Another way coffee pots and other heated home appliances
fail is a part called a thermal fuse. I ran across my first one
of those in the seventies when my sister brought me her hot
curlers and asked me if I could fix them as they had
spontaneously become cold curlers.

	There isn't much to those at all, there is a rail in the
middle with pegs on both sides. A curler sits on each peg or rod
and has something in it to store heat.

	Each peg at least in this model had a pair of wires
coming out the bottom so there was a heater in each peg. I
remember checking continuity on each heater and finding that they
all  had resistance.

	The inside of the unit consisted of all those heaters, a
small thermostat and some sort of roundish lump in a tube of what
might have been asbestos. I could follow the circuit up to that
mysterious lump so I pushed the insulation back and could put my
finger right on it.

	The lump looked like one of those silicon diodes that are
shaped like a bullet. This one had no continuity so I cut it out
and took it to somebody who can see and it had a temperature in
Celsius stamped on the bullet and it dawned on me that this was a
thermal fuse. Those devices are just like electrical fuses except
that they are designed to blow when the temperature goes past a
certain value.

	They are hard-wired in and they blow for a couple of
reasons.

	The little thermostat may have welded itself continuously
on which would make the temperature rise until something else
melted or caught fire so they are a safety device of last resort.
Usually when they blow, something else is wrong and a new set of
curlers or coffee pot costs maybe $30 to $50. The only time I
ever replaced a thermal fuse was when I worked as a repair
technician for OSU's Audio Visual Center.

	Slide projectors had those same fuses as well as several
other film projectors that could cause some serious fires if they
overheated.

	We bought the replacements directly from the projector
companies. You sure don't want to fool around with dodgy
suppliers and fuses that are probably okay but maybe not.

	Another reason why they fail may be metal fatigue or
maybe somebody blocked the heat vents with something in the case
of projectors. In the case of coffee pots and heated curlers,
they are made so cheaply that it is not work trying to fix them
especially if they are capable of burning down your house and I
really mean that.

	My wife's Mr. Coffee failed, however, and it must have
been the electronic clock as it had two thermal fuses and both
were still good.

	I took it apart to see what was there. The only
interesting bit of design was that there was a one-way valve
which allowed the boiling water to spew up through the basket and
back down but only circulate in one direction, sort of like a
heart valve.

	Anyway, toss it. It's junk now.

Martin WB5AGZ