Re: "Reverse engineering" burnt resistors

N Cook <diverse8@xxxxxxxxx> wrote in message
news:f7crqi$64s$1@xxxxxxxxxxxxxxxxxx
Can anyone assist in expanding this table.
If you know the maximum voltage likely to have fell across a resistor in a
fault condition - how to get an idea of the original value , if the
schema/parts listing is unavailable. Using V x V /R

Say for a 1/3 watt metal oxide resistor
1/ Discoloured unreadable bands , body of resistor browned, intact , but
ohmage may have varied
2/ Charred to the point of not being able to discern band positions , let
alone colours, may or may not be still intact in resistance of any value.
3/ Melted and fused into a small glob
4/ Exploded apart

For 1/ I would guess about 0.5 to 1W, value likely going up or down ? and
different result due to long term or catastrophic overheating ?
3/ from one recent example , dissipation of about 10 watts. The resultant
glob had resistance values radially from the remnant of one lead to
various
points on the glob of 1/5 to 1/2 of the original value for areas where
there
was conductance, some areas above 30 Meg ohm.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/




Destructing a few 1/3 R, for the 4.7 ohm ones I chose then about 1W with
constant voltage but the remnant resistance decreased so runaway at about
2.5W.
Dropping to about 80 percent of original resistance for 1W and discoloured
bands and only browned body. For the one about 2W then black body and about
70 percent of the original resistance, held for 2 minutes but a bit more
than that and it would have accelerated to complete destruction.

So for those end results, approximate wattages
1/ , 1W
2/ , 2W
3/ , 10W
4/ , presumably > 20W

Number 3/ I should have said one lead broken and the glob being retained by
one lead

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/


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