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I think I have a short circuit in my Arduino Uno R3. The symptoms are:

  • It does not seem to be responding as before.

  • The orange "L" LED does not blink on boot-up, but just stays on continuously.

  • If I plug in a 12V source, then the voltage between the 5V and GND pins starts out around 5V, but then starts dropping rapidly. In the meantime, the underside of the board heats up a lot. If I plug in the USB source, the voltage between the 5V and GND pins stabilizes at around 4.28V ... but again, nothing seems to work.

  • After all this, I also checked continuity between the 5V and GND pins, and heard a beep. However, subsequent reading told me that this is not a reliable (or well-advised) test.

It seems like I have a short on my Arduino, right? Any idea of how this could have happened? I can only think of the following possibilities:

  • I was running the Arduino off a lead-acid 12V battery. At some point, the negative terminal wire disconnected and there was some sparking at the battery when I reconnected it (the pattery was also powering other things). Could this have damaged the Arduino?

  • I am using the Arduino to run 3 IR LED . To minimize wiring, I had wires these LED in series, rather than in parallel, which (at least at first) seemed to work. Is there any possibility this would have damaged it?

Thanks for any help.

  • Did these 3x IR LEDs have current limiting resistors? – Nick Gammon Aug 12 '19 at 21:40
  • Yes, there was one resistor in series with them. – Artemy Aug 12 '19 at 23:28
  • the pattery was also powering other things - what other things? – Nick Gammon Aug 12 '19 at 23:52
  • A couple of video projectors, drawing about 4.5 amps total. – Artemy Aug 13 '19 at 0:08
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See my answer here. A 12V battery doesn't necessarily provide you with 12V any more than a 1.5V battery provides 1.5V.

See here for example. A 12V lead-acid battery may well provide 12.7V which is out of spec for the Uno.

See the page about the Uno.

You have too much voltage going into the voltage regulator, it tries to dissipate it, it gets hot and either shuts down or fails.

I suggested in the answer above to use a buck converter to down-convert the voltage, and my answer (which was accepted) said that "it is working like butter".

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  • "Working like butter" is an idiom I have never come across before. How does butter work? – Majenko Aug 12 '19 at 10:22
  • Smoothly. Like butter spreading on a nice warm piece of toast, where you have the marmalade ready to go on top. :) – Nick Gammon Aug 12 '19 at 11:09
  • I don't eat toast... too many carbs... – Majenko Aug 12 '19 at 11:10
  • The voltage limits for the Arduino are 6-20V. I know that 12V is max "recommended" but I thought the issue was heating (I'm running very few watts through the things, so I don't think it should be an issue normally). – Artemy Aug 12 '19 at 17:28
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If I plug in a 12V source, then the voltage between the 5V and GND pins starts out around 5V, but then starts dropping rapidly. In the meantime, the underside of the board heats up a lot. If I plug in the USB source, the voltage between the 5V and GND pins stabilizes at around 4.28V ... but again, nothing seems to work.

That sounds like the LDO is overheating and going into thermal shutdown.

After all this, I also checked continuity between the 5V and GND pins, and heard a beep. However, subsequent reading told me that this is not a reliable (or well-advised) test.

If it's just a brief "pip" then it's just a capacitor charging or discharging. To qualify as a short it has to be a continuous "beeeeeeeeeeeeeeeeeeeeeee".

I was running the Arduino off a lead-acid 12V battery. At some point, the negative terminal wire disconnected and there was some sparking at the battery when I reconnected it (the pattery was also powering other things). Could this have damaged the Arduino?

It sure could. If other things are connected both to the battery -ve and the Arduino then the power could have passed through the Arduino in many strange ways killing things internally.

Chances are it's dead, yes. It may just be the main MCU which (if it's a DIP version) you could pop off and replace (with one with the bootloader already installed). But there's no guarantee it's only that chip that's gone. The USB chips are also a common failure point, especially the cheap Chinese CH340G.

In the long run it's safer just to replace the entire board unless you've got plenty of spare components and the skills to diagnose and replace them as needed.

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  • Thanks! It was definitely beeeeeeeeeeeeeeee-type continuity. Can you elaborate "If other things are connected both to the battery -ve and the Arduino then the power could have passed through the Arduino in many strange ways killing things internally."? I have a large 12V battery powering the Arduino, plus some projectors (drawing about 5 amps) which run running through a relay on the Arduino. Is this a dangerous setup? – Artemy Aug 12 '19 at 9:47
  • It sounds safe enough, but without seeing a full schematic I can't really say for sure. – Majenko Aug 12 '19 at 10:11
  • If you have the Arduino connected to ground but not 5V, but other things are connected to 5V, then power can be fed into GPIO lines on the unpowered Arduino, whicih is bad. Similarly, when you did connect the battery and saw a spark you might have sent a spike into places a spike should not go. (Which is pretty much everywhere. The voltage regulator is only designed for 12V max, if memory serves, so pushing more than that into it might have fried it.) – Duncan C Aug 12 '19 at 14:24
  • Thanks for the suggestions. The voltage regulator is designed for 20V max. Is there a voltage spike that occurs when a battery sparks? – Artemy Aug 12 '19 at 17:34

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