Transformer on PSU question

Discussion in 'General Electronics Chat' started by surfline, Jan 8, 2010.

  1. surfline

    Thread Starter Active Member

    Aug 12, 2009

    Attached is a picture of a PSU that outputs 12 Volts. The front of the picture contains what looks like a transformer. I wanted to know exactly what type these are called or why they are used because they are also found right before the DC output. Why would you use a transformer with a filtered dc input and a DC output? Are these just two inductors wrapped around the same core to reduce noise / filter on the power and ground line? What device would that be called?


  2. beenthere

    Retired Moderator

    Apr 20, 2004
    The device shown looks to be an inductor that is part of an input noise filter, The yellow box capacitor next to it is part of the filter network. The inductor makes a high resistance to voltage spikes, while the capacitor provides a low resistance path to ground.
  3. SgtWookie


    Jul 17, 2007
    Actually, I'd say it's a toroidal transformer that's part of an offline flyback-type switching supply. Finding the part numbers on some of the ICs would help to confirm or deny this SWAG (Scientific Wild-A.. Guess)

    It's labelled as "L4", but you can see under the clear mounting compound that it definitely has two windings, which would make it a transformer. There is also what appears to be a piece of fiberglass separating the two windings
  4. t06afre

    AAC Fanatic!

    May 11, 2009
    Could it be a common mode choke. I have seen the same before the rectifier at the primary side on medical switch mode PSU
    Last edited: Jan 8, 2010
  5. S_lannan

    Active Member

    Jun 20, 2007
    it's a common mode choke and the capacitor is an x2 rated capacitor which is meant to go across active and neutral