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Fusible resistor characteristics query

Discussion in 'Electronic Repair' started by N_Cook, May 27, 2008.

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  1. N_Cook

    N_Cook Guest

    Found this info for one supplier
    http://www.peterparts.com/\CatalogPages\73\3123.pdf
    and 5 band colour coding

    Markings are to be shown on the resistor body by color coding.
    1st, 2nd and 3rd color codes : nominal resistance value.
    4th color code: resistance tolerance-J (±5%) - gold color.
    5 th color code : fusing characteristics
    green color -8 times
    blue color -12 times
    white color -16 times
    violet color -32 times

    if 1W then white band means subjecting to 16W will fuse in 1 minute

    Surely any normal 1W resistor subjected to that sort of overload will have
    burnt up quicker than in 1 minute or have I misread something ?
     
  2. James Sweet

    James Sweet Guest


    That's probably worst case.

    The biggest advantage of fusible resistors is they burn up gracefully,
    failing suddenly rather than drifting, and they won't catch fire or pour
    out nasty smoke.
     
  3. N_Cook

    N_Cook Guest

    Any suggestions what to look for on scrapping off the coating of what maybe
    a mechanically failed one. Where to find the fusing bit to check for
    fracture versus fusing ? or any other indications to look for ?
     
  4. N_Cook

    N_Cook Guest

    Removed the grey coating and remnant parts of the metalisation/ MO sum to
    about 1.5 ohm. The joining section is about 1 x 1 mm with a neat hair-line
    crack across it and no sign of any heating there.
    So mechanical breakage, I will dig out my microscope and take a photo of it.

    Before removing coating, was definitely 2.2 ohm markings with white band
    (if) for x16 presumably referring to that 1 x 1 mm section, so x32 violet
    banded one would have a something like 0.7 x 0.7mm intercept region and a
    green band x8 would have more like 1.4 x 1.4 mm fusing area.

    Anyone care to give a reason for shock induced fracture just at that point?
     
  5. N_Cook

    N_Cook Guest

    http://home.graffiti.net/diverse:graffiti.net/crack.jpg
    Is a microscope view of the crack. Now viewing magnified it can now be seen
    the fault developement.
    The just left of centre part of the crack must be part of a crack that
    formed initially but enough current flowed in the other parts making
    touching contact until there was too much localised fusing.
    1 to 1 marks the 1mm or so of conductor between the 2 straight line
    etching/millings ? into the conductor to form part spiral paths.
    The C marks the end of one of these formed breaks and there is another one
    on the other track , off the top left of the image, giving about 1 x 1mm
    central fusible conductor.
    Nowhere on the remaining conductor is there any discolouration due to
    overheating
     
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