Resettable fuse

A self- resetting fuse is an electrical component in the form of a PTC thermistor based on polymers with a nonlinear resistance curve, which is at high electrical currents, such as a short circuit, heated and consequently high resistance. After cooling, it is by itself again low, which is equivalent to a function similar to a fuse with self reclosing.

Unlike other overcurrent protection devices, such as fuses, resettable fuses can be used multiple times. They are in their switching behavior, however, very slow.

Other common names are PPTC or PTC fuse (English polymeric positive temperature coefficient) and resetting fuse. These components are offered by different manufacturers under various brand names: polyswitch ( Tyco International), multifuse ® ( Bourns ), Polyfuse or Everfuse.

Description

A self- resetting fuse with monolithic structure reacts somewhat like fuses in circuits for overcurrent. Unlike a normal backup, which does not reset by itself and which is not reusable, these backup sets itself back again as soon as the circuit is broken. Thus an exchange of the fuse element is not necessary.

Operation

Resettable fuses belong to the thermistor, which are also referred to as a PTC resistor. The solid material is a filled with elemental carbon and thus conductive polymer, which is also the name of Polymer PTC for the self- resetting fuse is derived. The electrical resistance R of the material increases with increasing temperature. The current flow I through the element causes Joule heat ( proportional to I2 * R), which leads to an increase in the temperature and the resistance. If the current exceeds its normal value, transferred to the Joule heat the element in a high impedance state, ie, the current in the circuit is reduced to a value not only to the amount of the normal value, but far below what the outer appearance gives: The element switches off. In this condition the voltage drop across the fuse element almost corresponds to the applied voltage in the circuit.

Electrical features

Through its maximum design features self-resetting fuses are not suitable for all applications. So they were available until recently, only up to 60 V nominal voltage, recently there are also versions for up to 240 V. The maximum current ( switching capacity ) must usually be only 40 amps. Therefore, often an additional fuse to hedge against higher current is necessary.

Trigger Behavior

The response time (English: Trip Time ) is in most models due to the system for several seconds and must therefore be described as extremely sluggish. The function is also affected by unusually high or low ambient temperatures. For some applications, the response time is too long or the temperature dependency can not be tolerated and must be used in conventional fuses.

Costs

Self-resetting fuses are usually much more expensive than conventional fuses and fuse resistors.

Application

Self-resetting fuses come from the reasons mentioned above, usually in small electrical appliances in the low voltage range as a low cost, maintenance-free short-circuit protection and overload protection built directly on the circuit boards in the unit for use.

A common use case is to secure the supply lines for USB ports in computers and some high-quality USB hubs.