Tingling or vibrating macbook when its charging

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Level 1

8 points

Tingling or vibrating macbook when its charging

Being an electronics engineer my IT colleague came to me today to ask why a macbook "vibrates" when one brushes the case with a finger and the unit is plugged in. Ill admit to being at a loss to explain the reason for the sensation because it existed only while the finger was moving across the case. When you stop the finger so does the sensation.

Reseaching the problem indicates this was noticed and reported in this forum back in 2007. I also found threads in other forums. The concensus indicates a grounding issue in using a 2 pin ac adaptor, which I would agree with. 3 pin ac adaptors apparently dont manifest this problem.

One test I did was to measure the voltage between case and the earth on the power point with the macbook plugged in and charging using a 2 pin charger. To my surprise I meaured 106Vac. (Australian mains voltage is 240Vac). One comment on the forum said the vibrating sensation were micro electrical shocks. With 106 volts on the case that would explain why. Even though its a small current due to the high impedance from the supply, my question to Apple is why? Its not that hard to make a supply without these problems. FYI 47V can be lethal with sufficient current drive.

Thanks for reading.

Posted on Oct 8, 2020 6:18 AM

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Answer 1

Top-ranking reply

John Galt

Level 10

146,306 points

Oct 9, 2020 7:53 PM in response to Jellybomber

The reason is that a Mac's case is capacitively coupled to ground, through the power adapter, when it's connected to ground (earth). If it's not connected to ground, and you touch the case, you become at least a part of that circuit.

Its not that hard to make a supply without these problems.

Correct. As a low power device, there is no regulatory requirement to ground a Mac's case, but that capacitive coupling results in less EMI radiation. That is a regulatory requirement. Without that capacitive coupling the Mac would need an extensive amount of shielding, adding weight and bulk Apple goes through Herculean efforts to minimize.

The sensation is more noticeable in 240 V systems than in the 120 V systems commonly used in most of North America. It can be annoying.

To my surprise I meaured 106Vac.

It can be a lot more than that. Voltage itself is not relevant; as you know a "carpet shock" can be thousands of volts. Any amount of voltage can be lethal given sufficient current. Lethality is better understood if you were to discuss it as energy in joules. As long as the power adapter isn't a fake (a significant concern) it is impossible for it to deliver a lethal amount of energy.

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Answer 2

Halliday

Level 4

2,677 points

Oct 9, 2020 8:06 PM in response to Jellybomber

Yes, Jellybomber.

I’ve noticed this for decades, not just with Apple products, but many others.

Now, I don’t know about Australian “mains”, but in the US two prong power has one “hot” and one “neutral”. As a result, typically, switching the polarity often eliminates the affect, because the “neutral” is (indirectly) grounded (usually out at the power pole).

Have you tried switching the polarity of your two prong plug?

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Answer 3

Jellybomber Author

Level 1

8 points

Oct 11, 2020 8:21 PM in response to John Galt

Many Thanks John, Ah capacitive coupling , yes that explains a lot. There is alway trade offs between over engineering the safety factor (which is primarily my experience) and reducing size and weight but still keeping it safe. I always marvel at the engineering that goes into a laptop and how they cram everything in. Even so in the old thread they mentioned that macbook ac adaptors with an earth pin never displayed this behaviour so that was my thought was that why could not the shielding be done in there and leave the laptop alone. I've tested many laptop supplies that have earth pins going in. In some the earth is brought through to the low voltage side and on others is a functional earth that only connects to the internal EMI shield inside the supply.

I will certainly pass this on to our IT people with the emphasis on ensuring users are using a genuine ac adaptor. We have many international people coming through our building and they bring all sorts of gear with them, and some of it unfortunately is of very questionalble quality.

Once again Thanks for replying :)

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Answer 4

Jellybomber Author

Level 1

8 points

Oct 11, 2020 8:30 PM in response to Halliday

Thanks Halliday.

Yep same with us too, but switching the active & neutral prongs is not so easy for us here in Australia. The prongs are angled so the plug can only go into the socket in one orientation unlike the US and europe where you can turn the plug upside down an plug in with 2 orientations. And I would not advocate getting inside either the ac adaptor or the power point to change the wiring over.

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Answer 5

Halliday

Level 4

2,677 points

Oct 12, 2020 5:01 AM in response to Jellybomber

Neither would I «advocate getting inside either the ac adaptor or the power point to change the wiring over», Jellybomber!

So, unfortunately, it’s not so easy for you to test the alternative, as it is for us.

Admittedly, the way my Mac mini is plugged in, right now, has this “tingle” affect. I simply haven’t shut it down, yet, to try the other orientation.

However, I have noticed that the orientation of the Apple USB charger changes this “tingle” affect for my iPads!

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Answer 6

Jellybomber Author

Level 1

8 points

Oct 12, 2020 3:02 PM in response to Halliday

Ha iPads and Mini's too. Interesting! Most likely Apple is using the same circuit architecture for these compact devices.

Thank you Halliday for the tips :)

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