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I have plugged one end of an LED into the ground port of an Arduino.

circuit picture

By touching the other end of the LED with my finger, the LED lights up dimly. LED lighting up from touching it

I can keep the LED lit as long as I am touching the LED, but it goes off if I also touch the USB port housing (ground) on the Arduino with my other hand. It comes back on when I let go of the USB port housing.

This implies that my body is somehow being kept at a positive voltage with respect to the Arduino ground and that the positive voltage cannot be discharged by touching Arduino's ground. But I don't know what is supplying this energy. I'm just sitting on my couch with my feet on the floor. The floor is made of wood and lifting my feet of the ground doesn't change the effect.

Any clues what is happening here?

Mark
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  • It doesn't imply that you're being kept at a positive voltage, because the "D" in LED is "diode", and because a light that turns on and off quickly enough looks like it's continuously illuminated. – hobbs May 15 '20 at 06:58
  • See [this](https://electronics.stackexchange.com/questions/268597/coupling-between-hot-ground-and-cold-ground) and [this](https://electronics.stackexchange.com/questions/478607/feeling-a-tingling-sensation-by-touching-stuff-is-this-safe/478626#478626) for explanation of why a small AC current can be produced that could light a LED. – Andy aka May 15 '20 at 08:42

1 Answers1

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This is probably AC leakage to ground trough your body.
You can measure this with an AC voltmeter, it often reads half the mains voltage.
This is not dangerous.

It is caused by the Y-capacitor in switch-mode power supplies used to prevent emitting lots high frequency of noise.

Example, CY1 2.2nF: enter image description here

The solution to this is a grounded AC outlet. Secondary DC negative is then connected to ground, removing the weak AC component on the secondary.

Jeroen3
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    Hmm where did the picture come from I wonder? See [this](https://electronics.stackexchange.com/questions/268597/coupling-between-hot-ground-and-cold-ground) and [this](https://electronics.stackexchange.com/questions/478607/feeling-a-tingling-sensation-by-touching-stuff-is-this-safe/478626#478626) for explanation of why a small AC current can be produced that could light a LED. – Andy aka May 15 '20 at 08:43
  • I would not leap to the conclusion that it is not dangerous. It might also relate to a defect in house wiring, and might be the "canary in the coal mine" of a more serious problem that *would* kill you if not attenuated by that capacitor. It's never a bad idea to go through the house's Grounding Electrode, wiring from there to panel, neutral-ground equipotential bond (singular), wiring from panel to receptacle. – Harper - Reinstate Monica May 15 '20 at 16:15
  • So there is a capacitor somewhere inside the Arduino. One end of the capacitor is directly connected to the Arduino's ground output and the other end is connected to A/C power from my house. This causes a sine wave signal on my ground rail and when I touch it, I discharge the capacitor and current flows through my body to the ground of my apartment. Is this correct? – Mark May 15 '20 at 18:23
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    @Mark no, the capacitor is inside the power supply of your pc. – Jeroen3 May 15 '20 at 19:23
  • This reminds me of the 'screwdriver with neon bulb mains tester'. – vu2nan May 16 '20 at 07:21