Electronic – Why does the electricity tester work on DC Adapter

voltage

I have a problem to understand how AC electricity tester works.
I have a 12V DC Adapter (from 220 VAC). The multimeter gives the results “12,2” for it.

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When I connect the tester to an output wire of adapter then the other wire, the neon glows at both situation. Why does this happen with 12V DC, both wires, while it doesn’t on a DC battery?

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When I connect a probe of multimeter to one wire, the other probe to my hand (my body is connected to ground), Why does the voltage change on the screen results?
https://www.youtube.com/watch?v=B3BciRd32Ag

Best Answer

That the neon glow discharge lights up when you connect it to either output wire of the DC adapter, is probably the result of the internal circuit of the adapter. If there is even a small capacitive coupling from the 220 V AC input to the DC output, this can be sufficient to produce a high AC voltage (but only very small AC current) on the DC output leads with respect to the ground which causes the neon tester to glow. For this only a very low AC current is necessary.