Is this diode broken?

Discussion in 'General Electronics Chat' started by tom66, Aug 1, 2011.

  1. tom66

    Thread Starter Senior Member

    May 9, 2009
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    It is a 1N5822 Schottky used in the secondary side of a power supply that gets excessively hot and has a reputation for toasting capacitors. So I am thinking it is dodgy. The power supply does not work at present due to either this diode or another fault.

    I am testing it in-circuit but as far as I understand the circuit, this should not be a major issue.

    Tested forwards using diode mode: 0.202V
    Tested backwards using diode mode: 0.365V

    Tested forwards on ohms: 588Ω
    Tested backwards on ohms: 489Ω (???)

    Although it does have a lower forward voltage drop it also conducts backwards. And yet it has a higher forwards resistance on the resistance range. I have never seen this before. Most diodes tend to fail shorted... and never retain asymmetry after failing.

    At the very least it seems leaky. I will probably replace it but I am wondering what is causing the very odd readings.
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    With it in circuit it is not really possible to check. Can you unsolder one end and repeat the tests?

    Only a schematic will tell you what the other paths are.
     
  3. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    It would have a forward voltage drop of 0.2V at a very low current according to the datasheet.
    Since you are measuring in-circuit there could be a lot of other components involved in your measurement.
    Why don't you dissolder one side and measure again?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Without seeing the schematic, all we can do is suggest that you lift one lead and test it without the "unknowns" in the rest of the circuit. That's the only way you'll be able to determine if the diode alone is faulty.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Holy cow, a dogpile! ;)
     
  6. iONic

    AAC Fanatic!

    Nov 16, 2007
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    No such thing as Forward or Reverse Resistance to my knowledge. It's kinda like saying that there's more than one "shortest distance between two points!" It's a fluke! Probably because it's "in circuit." Even that doesn't make too much sense.

    Dogpile..... the whole darn football team!
     
  7. tom66

    Thread Starter Senior Member

    May 9, 2009
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    The diode is in the flyback configuration - a transformer SMPS. I don't have a schematic for this device. I've decided I will replace it and see if it works.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    As long as you have the diode out, check to see if you can charge the output cap(s) and have an acceptable amount of leakage. If the leakage is high, the diode current could get excessive.
     
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