Thermal cutoff

A thermal fuse or thermal fuse is used for switching off a circuit if overheating. In contrast to an electrical fuse which is not triggered by the primary current flowing through it influences, but by their temperature.

Basics

Thermal fuses are therefore close to or housed in the monitored components. This can be protected windings of motors or transformers or also cooking appliances, hot plates or coffee machines where safety from fires is required.

A distinction is resettable and non -resettable thermal fuses or irreversible triggering.

Non- self-resetting thermal fuses

Non- resettable type

Irreversibly inducing thermal fuses can be realized with an electrical connection at a defined temperature via a refractory metal (such as Wood's metal ). The solder connection must be open during the melting, which is achieved by a spring. Such components often have the form of a one-sided soldered sheet tab, the tab at the same time the spring is (eg on electrical resistors with overload cut-off ). Another design are, inter alia, used in coffee machines and irons radial components that are fully umhaust. They are similar to a semiconductor diode, with which they are therefore sometimes confused. Another design are flat, wired components that are suitable for placement in a wire coil (motors, small transformers, among others in power supply units and for low-voltage halogen lamps).

Manual reset type

This thermal fuses are similar bimetallic, but can only be turned on with a button after cooling again.

Self -resetting thermal fuses

Self -resetting thermal fuses close the circuit after cooling again. They can be realized as a bimetal or a PTC resistor.

PTC thermal switch also respond to the current flowing through it, which, inter alia, the protection of transformer windings is quite desirable. They are therefore a combination of thermal protection and over- current switch.

To monitor the temperature in windings housed PTCs switch the circuit before you replace it when it was interrupted for a time, because they still have a higher resistance due to their residual heat even when turned off. The high resistance in the hot condition leads to even after elimination of the overload or winding has cooled further held heating of the component and thus a continued interruption of the circuit - it will only be a small stream that holds the component warm.

Temperature fuses are EN 60691:2003 ( Thermal fuses - Requirements and application notes ) recorded.