I have disassembled a phone charger today and I have found some interesting feature that I do not understand, I have marked it on the picture. Left side of this structure is connected to P1, that is pin which is inserted into the socket in the wall. Paths on the right side are connected to output wires. I can't figure out what is the purpose of this structure and why these pads look like forks. enter image description here

  • 2
    \$\begingroup\$ It looks like a spark gap structure to me: Spark Gaps. What is the actual distance separating the forks? \$\endgroup\$ May 14, 2015 at 18:35
  • \$\begingroup\$ About 5 mm between forks. \$\endgroup\$
    – Al Bundy
    May 14, 2015 at 19:19
  • 1
    \$\begingroup\$ What's on the other side of the board? \$\endgroup\$
    – The Photon
    May 15, 2015 at 21:06

2 Answers 2


They cross a line marked Primary / Secondary, so I assume they are a discharge path, to divert a lighting or static overvoltage so it doesn't burn through the insulation in the transformer. They're even labelled "SP1"

A discharge across the PCB may not necessarily be a disaster, while a discharge across the transformer will destroy it.

I think the lack of solder mask, and the round tips, are to try to make the breakdown voltage a bit more predictable. It will still depend on the air pressure and humidity.

Normally this would be done with a gas discharge tube, which has a much lower and more reliable striking voltage, but this might be too expensive in a phone charger.


Looks like some sort of spark gap protection, but why have that in a mobile phone? Where's the voltage coming from, lightening? If a phone gets zapped by lightening, it's definitely going to end up being a dead phone regardless of spark gap protection! And the person holding it is likely to be a gonna too.

  • \$\begingroup\$ Any mobile phone charger will have to pass ESD tests, where all of the pins are zapped with an ESD gun set to maybe 5 or 10 kV. This gap would help it pass that, keeping the discharge somewhere safe on the board. \$\endgroup\$
    – tomnexus
    May 16, 2015 at 10:45
  • \$\begingroup\$ OK, that's to do with CE marking on the equipment, checking conformance to electrostatic discharge and RFI regulations. So its really there to get through the certification process. \$\endgroup\$
    – Dean
    May 16, 2015 at 15:15
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    \$\begingroup\$ @Dean The certification process for ESD is meant to ensure that devices have a chance [tm] of surviving in the real world, so being designed to get through the certification process is, in this case, being designed to survive real world trauma. \$\endgroup\$
    – Russell McMahon
    May 20, 2015 at 2:20

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