Return to thread "Do thermal fuses just die"

[N_Cook]

Basic wall wart ps but awkward V for replacement, just died, not been left
on or abused .
Rated 115 deg C ,3A in the primary side, In use about 30mA of mains.
I don't think I've sliced into one before and am not sure what I would
expect if fused from overtemp or old-age etc.
Probably as all the body of the fuse would be at the same over-temperature
then all the Woods metal/ high bismuth/antimony solder or whatever, would
have all melted and coalesced into one lump or a lump either side , would
have been my guess.
If failed from current surge/ old age / shock/vibration/ chemical breakdown
or something then a tiny fracture or a change of texture to powder or
something , instead of shiny metal.
Looking under x30 mag
This one between the terminal wires about 3mm run of the low temp metal with
about 0.5mm gap on one side, the end of shiney "fuse-wire" run rounded as
though locally melted , all in its original posistion with no obvious
channel in the casing to mechanically retain it in place.

So not really any more forward on failure modes of these fuses.

 

[N_Cook]

There seemed to be grease around the Woods to avoid full mechanical contact
to the outer casing, greenish colour so may be leaching of copper or
something. Again emphasising that the 2.5mm run of Woods if near to melting
,
could have bent a bit while soft.
I'll have to slice open a new one (different make of course) to see if they
are made with a knecking to the Woods
This fused one marked (doubtfully relevant as seems to be a generic problem)
UM1 E1 X22
as well as 115 deg C and 3A

 

[N_Cook]

There seemed to be grease around the Woods to avoid full mechanical contact
to the outer casing, greenish colour so may be leaching of copper or
something. Again emphasising that the 2.5mm run of Woods if near to melting
,
could have bent a bit while soft.
I'll have to slice open a new one (different make of course) to see if they
are made with a knecking to the Woods
This fused one marked (doubtfully relevant as seems to be a generic problem)
UM1 E1 X22
as well as 115 deg C and 3A

I regularly have to replace these on Eiko overhead projectors I repair
for schools. The fuse, (rated 84 degrees C), is located over and just
to the side of the projection lamp and fan motor. Typically the fan
gets sluggish or just jams up. The inside gets too hot and the fuse
opens up. It's definitely not a current thing, its a heat thing. Lenny

+++++

I opened up a new one , 105 deg C marked WTC R105, so different make and the
fusing filament in a different place. The Woods element there was a regular
flat sided bar with no observable necking.
I would still have expected all the Woods to melt , once up to its critical
temp in a heat failure mode.
But along with the spherical surface of the end of the Woods metal, in the
one in question, It would seem to be failure from heat .
Perhaps next time you come across one of your projector failures cut along
the sides of the fuse to see if all or part of the Woods melted. I should
have said the ones here are "plastic" flat-pack not the cylindrical metal
clad ones.

 

[N_Cook]

I got back to this today. A wire in the mains plug was totally loose. Would
that making/breaking contact and multiple in-rush current be enough to break
the Woods. ?
I doubt it as less than 10mA sitting idle and 3 amp rating of the thermal
fuse in current terms, but what temperature would the transformer get to in
the worst situation of such make and break mains contact, probably enough
to get to 120 degrees C

 

[William Sommerwerck]

The make-and-break can cause an inductive spike that blows the fuse.

 

[N_Cook]

The make-and-break can cause an inductive spike that blows the fuse.

So multiple spikes?
I cannot see one spike producing 3 amps for long enough to melt the Woods

 

[N_Cook]

I decided to slowly increase the temperature of the new one I broke into. A
bit more subtle. The cross-bar is probably lead and just the ends , soldered
to the wires are Woods or low temp solder. The grease turns to oil at 80
deg C or so.
In the core is a strip of silicone rubber foam , doubled over. When the
joint fails on the foam pressured end , it fails at that end , and then
"hinges" quite neatly at the othe rend , that is equally softened but
without the foam pressure. The temp then goes down, in normal use, and the
lead bar is then held in the open posistion.