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Chicken or the egg ?

Discussion in 'Electronic Repair' started by N Cook, Oct 4, 2007.

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  1. N Cook

    N Cook Guest

    I know this is just a pc ps but I'd like to understand what is going on, or
    not going on, even if not repaird.
    There are 3 HF transformers and an opto-isolator bridging the HV and LV
    sides.
    The startup oscillator, on the HV side, is a KA1H0165R whose o/p crosses the
    divide to give a Vcc of 12V on the main SMPS controller a KA3511DS, both
    datasheets downloaded.
    This controller o/p should then cross the divide to drive the HV side main
    oscillator to drive the main transformer back to the LV side supply rails.
    The SMPS "flag" output pin goes high meaning no-function and no o/p pulses
    for either push-pull output of this KA3511.
    As the KA3511 monitors over-voltage, and more to this point, under-voltage
    of the supply rails , how should it ever get started if it never outputs any
    main drive pulses, so no main supply rails.
    If i find time today I will run the KA3511 from 12V bench supply and see
    what happens .
     
  2. Eeyore

    Eeyore Guest

    More likely one transformer and 2 chokes, unless you have a specific reason to
    say otherwise.

    Graham
     
  3. N Cook

    N Cook Guest

    Why would chokes bridge the "no man's land" isolation strip between HV and
    LV areas ?
     
  4. No, probably transformers instead of opto-isolators (on the feedback
    and drive).

    Personally, I wouldn't even attempt to do any serious troubleshooting
    on a supply of this type unless there was no inexpensive replacement.
    I would check the obvious - ESR of e-caps, open high value startup
    resistors, and such. But beyond that, these tend to be infinite
    frustration sources. :( :)

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
  5. N Cook

    N Cook Guest

    I don't mind challenges unless someone wants a turnaround repair, which this
    is not. Its just practise for some future dedicated complex supply that has
    no generic replacement.
    I can see the one large and two small HF transformers tied up with the above
    process but I wonder what the 4 pin opto device is for, perhaps an overall
    on/off killer: when the controller flag is high it locks out the main HV
    oscillator from making any blips.
     
  6. Franc Zabkar

    Franc Zabkar Guest

    If it's a typical ATX PSU, then one transformer is part of the +5VSB
    supply, a second drives the oscillator/chopper, and the third supplies
    the secondaries. The optoisolator may provide voltage feedback for the
    +5VSB rail, or it may be part of a protection circuit that shuts down
    the oscillator.

    See http://www.pavouk.org/hw/en_atxps.html.

    Otherwise, if you have an AT PSU, then AFAIK the PWM controller on the
    secondary side gets its supply rail from the very first "kick" of the
    main oscillator and then "bootstraps" itself into regulation.

    - Franc Zabkar
     
  7. N Cook

    N Cook Guest

    Didn't get around to powering from 12V dc but did trace a bit of the tracks.
    The opto would seem to link, DC fashion, from the 3.3V rail back to the
    feedback pin of the low power startup IC to shut that one down.
    This IC, with the reservoir caps, will keep the main smps IC running for 10
    seconds after removing the mains supply (in the present unfunctional as a ps
    state)
     
  8. Could be, since the transformers can't provide DC coupling.

    The problem is that with no schematic or service manual, there can be
    so many dead ends and confusion that the experience IMO may not transfer
    to other areas. It's not clear that the knowledge gained is worth the
    hair lost. :)

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
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