Electrovibration In Ungrounded MacBook Pros

My 2011 15” MacBook Pro

A few weeks ago, I noticed that the aluminum enclosure of my unibody Macbook Pro had a strange texture when I brushed my hand across the surface. After some tinkering, I noticed that this only happened when the device was being used while charging and that it only happened when using my shorter, 2-prong, power cable—leading me to believe there was some sort of current leakage happening.

After some searching, I realized that this doesn’t seem to be a unique problem. Posts on Quora and StackExchange describe a similar phenomenon:

Whenever my MacBook Air is connected to the AC power and I have my hands on the palm rest, I feel it passes some electricity to me. If I unplug it from the AC power it stops doing that.

The consensus seems to be that this phenomenon is the result of an ungrounded charging circuit. Rather than pass excess current to the ground pin of an outlet, the charger is incorrectly leaking some of the current into the aluminum chassis of the device. Since the Macbook Pro (and many other Apple laptops) have a body made almost entirely of metal, the leaked current passes through the locations where a user typically rests their palms at a frequency roughly around 50Hz.

The strange vibrating sensation is the result of a phenomenon known as electrovibration—the rapid attraction and repulsion of the skin to an insulated ground plane (such as brushed aluminum), due to the changing magnetic field produced by the alternating current. While it is an unintended effect in my Macbook Pro, this same technique of creating electrovibrations has been used to create prototypes of touchscreens that are capable of modifying the texture and friction of the surface in different locations, such as TeslaTouch

Author: Jake Rockland

Jake is currently a junior pursuing a B.S.E. in Computer Engineering at the University of Arizona. A hacker at heart, Jake has experience with firmware development, full stack web development, and iOS development.

9 thoughts on “Electrovibration In Ungrounded MacBook Pros”

  1. I’m actually a bit surprised at this. Since the actual power transformer is in the separate power supply, and the Magsafe connector only supports 16.5V DC. It’s possible that if you use the standard two prong connector, there’s actually no grounding/earth for the power supply (it’s actually the round metal bit holding the plug on), so there might be some leakage through the GND pins to the MacBook.

    Have you tried using the three pronged extension cable that Apple sells? It has a ground wire, and with it the sensation should disappear.

    1. Thanks for the comment admford! It is true that you should only experience this sensation when using the ungrounded 2-prong connector. With the 3-prong connector, the current is properly grounded and doesn’t pass into the device anymore.

  2. Wow just yesterday I was showing my friend about this vibration and we figured out that it must be due to leakage charges on the surface while charging and today I see your post.

    1. Hi Suzn! Though I have only felt mild current leakage through my computer and this seems to be a well-reported issue that is relatively benign, I would always take caution around ungrounded electricity!

  3. Hey, I actually have the same issue with my Surface Pro 3! When I’m charging and I brush against the back chassis, it feels, you know, bumpy or something. Unplugging the charger instantly makes it stop, and the chargers for Surfaces don’t have the third grounding prong.

    I had figured it was something related to current leaking onto the chassis, but cool to see that it’s not just me or my laptop.

    1. That’s very interesting! There might be a variety of other devices with a similar problem, since the unibody design is becoming more popular nowadays. The Surface Pro 3 definitely must conduct the ungrounded current.

  4. Appreciate the write-up.

    Same problem here. Tingling, unsettling sensation; clearly electrical. Varies with outlet and mostly goes away when using the 3-prong, long cord rather than just the 2-prong adapter brick.

    Computer also runs hot and battery life isn’t all that great.

    Much to like about the MBP, but it’s going back!

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