Electrical buzzing on MacBook Pro case when plugged in

Question

Electrical buzzing on MacBook Pro case when plugged in

I was touching the case to my new M4 MacBook Pro when I started to feel a vibration on my fingertips when I moved my fingers or palms over any part of the metallic case. I first I thought I was setting up resonant standing waves on the case from the friction of rubbing.

However, I finally figured out it only happens when the laptop is plugged in and my feet are grounded to the floor. Lift my feet off the floor and it stop. Unplug it and it stops. I figure it must be high frequency noise as I'm wearing socks so I'm only capacitively coupled to the floor...no DC current. Next, I should hook up a scope to measure it. I did use the multimeter and got 1V AC between the case and wall plug ground. I get 3V when I also touch the case with my feet on the floor and 4V when I lift my feet off the ground. It happens with both the apple supplied and third party chargers so I think it is from the electronics in the MacBook itself. However, If I flex the MagSafe plug down so its not charging but the ground is still connected I still get the effect, I only really feel it when I'm moving my fingers with a light touch. I think when I'm not moving I'm making a better ground the voltage must drop. Has anyone else notice this weird effect?

[Re-Titled by Moderator]

MacBook Pro 16″, macOS 15.2

Posted on Dec 19, 2024 10:25 PM

Reply

Answer 1

Dec 20, 2024 8:00 PM in response to AlanFromThree

Figured out what was "wrong". The EMI filters leak current onto the case. If you use the 2-prong plug on the charger there is not a ground to remove the leakage current. So you can become the ground. You also act as a capacitor to ground when you touch the case, and this extra capacitance from you add increases the leakage current. The currents are low, from between 5-50 microamps. I also measured voltages up to 46V. This is all at 60 hz. But the weird thing is that you only feel a buzzing sensation when you are moving your fingers on the textured part of the case surface. If you stop or move over the smooth apple logo on the top of the lid you cannot feel it. I also tested this out on two other new MacBooks Pros in the apple store with the same effect. The apple tech also felt the same thing so it's just not me. I tried it on a MacBook Air and got nothing. I see this more as a nuisance if I'm working on my laptop and I feel this constant buzzing on my hands as they rest on the case as I type on the keyboard. The simple solutions are either 1) Use the 3-prong cable to your charger so the case now has a good ground 2) Wear shoes so you are not grounded, assuming your floor is a good ground or 3) Don't work on it while its plugged into a 2-prong charger.

Reply

Answer 2

Dec 21, 2024 6:38 AM in response to AlanFromThree

Your assessment appears to be spot-on.

This effect is occurs rarely, but is reported on the forums from time-to-time. Since not all Macs and power adapters do it, this is considered a minor failure, and has an easy work-around. Apple sells a US$20 AC "extensions cord" that does provide a through ground to the power adapter, and stops the issue immediately in a properly-grounded home or office.

If your Mac is still under warranty, you can ask Apple to replace the power adapter (after inspecting your computer AND power adapter together) at your appointment at the Genius Bar of an Apple-owned store.

Apple does not consider this a safety risk that would force them to replace power adapters out of warranty.

Additional trivia:

the cord from the power adapter body to the MagSafe-end contains only two wires: an insulated center conductor, and its braided shield which is used as the return.

the MagSafe end has all the smarts in a little chip inside the magSafe-end. This accepts and responds to commands form the computer, responds to requests for power-adapter identity, negotiates power levels and provides that power ONLY on command from the computer. The LEDs in the MagSafe end are lit only under command from the computer.

The center pin is used a bidirectional bus for these exchanges. This is the pinout diagram of the previous version:

Reply

Answer 3

Dec 21, 2024 6:57 AM in response to Grant Bennet-Alder

The standard power adapter does not send ground all the way into the power adapter. The US version, for example, is only a two-prong input. The accessory AC extension cord DOES carry the ground all the way through. It is picked up by the raised chrome lug.

There are a few Power Adapters for entry-level Macs that do not have a replaceable AC input. the work-around in that case would be to replace that power adapter with a larger (higher capacity) one that DOES include replaceable input heads.

Reply

Answer 4

Dec 19, 2024 11:53 PM in response to AlanFromThree

I did hook a scope to up. To begin my laptop is pretty electrically isolated by sitting on a glass top desk. There is not much stray capacitance nearby either that could ground out an AC signal. The signal I measure is 60 hz and is the typical distorted shape you might expect from a power supply. The case by itself goes to a peak voltage of ~0.4V relative to earth ground. While I by myself go to about a peak voltage 1.5V. However, when I touch the case, I go to peak voltage of between 25-30V! (It maxes out the input of my scope at 20V). It only takes about 1mA at 60 Hz to feel a tingling in your fingertips and this is enough voltage to drive 1mA through my body resistance.

So is this laptop defective, or is this an issue with all recent MacBook Pros? I never noticed this with my old MacBook Air or M1 MacBook Pro.

Reply