I was observing some timer signals yesterday, earlier in the morning I had my oscilloscope on another lab workbench where I would first make some changes, reflash the board and then take it to that workbench to observe, while providing power use a switchable microusb cord connected to a usb wall connector. Like the setup shown here :

Updates : I was probing a SMT LED.

Nothing went wrong SuccessfulSetup

However, I was getting kinda lazy running back & forth so I thought maybe I could just have it connected to my PC so I can observe it right after flashing. So I hooked everything up, and then turned on the oscilloscope...and as soon as I pressed the power button, "zap" went my MCU, with a bit of smoke as well.
Setup shown here : FailedSetup

After some googling, I think it was maybe something to do with the scenario shown in this video (which is also the reference for my drawing), can anyone kindly help me figure out what the actual cause is? Though I have lots of spare boards, I still wanted to know how I can prevent this while also learn what had happened. Thanks in advance!

  • \$\begingroup\$ Did you connect your scope ground clip to anything not ground? What is that connection on bottom left of the ESK32? \$\endgroup\$
    – DKNguyen
    Nov 11, 2019 at 2:01
  • \$\begingroup\$ @DKNguyen It's a SMT LED, no I didn't clip anything not ground... \$\endgroup\$ Nov 11, 2019 at 2:15
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    \$\begingroup\$ Then what is the ground clip doing on the + of the LED? \$\endgroup\$
    – DKNguyen
    Nov 11, 2019 at 2:16
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    \$\begingroup\$ @JohnnyYeng - The diagrams are helpful. However, I believe your diagrams are slightly wrong. I think this is the same issue referred to by DKNguyen above. On the left side of both diagrams, the two connections to the oscilloscope are the wrong way round. The lower of the two wires going to the BNC connector is the earth (it goes to "Mains Earth" in the bottom left corner), but you show the upper of the two BNC wires going to "GND" on your board. I guess (?) it's the diagrams which are wrong and not your physical wiring. However the diagrams will confuse people in their current state. \$\endgroup\$
    – SamGibson
    Nov 11, 2019 at 2:44
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    \$\begingroup\$ @SamGibson I think I kinda know whats going wrong here... Here's my hypothesis : I had normal readings in the first place because the power source was a two prong which didn't share main earth as ground, but when I hooked it up to the PC, I shorted vcc to ground. Am I correct? \$\endgroup\$ Nov 11, 2019 at 3:33

1 Answer 1


You most likely have a ground issue on your board (non-continuous ground that is linked through the MCU) or somewhat connected the ground of the scope where it shouldn't be.

Your PC is most certainly grounded (sometimes, for laptops, it can be grounded in unintuitive ways through an external screen and through HDMI cables for example).

Your scope and computer are likely not connected to the same socket, you can be on a different line and thus have a voltage difference between the grounds which is probably caused by induced voltage.

This happened to me 2 weeks ago, as I found out that the ground between 2 adjacent sockets in my office had 100 V difference! My board was connected to my laptop (grounded through the screen) and a scope on another socket. Screen went black, it almost killed my laptop and my electronics, trying to understand when I finally went to measure the earth line on the sockets.


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