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Read value of SMD diode (mini-melf)

Discussion in 'Datasheets, Manuals and Component Identification' started by SebaX75, Feb 9, 2012.

  1. SebaX75

    SebaX75

    45
    0
    Jan 10, 2012
    Hi,
    I've a problem with a switching power supply and I've found a diode in short circuit. It's a SMD component, minimelf with only one blue band... I've found that it's a zener diode, but nothing other info on it's body as you can see from the photo (ZD1).
    How do I know its value to replace it?

    Thanks,
    Sebastian
     

    Attached Files:

  2. shrtrnd

    shrtrnd

    3,670
    453
    Jan 15, 2010
    If it's a zener, read the voltage across the good one.
    Zeners act like a simple voltage regulator in their effect.
     
  3. SebaX75

    SebaX75

    45
    0
    Jan 10, 2012
    Hi,
    I know as a zener diode works, but the problem is that they are not the same (you need to enlarge the photo to view differences).

    Thanks,
    Sebastian
     
  4. davenn

    davenn Moderator

    13,294
    1,758
    Sep 5, 2009
    cant see any difference

    care to tell us why you think they are different ? :)

    Dave
     
  5. SebaX75

    SebaX75

    45
    0
    Jan 10, 2012
    One of the two blue bands is smaller, the body inside the glass have different orange color (one more dark) and size (the interruption between the two bodies seems shifted)...
     
  6. Rleo6965

    Rleo6965

    585
    9
    Jan 22, 2012
    Remove the defective zener first then read hidden label around the zener using magnifying glass.
     
  7. SebaX75

    SebaX75

    45
    0
    Jan 10, 2012
    Hi,
    I've already desoldered the SMD diode, only the photo was done with component mounted, but there is no written on the body (I use a 10x magnifier) so I've decided to do a post here if someone can help me.
     
  8. Rleo6965

    Rleo6965

    585
    9
    Jan 22, 2012
    The only way we can assume the zener voltage value was you have to trace the portion of circuit.. Much better if you include voltage value of capacitors, name of ic, optocuplers etch connected to defective of the zener diode.. Post that portion of schematic.

    Then, we can help you.:)
     
    Last edited: Feb 11, 2012
  9. davenn

    davenn Moderator

    13,294
    1,758
    Sep 5, 2009
    could easily fall within manufacturing variations :)

    Dave
     
  10. SebaX75

    SebaX75

    45
    0
    Jan 10, 2012
    Hi,
    the object is a switching power supply, output 9V 1,3A.
    The mosfet is a STK0460F (removed because I must replace it), no optoisolator on circuit, precision shunt regulator TL431.
    The SMD diode removed is connected to the single pin of SMD transistor.
    If you need other info ask to me.

    Thanks,
    Sebastian
     

    Attached Files:

  11. marcdima

    marcdima

    1
    0
    Nov 4, 2012
    hi ,
    I found this information I hope it will help you to find the reference of this diode

    the color code follow the RESISTANCE COLOR CODE but, it doesn`t give the VALUE of the zener voltage but the PART NUMBER!

    Take the resistance code and use the color number and put “1N” in from of the code you found and you got the zener part number!

    I had the same problem the diode has 3 color violet=7 yellow=4 green=5
    1N745 zener diode and you search the datasheet
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,191
    2,693
    Jan 21, 2010
    In this case the zener has a single blue band, so that technique won't work :(
     
  13. SebaX75

    SebaX75

    45
    0
    Jan 10, 2012
    Thanks for the help, but as Steve wrote, only one blue band.
    On this power adapter I've a "new" problem... probably an SMD transistor (only it or some other components?) burned during measurement and due to a short circuit accidentally done, so it's open on my desk since 10 months, in waiting of some ideas... the transistor has no one identification... I'm not very lucky.
     
  14. Rleo6965

    Rleo6965

    585
    9
    Jan 22, 2012
    My last resort to repair that power supply was to reverse engineer the primary circuit portion of the circuit. The result drawn schematic will guide you how to understand the circuit and replaced parts even label was "erased". But this require you a very good background in circuit analysis.

    You can show us the schematic diagram and we will do the analysis for you.:)
     
  15. SebaX75

    SebaX75

    45
    0
    Jan 10, 2012
    I can try to obtain schematics from power supply, but I'm not sure to be able to do something like this; the PCB is double face with SMD components, and probably many tracks are covered by component... in the next days I'll try, if I don't post anything is because I was not able to do.

    Thanks,
    Sebastian
     
  16. henrypereira

    henrypereira

    7
    0
    Nov 7, 2012
    use a siple 9v zener it will do the work
     
  17. SebaX75

    SebaX75

    45
    0
    Jan 10, 2012
    Now the problem is bigger due to a short circuit done during measurement, probably an smd transistor was gone, but nothing is wrote on it... I'm tryng to get the schematics as Rleo6965 asked, but it's not simple due to the PCB design and used components...

    Thanks,
    Sebastian
     
  18. henrypereira

    henrypereira

    7
    0
    Nov 7, 2012
    you can use any transistor as it is used to signal control any will work as soon as you put the same type such as pnp or npn
     
  19. SebaX75

    SebaX75

    45
    0
    Jan 10, 2012
    Without anything wrote on it I can't be able to replace with the correct type...
     
  20. Rleo6965

    Rleo6965

    585
    9
    Jan 22, 2012
    To be able to have progress solving your problem. Remember "Know your enemy before you conquer" Draw the primary circuit actual component layout in a legal size paper. Just like it was position in the pcb. Then trace the component connection of pcb draw connection in your drawing. You need to confirm connection using ohmeter. Post the picture of your drawing and we will try to help you.
     
    Last edited: Nov 17, 2012
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