Let me set the scene:

  • Macbook with metal case plugged into the wall with 3 pronged plug
  • iPhone is plugged into an iPad charger with 2 prongs

When I touch both my iPhone and the case of my macbook I perceive a mild 'electric shock' which continues as long as I am touching both devices.

I assume that there is an air gap in the iPad charger so there should be no current flowing between the iPhone and the macbook.

I can't make any sense of what's going on and if someone could school me as to what's happening I'd be very grateful.

  • Why was this migrated to AskDifferent? It's obviously a question relating to power supply design and the fact that the devices are Apple products is incidental. The same thing happens in many contexts that don't include Apple products.
    – mjmdavis
    Sep 10 '15 at 6:42
  • It's an appropriate question on Ask Different, but that is a necessary but not sufficient condition to migrate; if it were off-topic on electronics AND on-topic here, that would explain migration. Not being familiar with what is on-topic for electronics, I don't know.
    – Daniel
    Sep 10 '15 at 10:40

Most switching power supplies use an EMI filter on the front, that looks like this:


The "ground" is connected to the "ground" on the secondary side as well as the earth pin on the plug. If the plug has no earth pin (or the socket does not have it), then when you touch it and an earthed object, a small current flows trough caps C2 and C3.

In most power supplies there is also a small capacitor bridging the primary and secondary windings of the transformer (again, for EMI reasons).

  • So on a MacBook you have the switching power supply in a white plastic rectangle with DC power coming into the MacBook via MagSafe. That means ground/power/sense/power/ground is DC in 14.5/16.5/18.5/20 V - is the "it" you are touching the metal chassis? If so, could you explain the current leakage in the DC system that is MacBook? It's not clear why the path to ground is the human instead of the neutral wire into the socket.
    – bmike
    Sep 6 '15 at 13:26
  • @bmike The path from the filter to ground, not the whole current. As you see, the filter has two capacitors between the AC input and earth (one between L and PE and the other between N and PE) and PE is connected to the ground on the secondary side, capacitors let a little current pass trough them. If the plug is grounded, then those caps are connected to earth. However, if the plug is not grounded, you get some voltage on the case of the device (and everything else connected to the "ground" of the power supply.
    – Pentium100
    Sep 6 '15 at 15:27
  • Thanks - I'm thinking that I'm not the target audience so the deficiency in understanding is mine, forgive me for not seeing the path from filter to the case evan after your excellent comment. If you wanted to add a second diagram to explain L, N (as opposed to L1/L2) and also diagram PE. I'm not able to apply this generic schematic to what Apple has actually engineered.
    – bmike
    Sep 6 '15 at 15:36
  • 1
    I do not know the specifics of the Apple power supply as I have never taken one apart (or own one). However. see the ground symbol? It is usually also connected to PE and the ground (usually, negative) of the secondary and the case (if the case is metal). If the plug does not provide the PE connection (as is in the case of OP) then you get some voltage on the case.
    – Pentium100
    Sep 6 '15 at 15:43
  • 1
    @pingbat, correct.
    – Pentium100
    Sep 8 '15 at 8:48

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