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


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  • \$\begingroup\$ Would Electrical Engineering be a better home for this question? \$\endgroup\$ – Qmechanic Mar 10 '18 at 15:32
  • \$\begingroup\$ I agree! Do I need to do it or the moderator? \$\endgroup\$ – Feliks Mar 10 '18 at 15:34
  • \$\begingroup\$ @Qmechanic This is a simple problem that any physicist should be able to solve . "Electrical engineers" are not better in this. If a physicist cannot solve such a common experimental question, he should better look for a different profession. \$\endgroup\$ – freecharly Mar 10 '18 at 15:40
  • \$\begingroup\$ @freecharly I expect every physicist to be able to use a washing machine, physics.se is still not the right site to ask how to use one. \$\endgroup\$ – Christian Mar 13 '18 at 14:18
  • \$\begingroup\$ @Christian Your comparison is not a good one. Physicist frequently have to design and use electrical measurements to make their experiments. For this they have to be able understand how this works. In particular, they have to be able to explain unexpected problems with their measurements. If they would need electrical engineers for that they would fail miserably in their profession. Thus I do not agree with your opinion, unless you deem physics.se to be only a chat for unworldly theoretical physicists. For using washing machines, also electronicxs.se is not the right place to ask questions! \$\endgroup\$ – freecharly Mar 13 '18 at 14:35

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.

  • \$\begingroup\$ Wow, It seems very clear for me. Is there any way to measure or identify the AC voltage on DC output leads? and can you explain the video? \$\endgroup\$ – Feliks Mar 10 '18 at 15:33
  • \$\begingroup\$ @Feliks It is probably not easy to measure this AC voltage. It depends on the internal resistance (impedance) of your AC voltmeter. If you have a very high impedance AC voltmeter, you might be able to measure the high AC voltage. In any case, the neon glow tester indicates a high voltage. If you ground one of the DC output leads (maybe even the housing of the adapter), the high AC voltage indication of your tester should disappear. The extremely short video was no clear to me. \$\endgroup\$ – freecharly Mar 10 '18 at 15:48
  • \$\begingroup\$ @freecharly, does it indicate a faulty adapter? \$\endgroup\$ – Babu James Feb 27 at 5:59

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