I took apart an office printer a while back and this is a photo of one of the power delivery PCBs:

enter image description here

(Please note I have removed some components from the PCB).

What is the reason for the slots in the PCB around the transformers?

  • 9
    \$\begingroup\$ Quick answer: They improve high-voltage isolation by increasing the creepage distance. I'll leave it to someone else to expand on this. \$\endgroup\$
    – Dave Tweed
    Jul 25, 2016 at 15:13
  • \$\begingroup\$ See also electronics.stackexchange.com/q/74244/2028 \$\endgroup\$
    – JYelton
    Jul 28, 2016 at 18:49

1 Answer 1


Those slots are isolation between various high-voltage parts of the board. Air is a very good insulator whereas the surface of the PCB can become low resistance in the presence of condensation or moisture.

The slots ensure that, at least, any current that does flow across the PCB will have a long path and, hence, a much higher resistance than it would otherwise.

Russell McMahon's answer to Creepage distance for PCBs handling line voltage AC? may be of interest.


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