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After dismantling an old PC power supply I recovered what seemed to be the primary transformer, after following the traces etc its purpose became less clear as it has two terminals and a ground connection , it receives ≈ 250VAC on one terminal and outputs an unknown voltage on the other terminal, which feeds into a bridge rectifier which feeds into 2 200v filter caps. This leads me to believe that it is used as a step down transformer but if so I don't understand its operation, could someone please clarify or explain the funtion, thank you.

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    \$\begingroup\$ I think you'll find that's the common mode choke. DC resistance measurements should be enough to confirm that. \$\endgroup\$ – Brian Drummond Dec 29 '16 at 13:03
  • \$\begingroup\$ So am I correct in saying it's filtering EMI out of the AC input before it is rectified to DC current? \$\endgroup\$ – Will D Dec 29 '16 at 13:08
  • \$\begingroup\$ Which also means the capacitors would have to be in series to handle the higher voltage from the rectified 250VAC \$\endgroup\$ – Will D Dec 29 '16 at 13:15
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    \$\begingroup\$ Yes, but mainly preventing EMI generated inside the supply from getting back out on the AC cable. It's a switching supply, which can generate a lot of EMI. \$\endgroup\$ – Brian Drummond Dec 29 '16 at 13:16
  • \$\begingroup\$ I think you have found a passive PFC choke. It usually has 2 terminals as a usual inductor, and the third (ground terminal) may be connected to a shield surrounding the choke to reduce EMI. See this schematic for reference - electronicproducts.com/uploadedImages/Power_Products/… \$\endgroup\$ – Todor Simeonov Jun 16 '17 at 6:43
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It's exactly what Brian Drummond says, it's a common mode choke: -

enter image description here

Some may have integral capacitors built in and thus you get the 3rd terminal that connects to ground. Some may not and thus rely on seperate capacitors to ground. Some also have an additional capacitor across live and neutral terminals like this: -

enter image description here

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