Thermal Fuses
These single operation devices work primarily as a temperature sensitive fuse. Due to their compact size and low cost they can be used as a primary protection device in applications where there is little thermal inertia experienced during normal operation conditions. Thermal fuses are also valuable as a cost effective added safety back-up to a primary operating control.
The construction of the most common form of thermal fuses basically uses a contact spring encapsulated into a wax pellet. The pellet is formulated to melt at a predetermined maximum temperature. As the wax melts the spring stretches until it breaks the circuit in the process.
Other types of thermal fuses incorporate the use of a specially formulated solder that effectively melts when exposed to given maximum temperature. These devices are non-resettable and react primarily to changes in temperature. There is some self-heating effect for these devices under higher current loads which may cause the operating characteristics to change over time.
Thermal fuses are also referred to as thermal links, thermal cut-outs (TCO's), or one shots.