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Shorted Secondary Winding In SMPS

By on October 27, 2009

switch mode transformer

For your information, i came across shorted primary winding in SMPS transformer but rarely on the secondary winding. So far i came across only few SMPS transformer that have bad secondary winding. If you come across any SMPS that produce lower output on all the output lines, don’t just suspect components fault in the primary section or even in the Optoisolator circuit- check also the primary winding. A partial short in the primary winding not necessary will cause the Main fuse and power FET to blow. Sometimes it will cause all the output to have lower voltage. You have to test the primary winding with a coil/flyback tester and the reading should light up 4 LEDs and above. If it has only one or two LEDs lights up then you know the transformer have a partial short. If it has no light at all then the primary winding have a severe short circuit.

So, first thing you must do when you come across any SMPS that produce low output, check on the primary winding before checking on other components.



  1. Waleed

    October 28, 2009 at 1:28 am

    thanks for the tip. it is good to know these things.

  2. admin

    October 28, 2009 at 10:26 am

    Hi Waleed,

    You are welcome!


  3. behzad

    October 29, 2009 at 11:52 am


    thank you so much of good subject but you did not tell if the transformer is open or short . where i can get one
    what about new book no news from the book

  4. admin

    October 29, 2009 at 8:25 pm

    Hi Behzad,

    You are welcome and the transformer had a short circuit in the primary winding. As for the book, it is now going for proof reading for the second time and will let you know once it is out.


  5. jgmanza

    October 31, 2009 at 3:55 am

    Hello Jestine, so I cannot fix the crt tv as yet, because my scope has not arrived. Also I can't find who sells Blue ESR Meter as the company electronix on ebay says that they ship only to the US. So I am scratching my head when I remember Anatek Corp I saw some time ago on this site. I says "Hello Sir (John Bachman) please send me a Blue Ring Tester, a Blue ESR Meter and an Atlas component analyser because sir I really need this stuff" So he says ok, but I will send the invoice when the Component Analyser is arriving as I have to get that in stock. So I said Ok sir thank you for quick service, I read about your site on So he says oh, that is right, I know Jestine he is a close personal friend.
    Ha, small world.
    Anyway I didn't know about that low voltage due to secondary. Thank you, that is a tip and secret.


  6. admin

    November 1, 2009 at 8:25 pm

    Hi Jorge,

    No problem, in fact one has to have the ring tester in order to successfully test the primary winding of SMPS transformer.


  7. leonard williams

    November 6, 2009 at 3:34 am

    good day mr young i have mcr51r410 rca tv working on the sound is very low could tell me how to get into the service moode to ajust the sound please thanks to you .

  8. admin

    November 6, 2009 at 8:46 am

    HI Leonard,

    Sorry no information about this TV. In some TV, one has to press few buttons in the remote controller in order to access the service code.


  9. Bais Ahmad

    November 6, 2009 at 10:24 am

    Thanks useful info for LCD mon troubleshooting.

  10. k.selvaraj

    January 11, 2011 at 9:12 pm

    very usful for yolur informaction

  11. roshan

    April 6, 2011 at 9:36 pm


    i wont how switch mode transformer calculator..?

  12. admin

    April 6, 2011 at 10:27 pm

    Hi Roshan,

    Sorry i do not have the info.


  13. Mhel

    May 22, 2012 at 10:30 am

    Hi Mr. Yong I have here a ibm 15" lcd monitor w/ power input of 12v using adaptor and a dvi input, my problem is when ever I'm going to connect my dvi cable to pc my screen goes black and no response but sometimes it shows some image en then blank again. With only a 12v adaptor connected it working fine and shows the no cable connected message, I tried replacing the lcd panel but still the same, even with new 12v adaptor, hope you can help me with this. Thanks in advanced

    • Jestine Yong

      May 27, 2012 at 4:17 am

      Hi Mhel,

      If you connect the VGA port and it is working fine then the problem is in the DVI eeprom or even a bad MCU program. Before that make sure all the voltages from the voltage regulators ICs in the mainboard are good.


  14. silverthana

    July 24, 2012 at 2:49 am

    Hi Sir,
    Can you recommeded me where I can buy the coil/flyback tester to check SMPS transformer
    in Malaysia or Singapore or any Chinese brand.

    • Jestine Yong

      July 24, 2012 at 7:48 am

      Hi Silverthana,

      You can buy from me at this link


  15. Mubashir

    December 3, 2012 at 7:26 am

    Dear Jestine, Good day
    I want to ask a question regarding Switch Mode Power Supply Trasformer.  is it possible that normal ATX smps Supply transformer can use in other or all smps of computers. Voltages and pin configuration is same like other smps transformer.

    • Jestine Yong

      December 3, 2012 at 11:19 pm

      Hi Mubashir,

      All smps (including atx) transformers are unique and it can't be used on other equipment except on the same model.


  16. Tshering Tobgay

    February 1, 2016 at 1:19 pm

    Hi Jestine Yong sir,
    i have tube samsung tv, problem with power supply, I checked 300 v dc good but there is noise from SMPS transformer,

    • Jestine Yong

      February 2, 2016 at 8:06 am

      Hi Tshering,

      What was the first complaint? Have you checked if the output voltages good and stable?



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