Thermofuses

A thermofuse, also called a thermal fuse, is a small temperature-sensitive electrical device that provides overheat protection by permanently interrupting current when a defined temperature is reached. It is designed to prevent damage to components or fires by acting as a one-time safety stop in the circuit.

Construction and operation: A thermofuse consists of a sealed body containing a fusible element, typically a low-melt alloy or a pellet, wired in series with the load. When the device temperature rises to its rated opening temperature, the fusible link melts or deforms, breaking the circuit. Because the opening is permanent, the device cannot be reset and must be replaced after triggering.

Ratings and placement: The key specification is the opening (or rated) temperature, commonly specified in degrees Celsius. Typical values range from about 60°C to 210°C, with common ratings such as 72°C, 90°C, 125°C and 180°C. Some designs may specify a time-to-open at a given temperature. Thermofuses are chosen to protect specific components or insulation, and are often placed near heating elements, power supplies, motors, transformers, printers, or other heat-generating parts.

Applications and safety: Thermofuses are widely used in household appliances and consumer electronics as a final line of defense against overheating. They comply with safety standards from organizations such as UL, CSA, and IEC, and should be replaced with an identical rating after activation. Improper replacement or use can compromise protection.

Limitations: Because they are single-use devices, a thermofuse does not provide ongoing protection after an event and may require investigation into the cause of overheating. They are distinct from resettable devices, such as PTC thermistors, which can recover after cooling.