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Recently, I noticed my MacBook gave me a small shock when it was plugged in and I only touched the very corner of it with my palm. It was fine if I gave it enough contact by resting both palms on the shell. Intrigued, I went to investigate this issue and with my multimeter, I am measuring 80 V AC between my hand and the laptop cover:

80V AC between hand and laptop

This seems a bit concerning. I also remembered that my USB power source also gives me a similar shock. It's a bit annoying as I get shocked every time I touch either the 5 V or the ground. I can light an LED by connecting any one lead to either rail and simply holding the other side, but the LED turns off if I jump off the floor. (I made a cool circuit that lights the LED when I touch the handle on my desk:)

schematic

simulate this circuit – Schematic created using CircuitLab

I also remembered that our iMac would give you a nasty Jolt whenever you touched the cover, making it inconvenient to use the power button on the back.

I imagine this is all linked to how the house is grounded. I live in India, and I don't imagine the ground prong of the sockets is actually grounded; i.e. connected to a metal bar in the ground. Also, we don't have any of these problems in North America.

So my question is this: What is this effect, and how can I stop it?

[Edit:] There were some comments that were very helpful in understanding the problem: The main cause due to power supply design ("Y-Capacitor issue"). Can anyone explain how this problem arises and how to counter-act it in my own designs?

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  • \$\begingroup\$ Obviously the details of the power supply is missing. Does the power supply have ungrounded mains plug with 2 contacts or grounded mains plug with 3 contacts? \$\endgroup\$
    – Justme
    Dec 6, 2023 at 10:14
  • \$\begingroup\$ My 5V power supply is simply a USB connector on a power bar with 3 contacts, however, this issue is present with both 2-contact and 3-contact plugs. \$\endgroup\$
    – Klumpy7
    Dec 6, 2023 at 10:16
  • \$\begingroup\$ I meant the laptop supply. You obviously don't know how your power strip USB charging is implemented, unless you tear it apart and see. Many 2 prong USB chargers and other 2 prong power supplies may give shocks due to their internal design. \$\endgroup\$
    – Justme
    Dec 6, 2023 at 10:26
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    \$\begingroup\$ Is your adapter genuine? My old 60W Magsafe 2 Macbook adapter (no ground pin) has less than 1nF between primary and secondary which is not nearly enough to cause a tingle typically. Yours looks similar from what I can see of it. Aftermarket and fake adapters vary from okay to criminally bad. \$\endgroup\$ Dec 6, 2023 at 11:48
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    \$\begingroup\$ I measure over 100VAC on mine - all official MacBook Pro 2015 and MagSafe psu. It’s the classic ‘Y’ capacitor issue. There’s plenty of equipment that have the same issue - TV’s, AV gear…… \$\endgroup\$
    – Kartman
    Dec 6, 2023 at 12:21

1 Answer 1

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Many 2 prong USB chargers and other 2 prong power supplies may give shocks due to their internal design. It has nothing to do with grounding in your house. These adapters couple mains to low voltage side capacitively. Either intentionally with an Y cap or unintentionally via transformer winding capacitance.

If the laptop charger has a an earthed mains plug and it is connected to properly earthed wall socket, it should not give a shock. But if the laptop has an earthed mains plug and it is connected to an 2-prong unearthed wall socket, or the 3-prong wall socket has missing grounding, then the laptop supply may show high values such as 80 VAC on output and may give shocks. The mains input filter has Y caps from live and neutral to ground so it floats at half mains if wall socket does not provide ground.

So two pronged laptop chargers tend to give shocks anyway due to their internal design. Three pronged usually don't when connected to 3 pronged outlet as intended, but will shock if wall socket does not give the required ground.

As an example, a very big and expensive top of the line model flat TV from a very known brand has a black anodized metal frame and two prong plug. I don't want to touch the metal frame or do any connections to the TV unless it is unplugged or has connections through another device to mains earth so TV frame stays at earth potential.

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  • \$\begingroup\$ The issue being due to capacitive coupling across the switching transformer, with can't be bypassed to ground as there is no ground wire. On the positive side, whilst the current is enough to feel a tingle (especially if you are touching a real ground), the magnitude is too limited to be a safety risk. \$\endgroup\$
    – colintd
    Dec 6, 2023 at 10:39
  • \$\begingroup\$ @colintd There are different mechanisms: 3 prong input used without ground wire, or 2 prong input with EMI preventing Y cap deliberately between mains and 5V side. In any way, poor quality devices may be unsafe, I have witnessed a power supply to a device to feel much worse than a slight tingle, it can scare you if you don't expect touching a metallic device will bite you. And connecting two devices together with some data cable while being powered in such way can damage them. \$\endgroup\$
    – Justme
    Dec 6, 2023 at 10:46
  • \$\begingroup\$ Agree on magnitude. I remember finding this first with a Toshiba laptop and a grounded token ring cable. If you brushed the token ring connector housing with a finger, whilst your wrist was on the metal laptop, there was a real burning feel. On a related scenario, I've seen a serial cable melt like fuse wire when connecting telco kit on mixed -48V & 24V supplies, where "Gnd" of the serial link on the 24V kit was actually at -24V not 0V (mix of mesh grounding and an incorrect 48V-24V converter), and the -48V end had "Gnd" at 0V. Very lucky noone was injured \$\endgroup\$
    – colintd
    Dec 6, 2023 at 11:08
  • \$\begingroup\$ @Justme yes, I put it in the wrong spot. Will delete it and move it. \$\endgroup\$ Dec 6, 2023 at 11:48
  • \$\begingroup\$ This answer doesn't explain why it never happens in North America though... \$\endgroup\$
    – Klumpy7
    Dec 6, 2023 at 12:40

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