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Flyback reference not correct #6

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Flyback reference not correct#6
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@clayton13
clayton13
opened on Feb 3, 2021

The U302 reference was correctly chosen as a TL431 (TL431ACDBZR) which has a reference voltage of 2.5V instead of the expected 1.24V. This has the effect of doubling the expected 5V output from the flyback.

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  1. clayton13 commented on Feb 3, 2021

    @clayton13
    clayton13
    on Feb 3, 2021
    MemberAuthor

    I see two possible solutions:

    1. Recalculate the feedback resistors to match, R315 and R324
    2. Purchase the expected TLV431 and put in its place.
  2. clayton13
    added
    bugSomething isn't working
    schematic
    on Feb 3, 2021

  3. clayton13 commented on Feb 11, 2021

    @clayton13
    clayton13
    on Feb 11, 2021
    MemberAuthor

    As option 2 does not work due to incompatible footprints so we will proceed with option 1.

    This is an easy calculation to get 5V as it is exactly double the 2.5V reference so if two identical resistors are used in the range of 30k - 100k the output voltage would be suitable. However, under load this voltage my droop so we want to aim a little higher.

    Taking advantage of the existing 41.2k and a parallel combo of common 75k and 100k (42.857k equivalent) generates an output voltage of 5.10V.

    Original Modded
    1. Desolder R324 (marking is 13C), discard
    2. Move R315 to R324 (marking is 60C)
    3. Solder 100k on R315
    4. Solder 75k ontop of R315
    Original PCB schematic image
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