In another question someone ask about repair of laptop PSU. I cannot find it ATM - possibly because it was closed as about repair and off topic.

However an attached schematic to one of the answers had a sideway transformer (XFRM1 on schematic below)) surrounded by 2 caps (C1 and C2) before transformer to (I presume) lower the voltage. As far as I can tell it doesn't 'do anything' which leads me to think it may be a safety feature. What may be a purpose of it?


simulate this circuit – Schematic created using CircuitLab


1 Answer 1


It is a common mode choke, and it works to prevent high frequencies conducting back to mains. It is required for switch mode power supplies to pass the required electromagnetic interference tests.

  • \$\begingroup\$ except that this is not a switched mode power supply, and perhaps it is used as a line filter to reject incoming line noise, as common in audio applications and measurement equipment \$\endgroup\$
    – P2000
    Jul 19, 2020 at 3:37

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