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 ?

  • \$\begingroup\$ Measure it with an AC voltmeter. I expect you'll see perhaps a dozen volts RMS, maybe more. A capacitor is a likely culprit, bridging primary to secondary. Look for it by tearing down your charger. See: electronics.stackexchange.com/questions/216959/… \$\endgroup\$
    – jonk
    Commented Aug 31, 2017 at 5:27
  • \$\begingroup\$ Y-caps! There are several questions about this already. \$\endgroup\$
    – winny
    Commented Aug 31, 2017 at 12:47
  • \$\begingroup\$ See electronics.stackexchange.com/questions/216959/…. \$\endgroup\$
    – winny
    Commented Aug 31, 2017 at 12:48

2 Answers 2


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: -

enter image description here

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.

  • \$\begingroup\$ That's a nice and clean and practical SMPS design, BTW :-) Complete with silicon part models and values of all the passives. Thank you. \$\endgroup\$
    – frr
    Commented Sep 1, 2017 at 13:09
  • \$\begingroup\$ @frr if it pleaseth you then your upvote would pleaseth me. \$\endgroup\$
    – Andy aka
    Commented Sep 1, 2017 at 13:35

That's simple. It's an inherent downside of switch-mode power supplies. I'd assume yours doesn't have an earth (PE) terminal into the wall.

This is what it would look like without countermeasures: SMPS without RF EMI suppression

This is where you are now, probably: SMPS with an RF short between primary and secondary

This can be done, but it would take a PE terminal on your PSU: SMPS with an RF short between primary and secondary and a secondary - earthed (And it has residual side-effects still: can contribute to a ground loop, can help trip a "Residual Current Circuit Breaker" etc.)

  • \$\begingroup\$ 50 Hz fron Y-caps/transformer leakage is the problem, not HF. \$\endgroup\$
    – winny
    Commented Aug 31, 2017 at 12:46
  • \$\begingroup\$ @winny The schematics show the y-cap and why it's there. \$\endgroup\$
    – Jeroen3
    Commented Aug 31, 2017 at 13:00

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