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I am currently digging into electronics and today I experienced a strange problem.

I rebuild an oscillator circuit with the 30106 IC and I wanted to measure with my new oscilloscope (it is a china thing called DSO FNIRSI-150), whether there is something oscillating. As I connected only the oscilloscope's ground crocodile clip to the ground on the breadboard, the IC started to produce steam like an old locomotive and the 3 upper left pins of the IC produced so much heat that the breadboard was melting too in this area.

Why was this happening? - I could not find any explanation online. There are some threads discussing grounding problems with AC ground, but since the power supply of my very cheap oscilloscope does not utilize the ground connection of the power outlet, they might not be applicable here or am I wrong?

Also why does the IC melted down? - If I had (I don't know how) produced a short circuit with the ground of the switching power supply that powers the circuit and the ground of the oscilloscope, then why wasn't the oscilloscope damaged but the circuit IC?

I would highly appreciate any answer that gives me some clarity so that I won't burn any chips in the future :)

Here is an annotated image of the circuit:

burning circuit

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    \$\begingroup\$ Please, Show a picture of what you "did" on the breadboard ... \$\endgroup\$
    – Antonio51
    Feb 15, 2022 at 21:29
  • \$\begingroup\$ Does your scope plug into the wall at all? How was your test circuit being powered? \$\endgroup\$
    – DKNguyen
    Feb 15, 2022 at 21:35
  • \$\begingroup\$ I added a picture of the breadboard. The scope plugs into the wall (but the ground connections are not used, it is a "small european plug"). The circuit is powered by a grounded switching power supply (Meanwell RB65). \$\endgroup\$
    – hitchdiddy
    Feb 15, 2022 at 21:50
  • \$\begingroup\$ I just realized that I did a mistake connecting a wire: The brown wire in column 23 has to be connected to column 24 to ground the input of the inverter. Instead, I have connected the output of the inverter to ground, which may caused the short circuit? But why was this triggered as I connected the oscilloscope ground? \$\endgroup\$
    – hitchdiddy
    Feb 15, 2022 at 21:58
  • \$\begingroup\$ You could have at least mentioned that the scope does not have a 2-prong mains plug and it is powered by a 9V mains adapter. \$\endgroup\$
    – Justme
    Feb 15, 2022 at 22:36

1 Answer 1

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The likely cause is that your oscilloscope ground and your circuit ground are not at the same potential.

Usually, a bench oscilloscope is grounded to the ground wire of your building's main power. There are a variety of reasons why it might not be. One common reason is the use of "cheat" adapters that allow a three pronged plug to connect to a two pronged socket (without grounding). I recommend that you not use cheat plugs, as there are serious dangers associated with their use, such as electrocution and fire.

A USB oscilloscope will probably be grounded to the ground of the computer to which it is connected.

The ground of your circuit will (almost certainly) be the ground of your power supply. If your circuit is stand-alone, using an isolated supply will likely solve the problem. An isolation transformer can effectively turn a non-isolated supply into an isolated supply.

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