What is this element and how to check it?

Discussion in 'Test & Measurement Forum' started by UnnamedUser159, Nov 29, 2017.

  1. UnnamedUser159

    Thread Starter Member

    May 3, 2016
    436
    2
    IMG_20171129_103010.jpg

    Thanks in advance
     
  2. LesJones

    Well-Known Member

    Jan 8, 2017
    1,625
    400
    It is an input filter choke on the mains input. It has two windings. One for live and one for neutral. It is part of the circuit to reduce electrical noise at the switching frequency from getting into the mains supply and causing interference with other equipment. It will also attenuate electrical noise that is on the mains supply entering the supply. To check it measure the resistance of each winding. They should have a low resistance reading of less than a few ohms. Both windings should have the same resistance. You then need to use a high voltage insulation tester (Megger.) to test the insulation between the windings and between each winding and the core. You could also check that the inductance of the windings matched the manufacturers data sheet.

    Les
     
    Last edited: Nov 29, 2017
  3. UnnamedUser159

    Thread Starter Member

    May 3, 2016
    436
    2
    Hm. without desoldering the item measuring on Range 200 Ohms i see "1" between begin and the end of the windings - between 1 and 3 AND 2 and 4
     
  4. DickCappels

    Moderator

    Aug 21, 2008
    5,015
    1,524
    200 ohms (400 ohms total) sounds unusually high for a common mode choke.

    I you would, check all of the possible pairs of contacts. The two pairs that show the lowest resistance are the windings.

    Just in case it is helpful:

    upload_2017-11-29_18-7-3.png
     
  5. UnnamedUser159

    Thread Starter Member

    May 3, 2016
    436
    2
    i am not telling 200Ohms but "1"

    Now am desoldering the 4 legs to measure out of circuit because i see Resistances on the Winding`s ends
     
  6. UnnamedUser159

    Thread Starter Member

    May 3, 2016
    436
    2
    yes. the both of them are 0.7 Ohms. There is 1 value question:
    IMG_20171129_134715.jpg

    on the previous image i have showed the 4 diodes from bridge rectifier

    above them is this item - L1. About it there are 2 resistance and one of them is missing. It should be like the other one ?

    thanks

    * i am the reason for its missing !
     
  7. SLK001

    Senior Member

    Nov 29, 2011
    1,187
    441
    Why don't you share with us the reason you are tearing apart your power supply?

    It does look like R14 is missing - my guess is that it is the same value as R17. Probably a pair of bleeder resistors in the MEG range.
     
  8. UnnamedUser159

    Thread Starter Member

    May 3, 2016
    436
    2
    Hi @SLK001

    In short - this is power supply from audio amplifier. As usual i sometimes take devices for repair that i cannot repair "nice and easy".

    The idea was element by element check unfortunately...
     
  9. ian field

    AAC Fanatic!

    Oct 27, 2012
    6,252
    1,131
    The only one I ever saw fail, was breakdown between the windings - I thought the design and construction was supposed to make that impossible, but it happened.
     
  10. LesJones

    Well-Known Member

    Jan 8, 2017
    1,625
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    Hi Ian,
    That was the mode of failure of the only one that I have seen fail. There was no plastic former for the windings and the winding insulation was breaking down to the core. The unit was blowing the input fuse so I was expecting The main switching device to have failed short circuit. It made for a nice easy fix as I found a suitable replacement in my junk box. I find the TS's fault finding method rather odd just testing components without logically locating the fault.

    Les.
     
  11. ian field

    AAC Fanatic!

    Oct 27, 2012
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    Mine was a TV with a VGA connector that was being used on the PC. Always unplugged when not in use, it made a funny smell one time I plugged it in. Fully prepared to scrap it, I opened it in case of useful salvage - saw how easy, and fixed it.

    Used a toroidal type to replace it, I could better see the plastic shell under the windings.
     
  12. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    7,083
    2,002
    Why do you think this power supply has failed?

    ak
     
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