Excellent resource for polyswitch sourcing & designing.

A Polyswitch (Raychem) / Multifuse (Bournes) / Polyfuse (generic) is a
self resetting thermal fuse used to provide both overcurrent and over
temperature protection. Basic principle has a conductive powder
(typically carbon) embedded in a polymer matrix. When the matrix is
heated above its phase change temperature by the combination of the
current through it and the ambient temperature it "swells" and
separates the conductive particles, thereby causing a significant rise
in resistance. A much reduced 'holding current" is sufficient to
maintain heating due to the higher resistance. The device is (usually)
reset by depowering it and allowing it to cool. (A blast with a CO2
extinguisher may also work).

Polyswitches are extremely useful, but also very nasty thermo
mechanical devices. Basic operation is very straight forwards.
Understanding them fully is an arcane art. They are far from just
being the basic "trip at X amps / reset when power removed" devices
they may initially seem top be. Variables which may vary relevantly
include attack times, current/temperature/time curves, holding and
trip currents, max allowed tripped voltage  and more.

Excellent Tyco / Raychem Polyswitch page
Selection guide, catalog, order samples.
App notes.
Some links broken.
Also has links to other Tyco protection systems.

        http://www.circuitprotection.com/polyswitch.asp

Tyco / Raychem documentation

        http://www.circuitprotection.com/literature.asp


Extremely good 12 page design guide.

    http://www.circuitprotection.com/06Databook/fundamentals/PSWFundamentals.pdf

Basic overview and useful selector guide
4 pages

        http://www.circuitprotection.com/06Databook/PSWFundamentals(80-83).pdf


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