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Power Supply Blow Again After Sometime

By on November 16, 2009

If you have replaced power components (be it a power IC or power FET) and the power supply blow again after sometime, then i suggest that you check on the secondary side. The secondary side that I’m referring to is the legs of the power transformer. I came across the power transformer secondary pins that had intermittent dry joints problem and the power supply could work (may be few hours ) but after that it will blow again. It is interesting to know that the blown parts were exactly the one that were replaced earlier. It will cause overvoltage, because no load at the secondary side. This tiny dry joints were quite hard to see and if possible get a magnifying glass to help you out.

Sometimes it can be your fault  too because you did not apply the heat compound to the power IC (power IC that attach to heatsink) or even to the power FET. Always check corresponding components if you found that the power FET had shorted.



  1. jgmanza

    November 17, 2009 at 2:36 am

    Hello Jestine, when you say power IC is it the opto?
    Thanks about the info about the secondary, I already met legs of secondary that had hairline fracture, but it had not caused problems as yet.

    How is the SMPS book coming.

    Best Regards

  2. admin

    November 17, 2009 at 8:20 am

    Hi Jorge,

    No it was not the opto is. It is the STK type (standing type of IC) of power IC that attach to heatsink. As for my book, it is done except that now i need to convert the file into html (for protection purposes). I just bought the protection software yesterday and guess got to play with it and it may takes time.


  3. saif

    November 19, 2009 at 5:58 am

    DEAR JESTINE YONG PLS TELL ME ABOUT PWM FAULTS IN CRT MONITOR, MY MONITOR B+ voltage is high, smps control circuit not working, in result FET & HOT blown in hours, my monitor is mitsubishi 900 u flatscreen 19", having njf9905 chasis, pls reply soon , thanking u very much again,

  4. admin

    November 19, 2009 at 7:25 am

    HI Saif,

    Try use this method to isolate the fault:



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