I was bored one day so I took apart an old (as in no longer needed, late 2013) phone charger.

Everything looks well designed except for the spark gap between neutral/live and the low-voltage side. I'd understand if it was placed between live and neutral, but this one makes it look like you could get zapped if you're holding the phone while it's charging and a massive voltage spike occurs.

What good is it doing where it's placed?

The black thing that sticks out on the top left side is two wires for 5V DC out. AC in is at the bottom fork-looking pads.

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Side view

Top view with plastic cover-stuff

Top view without plastic

Attempted to draw a schematic of it

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    \$\begingroup\$ Could it be to protect the charger from the user? \$\endgroup\$ Feb 1, 2017 at 16:38

1 Answer 1


It's simply there to prevent the secondary side from building up arbitrarily large static charges, nothing else.

Common-mode primary-side surges large enough to jump the gap are vanishingly rare — and if one occurs, you've got bigger problems anyway. Differential-mode surges are probably handled by other components, such as a MOV, TVS or gas discharge tube.

  • \$\begingroup\$ That would explain it, I'll mark this as correct unless someone else has an even better answer. I think the differential mode surges are handled by either the 12 Ohm 1 W resistor in series with live/hot, or by toasting the circuit in its almost airtight environment. \$\endgroup\$
    – Oskar Skog
    Feb 1, 2017 at 16:55

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