Electronic – How to get shocks while charging the phone in the train


Ever since I got a phone with a metal chassis I get this issue : if my phone is plugged to a train electric outlet and I simultaneously touch my phone and any metallic part of the train (such as an ventilation grid) I get a small, but painful jolt.

Now, as I understand it, insulation between the phone chassis and the train should be provided by

  • the train electric supply system
  • the charger's transformer
  • the separation between the phone's battery and chassis

So why do I get those shocks ?

Best Answer

It can be more than just capacitive coupling between the windings of the transformer. Some designs couple high frequency output noise (due to the switching action of the controller) back to the nearest thing to ground that is available and that generally means the high voltage DC supply line after the AC power rectifier. Coupling is done with a capacitor like this: -

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So, it trades better EMC performance for a slight tingle when you touch the DC output wires or you touch the metal parts of the device it is charging.