Thermal fuse
A temperature fuse or thermal fuse is used to switch off a circuit in the event of excess temperature. In contrast to an electrical fuse , tripping is not primarily influenced by the current flowing through it , but by its temperature .
Basics
Thermal fuses are therefore located close to or in the components to be monitored. These can be windings of electric motors or transformers that need to be protected, or cooking appliances, hot plates or coffee machines that require protection against fire.
A distinction is made between resetting and non-resetting or irreversibly triggered thermal fuses.
application
Thermal fuses or thermal fuses are used for the safe and one-time shutdown (disconnection) of electrical circuits when the permissible operating temperature is exceeded.
Frequent applications are in transformers, power supply units, heating elements, electric motors, solenoid valves, fans, water heaters, ballasts, alarm systems, electronic circuits, power transistors and many more.
Important parameters
Rated switching temperature T f
Switching temperature at which the thermal fuse switched off under specified conditions. The T f has no positive tolerance (EN 60691).
Operating temperature ( Fusing Temp. )
The trigger temperature indicates the temperature at which the fusible alloy in the thermal fuse begins to melt.
When selecting a suitable thermal fuse, the tripping temperature is less relevant compared to the rated switching temperature (Tf).
Holding temperature T h
The highest temperature at which the thermal fuse does not change the switching state. If the holding temperature is repeatedly reached, the fuse can be damaged and trigger prematurely.
Maximum limit temperature T m
Above the T m , the temperature fuse can be completely destroyed and contact again if necessary.
Rated current I r
The rated current indicates the maximum current that the thermal fuse can safely switch.
Transient overload current I p
The I p indicates the maximum pulse current that the thermal fuse can withstand without damage.
Rated voltage U r
The rated voltage indicates the maximum voltage that the thermal fuse can safely switch.
Thermal fuses not self-resetting
Non-resettable type
Irreversible thermal fuses are implemented with an electrical connection via a metal that melts at a defined temperature (e.g. Wood metal ). The solder joint must open when it melts, which is achieved by a spring. Such components often have the shape of a sheet metal tab soldered on one side, the tab also being the spring (e.g. on electrical resistors with overload cut-off). Another design are the u. a. Radial components used in coffee machines and irons that are completely enclosed. They are similar to a semiconductor diode , which is why they are sometimes confused with. Another design are flat, wired components that are suitable for being housed in a wire winding ( motors , small transformers, among others, in plug- in power supplies and for low-voltage halogen lamps ).
Manually resettable type
These thermal fuses are similar to bimetal switches, but can only be switched on again with a button after they have cooled down.
Self-resetting thermal fuses
Self-resetting thermal fuses close the circuit again after cooling down. They can be implemented as a bimetal switch (see also Klixon ) or with a PTC resistor .
PTC thermal switches also react to the current flowing through them, which u. a. is quite desirable when protecting transformer windings. They are therefore a combination of thermal fuse and overcurrent switch.
PTC resistors housed in windings for temperature monitoring only switch the circuit on again when it has been interrupted for a while, as their residual heat means that they still have a higher resistance even when switched off. The high resistance in the hot state leads to the heating of the component, which continues even after the overload or cooling of the winding has been eliminated, and thus to a continuous interruption of the circuit - only a small current flows that keeps the component warm.
Thermal fuses are recorded in EN 60691: 2003 (Thermal fuses - requirements and application notes).
literature
- Enno Folkerts, Werner Baade, Horst Friedrichs: The master's examination for household appliances, lighting and air conditioning technology. 10th edition, Vogel Buchverlag, Oldenburg and Würzburg, 2000, ISBN 3-8023-1816-1
- Günter Springer: Expertise in electrical engineering. 19th edition, Verlag - Europa - Lehrmittel, Wuppertal, 1989, ISBN 3-8085-3018-9