Polyswitch fuses miniSMD series will protect not only

Polyswitch-es provide a big service at protection of devices, where they behave like „resettable“ fuses. How many times it is suitable or necessary to use a fuse in a device, but a relatively bulky fuse socket simply doesn´t have a space in our device (or it would spoil a device design) and we also don´t want to use a PCB fuse holder, because a device would have to be constructed as user accessible... Then and not only then, is the ideal solution to use a Polyswitch. Such „fuses“ are available from various producers (for example also under names Polyfuse, Multifuse), Polyswitch is a trade name of company TE Connectivity/ Raychem, representing a top level in this segment.

In a principle, a Polyswitch is a PTC resistor with a Polymeric structure designed for a big change of resistance in a narrow temperature range. Conductive particles are dissipated in an amorphous polumer, which changes its structure at a higher temperature (melts), increases its volume resulting in a rapid increasing of a component resistance. In this point (tripping point) a very rapid increasing of resistance appears, which is high enough, so that a leakage current is practically negligible and won´t cause a damage to a protected circuit. Each Polyswitch has a defined holding current (I hold), nominal resistance at a given temperature and tripping (switch off) current (I trip). By a current flowing through a Polyswitch, it gets calorified (I2R), until a certain current it will reach a level, where a rapid increasing of its resistance appears. An exact current at which this change arises naturally depends also on an ambient temperature (at higher temperature a lower current is sufficient). This behavior is digestedly illustrated on an enclosed graph (Fig.4). In the A region, a Polyswitch is switched off (tripped) and a device is disconnected. In the region B the Polyswitch allows for normal operation (on) at any combination of a current and ambient temperature. In the region C can the component go tripped (off) so to say “anytime” depending on a concrete resistance value of a given piece.

Despite maybe seemingly complicated choice of an appropriate Polyswitch, in reality is the choice quite simple. If we want to be sure, that the component will withstand a given current (without tripping) in a wide temperature range, we´ll choose a Polyswitch with somewhat higher I hold. If we suppose, that a device will work only at a room temperature and doesn´t generate too much heat, it´s possible to choose an I hold closer to a desired tripping current. It´s worth to mention, that various producers declare I hold in a slightly different way, or they indicate the Itrip as a main parameter etc. That´s why when comparing various types from various producers, it´s suitable to check how a given parameter is defined.

In many cases it can be advantageous, that a Polyswitch needs for warming and a possible trip to a high-resistance status some time. That´s why a short-time increased current - for example at a device start-up won´t cause immediate switching off. However in general the Polyswitch is a quite fast component and at a hard shortcut for example type MINISMDC020F-2 (RF1335-000) will switch off within scant 10ms.

From our offer you could be familiar so far mainly Polyswitches in a THT version (with wire leads). Newly we added to a standard stock offer also small SMD versions - miniSMD ideally suitable for applications where there´s not too much space left. Those are types MINISMDC014F-2 (RF1334-000), MINISMDC020F-2 (RF1335-000) and MINISMDC050F-2 (RF1403-000), with Ihold currents of 0,14A, 0,2A and 0,5A/60V. Their miniature dimensions 4.7x3.4x0.9mm (SMD 1812) really save space and enable a automated machine assembly.

Detailed information will provide you the Polyswitch Fundamentals document as well as SMDC014F, SMDC020F and SMDC050F datasheets. In the SMD Polyswitch series overview you´ll also find recommended types for various applications.

In case of interest in any product from the TE Connectivity portfolio, please contact us at info@soselectronic.com.