Need to identify SMC diode for repair project

Discussion in 'General Electronics Chat' started by Simon Boyd, Mar 22, 2016.

  1. Simon Boyd

    Thread Starter New Member

    Feb 9, 2016
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    Hi all,

    I am trying to fix a board, looks like an easy fix as a screw driver broke a diode in half.
    On my meter set to diode test I get a value of 278 on the good diodes. Picture attached.
    I am not sure what diode to order to replace the broken one and the suspect one.

    Any advise much appreciated.

    Simon
     
  2. ian field

    Distinguished Member

    Oct 27, 2012
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    If you've destroyed the diode, there isn't anything left to measure - and the diode test function should display a volt drop, not a resistance.

    If you're sure the adjacent diodes are identical, you can take one of those off the board and measure it.

    A Vf of anything under about 0.4V is probably Shottky-barrier. You have to watch out for reverse voltage rating - they start at only 20V and anything over 60V starts getting expensive.

    Zener diodes come in closest to the much mentioned magic figure of 0.7V - if you've destroyed the original; you're probably screwed.

    Standard recovery rectifiers are usually somewhere around 0.65V. Faster recovery diodes tend to have lower Vf - in extreme cases they can be hard to distinguish from SB.
     
  3. Simon Boyd

    Thread Starter New Member

    Feb 9, 2016
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    I have a mirror of that board on the other side of the equipment.
    Testing of the 5 remaining good diodes all give a read of between of 278 - 284
     
  4. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    InspectorGadget likes this.
  5. ian field

    Distinguished Member

    Oct 27, 2012
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    The TS has repeated what I assume are resistance readings for a meter that's claimed to be set to diode test after I've pointed out that normally produces a Vf reading.

    My suspicions have been raised...........................
     
  6. Simon Boyd

    Thread Starter New Member

    Feb 9, 2016
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    The reading is consistent with a germanium diode I believe.
    As far as I can tell germanium diodes will give a reading of between 200mV and 300mV or a Schottky
     
  7. ian field

    Distinguished Member

    Oct 27, 2012
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    So do Shottky barrier types.
     
  8. Simon Boyd

    Thread Starter New Member

    Feb 9, 2016
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    Yeah I meant to put that, I edited my post straight after but obviously too slow lol.
     
  9. InspectorGadget

    Active Member

    Nov 5, 2010
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    Germanium diodes would be unusual in digital electronics, but I have to admit that we don't really know what the circuit is. Can you give us a good picture of the whole PCB?

    From what I can see, the right-most two diodes (one of which was broken) look like they're doing some kind of rectification, or perhaps a diode-or combiner of two power sources. Or some kind of AM detector.

    But my best bet would be on a 1N4148 SMT type like Dave says.
     
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