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Replace an LL4148 with a zener diode?

Discussion in 'Repairing Electronics' started by halleffector, Jun 20, 2017.

  1. halleffector

    halleffector Member

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  2. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    The LL4148 diode has the same electrical specification as the regular 1N4148, why do want to use a zener.?

    E
     
  3. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Why would you want to replace a signal diode with a zener diode?
    What part of the PSU circuit is involved (which would determine the voltage rating)?
     
  4. dave

    Dave New Member

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  5. halleffector

    halleffector Member

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    Thanks. I want to use a zener diode simply because I don't have a signal diode at hand to use.

    Edit: Actually I don't have 1N4148 either, guess I have to order something anyway.
     
  6. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    A 4148 is a bog-standard small silicon diode, you can use pretty well any little silicon diode to replace it.

    However, you don't give any indication of what the diode does?, the diodes datasheet is no use at all.
     
  7. halleffector

    halleffector Member

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    Sorry about that.

    https://www.scribd.com/doc/207782153/Dell-2407-WFP-Power-Supply-Rev-A11-Schematic-Diagram

    The diode in question is D655, which is found on the first page, connected to pin 7 of the L6561D PFC.

    It is for the same PSU I've posted about before, http://www.electro-tech-online.com/threads/repairing-a-dell-2407wfp-lcd.149287/

    All I have as far as SMD diode parts go are various zener diodes. In particular, these are all I have: http://www.ebay.com/itm/1280Pcs-080...169824?hash=item2a75b12420:g:eSEAAOSw2gxYuQzB

    As you can understand, I'm a beginner when it comes to repairs of SMD stuff, I guess I should spend my time in a better way than attempting to repair that PSU but I want to do it. It is my own device, and I don't intend to assemble it fully and run it permanently with a non-optimal part that may be a hazard. But if I could just use something and at least test it to see if it is working, then I can always order what it actually should have and replace that final part before it put it back together. Or would it be fine to use one of the zener diodes that I have? What voltage should I use in that case?
     
  8. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    IMO a fast switching signal diode is required, in order to quickly discharge the MOSFET Gate capacitor.
    Zener diodes are inherently slow and 'soft' and are not suitable for this repair.

    If you have access to any scrapped electronic modules, you should be able recover a suitable signal diode
    E
     
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  9. hyedenny

    hyedenny Member

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    The link you mentioned is for SMD resistors. I hope you don't think resistors and Zener diodes are the same thing...
    In any case, you could probably just leave the diode and the resistor out -- the datasheet for the L6561 shows neither on that pin in the applications notes. It looks like the gate driver circuit in the L6561 would make anything else redundant.
     
  10. Willen

    Willen Well-Known Member

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    Do you have scrap boards? I can get hundreds from such boards.
     

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