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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?

schematic

simulate this circuit – Schematic created using CircuitLab

Maciej Piechotka
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    It is a common-mode choke. [This post](https://electronics.stackexchange.com/questions/202267/common-mode-chokes-and-their-selection) might have some useful details – nanofarad Jul 19 '20 at 00:10
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    see also here https://electronics.stackexchange.com/questions/331177/why-are-the-transformer-windings-in-series-with-line-and-load-in-this-circuit – P2000 Jul 19 '20 at 03:38
  • @P2000 voted to close as duplicate – Maciej Piechotka Jul 19 '20 at 06:37

1 Answers1

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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.

Justme
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  • 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 – P2000 Jul 19 '20 at 03:37