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If I put the positive probe of my multimeter on earth, and touch the negative probe with my hand, while another hand is touching the metal surface of my MacBook, I get around 22VDC or 69VAC. I cannot figure out why this happens. If I replace the MacBook with a li-ion battery, I get floating readings, if I try with a DC power supply, either positive or negative, I get nothing else.

This even happens when the MacBook is not plugged into it's charger, so it's not being connected to earth at any point. This behaviour defies my understanding of electrical engineering, and I would appreciate if someone could shine a light so I can understand how this is possible.

Thanks

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  • \$\begingroup\$ you are a big antenna \$\endgroup\$ – analogsystemsrf Jun 22 '19 at 2:27
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    \$\begingroup\$ this is a FAQ, but I can't find the other. \$\endgroup\$ – Jasen Jun 22 '19 at 2:36
  • \$\begingroup\$ Ohm’s Law says your lower impedance relative to the 10MOhm DMM couples this Voltage as a voltage to the DMM. Understanding the stray impedance and stray voltage present is the result of these experiments. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 22 '19 at 3:18
  • \$\begingroup\$ What 'ground' is your DMM touching? Ground is a relative concept... \$\endgroup\$ – hekete Jun 22 '19 at 4:21
  • \$\begingroup\$ The EMI filter in your PSU has Y-type caps connected from live to earth and neutral to earth to keep switching noise out of the power line. Connected one way, the Y cap to live will bring the case to line potential (at very low current insufficient to hurt you.) For two-prong supplies where possible, simply reverse the plug connection to the outlet and the high measurement will greatly decline. This is exactly how my Microsoft Surface book (titanium case) operates. I feel a shock with the plug inserted one way. No shock the other way. An AC-hot tester will alarm one way and not the other, too. \$\endgroup\$ – jonk Jun 22 '19 at 6:08
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Unfortunately my assumptions were incorrect. The part that defied logic to me was that the reading would still persist when the MacBook got unplugged from the charger. However the metal case around the charger plug is the culprit. When unplugging the plug, I was touching both the computer and the charger with the same hand, leading to the wrong result.

If properly unplugged and while not touching the charger plug itself, I do not read 23VDC/69VAC. The explanations in the comments do apply in that case, thanks. It's only leakage current being seen from the charger.

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