Self-resetting fuse
A self-resetting fuse is an electrical component in the form of a polymer- based PTC thermistor with a non-linear resistance curve , which heats up in the event of high electrical currents , such as a short circuit , and consequently becomes highly resistive. After cooling down, it becomes low-resistance again by itself, which is equivalent to a function similar to a fuse with automatic reclosing.
In contrast to other overcurrent protection devices such as fuses , self-resetting fuses can be used multiple times. However, they are very sluggish in their switching behavior.
Other common names are PPTC or PTC fuse ( English polymeric positive temperature coefficient ) and resetting fuse . These components are offered by different manufacturers under different brand names : Polyswitch ( Tyco International ), Multifuse (Bourns), Polyfuse or Everfuse .
description
A self-resetting fuse with a monolithic structure reacts to overcurrent in circuits similar to a fuse. In contrast to a normal fuse, which does not reset itself and is not reusable, this fuse resets itself as soon as the circuit is interrupted. It is therefore no longer necessary to replace the fuse element.
functionality
Resettable fuses belong to the group of PTC thermistors , which are also known as PTC resistors . The solid material is a polymer filled with elemental carbon and therefore conductive , which is why the name polymer PTC is derived for the self-resetting fuse. The electrical resistance R of this material increases with increasing temperature . The current flow I through the element causes Joule heat (proportional to I 2 · R), which leads to an increase in temperature and resistance. If the current exceeds its normal value, the Joule heat converts the element into a high-resistance state, i.e. the current in the circuit is throttled to a value not only at the level of the normal value, but far below it, which gives the appearance: the element switches from. In this state, the voltage drop across the fuse element almost corresponds to the voltage applied in the circuit.
Electrical features
Due to their special design features, self-resetting fuses are not suitable for all applications. Initially, they were only available up to a nominal voltage of 60 V, later for up to 240 V. The maximum permissible current strength ( switching capacity ) may usually only be 40 A. Therefore, an additional fuse is often required to protect against higher currents.
Release behavior
The response time ( triptime ) is several seconds for most models, depending on the system, and can therefore be described as extremely sluggish. The function is also impaired by unusually high or low ambient temperatures. For some applications the response time is too long or the temperature dependency cannot be tolerated and conventional fuses have to be used.
costs
Self-resetting fuses are usually significantly more expensive than conventional fuses and fuse resistors .
application
For the reasons mentioned above, self-resetting fuses are mostly used in smaller electrical devices in the low-voltage range as inexpensive, maintenance-free short-circuit protection and overload protection built directly on the circuit boards in the device.
A common application is the protection of the supply lines for USB ports in computers and some high-quality USB hubs.