Eeyore wrote in message
...


"Michael A. Terrell" wrote:

N_Cook wrote:

W for Wood's metal for the fusible part ?
Googling "wood's metal" AND "fusible resistor" produces nothing

It shouldn't. You don't want molten metal loose inside equipment.
Older fusible resistors were nichrome wire in a fiberglass, asbestos or
sand housing. A few manufacturers used carbon resistors, but running
them right at the rated current caused them to age, and the resistance
to rise, which caused false trips. The original Motorola 'Quasar, Works
in a drawer' TV chassis did this on the audio output transformer. You
had to use the specified brand and wattage resistor for repairs, because
of variations between brands. Metal film has replaced nichrome wire in
modern equipment. You wand the failed fuse to vaporize the conductor,
not melt it and let it still make contact, or worse, create plasma which
will carry a lot higher current than the fuse did.

Yes, modern 'fusible resistors' are typically made using metal film over a
ceramic former and covered with a 'cement' coating. All of which is
non-combustible and flameproof..

In comparison, carbon film covered with laquer is very combustible.

Graham


Fusible resistors are what it says on the tin - a resistor and a fuse.
The MO or metal bit and then the fusible bit, so avoiding the possibility of
seriously high temperatures building up in a fault condition, rather than
just non-combustibility.
I've only ever scraped one apart, long ago , out of curiosity, and there was
a definite fusible part that I remember as a metalic? blob rather than a
necking.
Unfortunately I did not know of Wood's metal, then, and didn't try heating
it up.
I'll try scraping back a 2.2 ohm one, tomorrow.

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