connected to the GND of the Arduino.
When you supply 5V to the Arduino ground and connect the usb, you have created a shortcut.

You should never connect the Arduino directly to the power of a computer. The Arduino is about prototyping so a mistake is easily made.

This is the shield ? http://www.dfrobot.com/index.php?route=product/product&product_id=673

Can you get a regulated power supply of 7.5 V ?

Ohhh, that makes sense. I was just looking at it as a 7V source and a 5V source, but they're both coming from the same power supply so yes, that can easily cause problems.

Thanks a lot for your quick reply. I knew it was my fault, I just couldn't pin point exactly what I did wrong. Serves me right for not just buying a proper AC adaptor, haha.

And yes, that is the shield I’m using. On that link it says 100mA@7V and in the link to the wiki it says 200mA@7V when making calls. But I was told to use a 1A power supply as a weak signal could actually cause it to draw a fair bit more than 200mA, and the 1A rating on an AC adapter is just a maximum, so it won’t cause any problems (is this correct?).

There aren’t many options near where I live for buying power supplies, but I can easily get this selectable 3-12V, 1A regulated power supply. Would this be alright? And would it be able to achieve 1A at any of the selectable voltages?

Thanks again for your answers.

Those switching power supplies are perfect. I use them myself.
They can supply 1A for every voltage setting. Set it to 7.5V

There are very cheap switching power supplies, and I read they can catch on fire. So I bought a few good ones (according to the regulations and with the quality labels) and I got rid of those brandless cheap ones.
I think that is a good one, but it costs twice as much as I am expecting.

Yeah, they aren't too cheap around here, that's why I decided to use my PC power supply instead of buying one. But now I have to buy one of these and I had to buy a new Arduino as well, lol. Oh well, at least I learnt something from the experience, haha.

I'll buy one of these and never use my PC power supply for the Arduino again.

Thanks for all your help :slight_smile: