USB C Power Adapter Short/Voltage Leak

Discussion in 'MacBook Pro' started by Kahalu'u Bob, Jan 13, 2017.

  1. Kahalu'u Bob macrumors 6502a

    Kahalu'u Bob

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    Honolulu
    #1
    I have a new 2016 MBP and could feel a tingle from the metal frame when charging, but not when running on battery. Curious, I put a digital multimeter on the USB C cable. The shield (the metal shell on the exterior of the plug) shows 35 to 50 Volts AC to ground when the power adapter is plugged in using the supplied duck bill plug! When I attach an old grounded plug adapter from my previous MBP to the power adapter, instead of the two prong duck bill plug, it goes to zero (as one might expect) and there is no tingle from the metal frame of the MBP while charging.

    I assumed it was a defective power adapter and got a new one from Apple, but it has the same issues. I have read in older threads about previous MBPs that others have claimed this is common when using an ungrounded plug, but I have never experienced it before on any previous MBP that I have owned. I have not seen any reference to this issue with the new MBP.

    Has anyone else noticed this? Does anyone have a digital multimeter they can use to check their USB C plug? If so, do you have AC voltage on the shield of the USB C cable while plugged into the power adapter?

    I suppose this is not a huge deal, but it seems odd (and potentially unsafe) to have that much AC voltage leaking through the power adapter to the USB C cable shell.

    Thoughts? Comments? Thanks for any help or insights anyone can offer!
     
  2. jerryk, Jan 13, 2017
    Last edited: Jan 13, 2017

    jerryk macrumors 68040

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    #2
    This is common to all aluminum Macbooks. Current leaks into the chassis when plugged in with the duckhead. And as you pointed out, does not if the charger is connected to a grounded plug. I have noticed it with a 2012 cMBP and my 2015 15" rMBP. I assume as long as the amps are low we are OK. Or as they say in the movies, "Not dead yet"!
     
  3. Kahalu'u Bob thread starter macrumors 6502a

    Kahalu'u Bob

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    #3
    Thanks jerryk. I have read others in older threads that have said the same thing. But I have had two prior all aluminum MacBooks and neither had a discernible current leak into the chassis. And 50 volts AC on the USB C cable shield seems like an awful lot to me under any circumstances. But as you say, we are "Not dead yet"!
     
  4. jerryk macrumors 68040

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    #4
    I have the same concerns about having that much power on the chassis. I really wish the duckhead had a ground.
     
  5. BorderingOn macrumors 6502

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    #5
    Try measuring neutral to ground at your outlet to see if voltage is present there.
     
  6. Kahalu'u Bob thread starter macrumors 6502a

    Kahalu'u Bob

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    #6
    Doesn't seem to be the outlet. Thanks for the suggestion. Do you have a 2016 MBP? Do you show AC voltage on the USB C shield? It seems very odd to me that there should be AC voltage there . . . . .
     
  7. curmudgeonette macrumors 6502

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    Jan 28, 2016
    Location:
    California
    #7
    The power brick will have an RF filter on the AC wires. The filter will have a capacitor from hot to neutral, another from hot to ground, and a third from neutral to ground. When the ground wire isn't connected, the two capacitors going to it will form a voltage divider. Hence, you see something like 60 VAC on it.

    Further, in computers it is customary to connect logic ground and shield ground to the AC ground. Thus everything grounded on the computer will show the 60 VAC. This is nothing new. My G4 Powerbook has this issue. Heck, I observed this in the 1980's.
     
  8. jerryk, Jan 22, 2017
    Last edited: Jan 22, 2017

    jerryk macrumors 68040

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    #8
    Thanks for explaining how this circuit is defined.
     
  9. Kahalu'u Bob thread starter macrumors 6502a

    Kahalu'u Bob

    Joined:
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