Fusible resistor characteristics query

May 27, 2008 4 Replies
N

Found this info for one supplier

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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 ?


-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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J

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.

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N

have

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 ?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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N

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?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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N

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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

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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