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    #1

    Increasing voltage

    I recently recapped some Pentium SBC boards.
    When testing one today, it halted after 8 hours. When checking the voltages,
    I saw 3.3v was 3.6v. Turned off power, turn on again, now it was 3.4v.
    Then keeping an eye on it, it was very slowly increasing..
    I then tried replacing the 3.3v regulator, no dice, still the same.
    The board gets power (5v only) from a 12v-5v step down module.
    It just sits calm at 4.9 volts ALL the time, so powersupply is no problem.
    Any idea, why the 3.3v is slowly increasing ???
    Tags: None
    #2
    Re: Increasing voltage

    Is there a zener diode between 3.3v and ground or between 5v and 3.3v?
    It may be drifting w/temp.
    Mann-Made Global Warming.
    - We should be more concerned about the Intellectual Climate.
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    Be who you are and say what you feel, because those who mind don't matter and those who matter don't mind.
    - Dr Seuss
    -
    You can teach a man to fish and feed him for life, but if he can't handle sushi you must also teach him to cook.
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      #3
      Re: Increasing voltage

      Thanks for the reply!

      Just checked, and no zener.

      I come to think of a thing. I replaced an old Panasonic 10v/1000mf cap
      GE series. I could not find any info on GE series, so I went for Panasonic FM.
      As it turns out, I measured old cap, it had ESR 0.20 Ohms. New is ESR 0.08 Ohms.
      The 3.3v regulator output, is connected directly to the positive leg of the cap.
      So, can the lower ESR be a problem ?

      Comment

        #4
        Re: Increasing voltage

        In that application I can't see how ESR would do that.
        ESR affects AC and ripple [noise] and not DC.
        I can't see how it would cause a static DC voltage to slowly drift.
        .
        Mann-Made Global Warming.
        - We should be more concerned about the Intellectual Climate.
        -
        Be who you are and say what you feel, because those who mind don't matter and those who matter don't mind.
        - Dr Seuss
        -
        You can teach a man to fish and feed him for life, but if he can't handle sushi you must also teach him to cook.
        -

        Comment

          #5
          Re: Increasing voltage

          Maybe leaky cap if Vref to VR chip?

          Adding a zener would be cheesy but should fix it.
          Mann-Made Global Warming.
          - We should be more concerned about the Intellectual Climate.
          -
          Be who you are and say what you feel, because those who mind don't matter and those who matter don't mind.
          - Dr Seuss
          -
          You can teach a man to fish and feed him for life, but if he can't handle sushi you must also teach him to cook.
          -

          Comment

            #6
            Re: Increasing voltage

            Originally posted by PCBONEZ View Post
            Maybe leaky cap if Vref to VR chip?

            Adding a zener would be cheesy but should fix it.
            I replaced all electrolytics, there are only some VERY small SMD caps left.
            The zener sounds like an interesting idea.
            Thanks for the advice!

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