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I'm tinkering a bit with a new printed circuit board I made and I already lost two boards (one ESP32 and one Arduino WiFi 1010) and I'm stumped because I don't even know where to start to look for a cause - it just doesn't make sense to me. Let me explain...

Here is the circuit diagram with a portion marked that I believe to be unrelated to what happened (I might be wrong):

enter image description here

(power supply set to 24V and 0.1A, the thing in the middle [microcontroller] is an ATTiny2313, WAGO is a wire-to-board connector)

What's a bit sketchy I would say is that the ground of the Arduino WiFi (which is provided via USB) is connected to my floating lab power supply. It also has an earthing terminal as in the picture, but it is not connected to anything.

The laptop is running on battery, so it is not connected to mains voltage in any way.

Now I can (sadly) only recall what happened in terms of when I realized (through coincidence) that my Arduino got unusually hot *. So hot in fact, I'm pretty sure it's done (I used it as an ISP programmer and now the arduino is not recognized anymore).

*this happened not immediately but after some time ; I can't tell how long.

Anyway, this is less about the dead Arduino (and ESP) but more about what went wrong because, as previously stated, I have no idea where to even begin looking for something that might have caused this.

The DC-DC converter might have something to do with it, although I can't imagine how it might have damaged my Arduino.

The problem is, I would like to investigate (i.e. replicate) what happened, but I don't want to risk losing another Arduino Board because they are really expensive (+40$).

Why do I not understand what happened?

1.) I only connected GROUND between the two circuits, so it cannot (likely) be something to do with the +24V rail or +3.3V rail.

2.) MISO, MOSI and SCK are I/O pins that I don't think can provide that much current to my Arduino (my Microcontroller did not get hot at all).

3.) RESET is a bit different since IT IS connected to the +3.3V rail but through a 4.7k resistor.

In the end, I'm really hoping I'm missing something obvious here.

Any pointers or common pitfalls that one might fall into when using a USB powered device with another device that isn't on the same supply rail?

Edit: And even if you can't explain what happened, do you have any ideas on how I could (potentially) replicate my circuit in a "safe" or "safer" way?

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    \$\begingroup\$ Funny that the only part you think is sketchy is actually mandatory. Connecting grounds of different chips/boards that is. \$\endgroup\$
    – Justme
    May 22, 2023 at 20:53
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    \$\begingroup\$ When troubleshooting, always, without exception, confirm the supplies are working as expected (the proper voltage), and the supply connections are also at the right level (connections on the chips and modules too). While there, probe the gnd pins as well. \$\endgroup\$ May 22, 2023 at 20:54
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    \$\begingroup\$ When you say fail, do they heat up, smoke, sparks, flames or do they go dead silently? What current do you observe on your lab power supply? What voltage do you measure on the 3.3 V rail? \$\endgroup\$
    – winny
    May 22, 2023 at 20:54
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    \$\begingroup\$ The usual suspect is some kind of short circuits, including accidental short circuits through grounds, but you are using floating ground for everything so that can't have been the problem. Or connecting +24V to the Arduino by mistake, but +24V only goes to the converter so that wasn't the problem either. I think even shorting the data pins on most Arduinos won't cause them to get hot (it sometimes breaks the pin so still don't do it). \$\endgroup\$
    – user253751
    May 22, 2023 at 21:07
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    \$\begingroup\$ A 5 V crowbar on the USB power connection should protect the Arduino if the voltage somehow goes to 20V or anything fatal to the components. Also, as mentioned, having two separate power supplies can cause problems with synchronization of the two circuits. I think I smell a rotten Apple! :) \$\endgroup\$
    – PStechPaul
    May 22, 2023 at 21:23

1 Answer 1

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Quite some time ago I had encountered a similar issue with the Arduino Mega board where the USB connection could back-feed the external supply circuit through a MOSFET body diode on their PCB. This cause various issues for me...

My solution was to remove a SMT fuse that was originally in-line with the 5Vbus USB input and connect the in-circuit side of the fuse pad to my external 5V supply.

Perhaps something similar is going on with the Arduino Wifi board?

See the note at B3 in the print below.

enter image description here

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    \$\begingroup\$ But in this case the Arduino does not have two power supplies connected. \$\endgroup\$
    – user253751
    May 23, 2023 at 20:56
  • \$\begingroup\$ I was about to comment what @user253751 said. It's a good thing to know, but as stated in my post, only GND and the SPI (incl. RESET) pins are connected. I'll probably take the risk and re-do the setup exactly as shown in my post to see if I can reproduce the issue. I'm a bit worried that I won't be able to reproduce the issue though. It could also have been a wiring mistake, although I see the chances of that being the case to be really low since the circuit has worked all day without an issue. To add to that, a similar thing happened to my ESP (but not connected the same way as the Arduino). \$\endgroup\$
    – Marco
    May 25, 2023 at 8:18

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