Bourns® Multifuse® Polymer Positive Temperature Coefficient (PPTC) “resettable fuses” are used in a wide variety of circuit protection applications. Under fault conditions the device resistance will rise exponentially and remain in a “tripped” state, providing continuous circuit protection until the fault is removed. Once the fault is removed and the power cycled, the device will return to its normal low resistance state.
How Bourns® Multifuse® products work
Resettable fuses are manufactured in the form of a conductive plastic, which is comprised of a nonconductive crystalline polymer with highly conductive carbon black particles impregnated throughout the crystal lattice. Because of the close proximity of the carbon black particles within the crystal lattice, under normal conditions current is allowed to flow easily through the conductive plastic. However under a fault condition, when there is an increase in current, the conductive plastic heats at the rate of I2R. As the material continues to heat, it eventually reaches the phase transformation temperature, which changes the crystal structure into an amorphous structure. Once the material has transformed into this amorphous structure, the conductive particles become isolated and are unable to conduct current hence the drastic change in material resistance. It is only when the current is removed that the material is allowed to cool and return to its original crystal structure.
It is the materials used in resettable fuses that allow them to reset after a fault condition has been removed. Resettable fuses exhibit a positive temperature coefficient effect when heated. While many materials exhibit a PTC effect when heated (an increase in resistance in response to a positive change in temperature), what makes the material used in resettable fuses unique is the fact that the increase in resistance changes exponentially rather than in a linear manner.
It is because of this transformation from a low resistance state to a high resistance state that allows the resettable fuse to protect loads. It is this transition from the low resistance state to high resistance state that is referred to as tripping. The time it takes for a resettable fuse to trip is relatively quick, depending on how high the fault current is and it can be as quick as a fraction of a second.
Typical application circuit
Bourns® Multifuse® PPTC resettable fuses are an excellent form of protection in most applications where sensitive devices need extra protection. Some typical applications where Multifuse® products are already successfully employed include data processing equipment, automotive, telecommunication, industrial, consumer and battery protection circuits.
One of the key application areas for Bourns® Multifuse® products is the protection of sensitive battery packs and cells, in particular those using rechargeable Lithium technologies. A wide range of Multifuse® PPTC resettable fuse devices are available to meet the needs of Li-ion battery protection. Bourns has been designing its PPTCs for battery cells for well over 10 years and the range today covers the traditional axial leaded strap, the bare disk type devices for cylindrical cells (e.g., AA & AAA) and the latest low resistance, small sized MF-LL series using ultra-high conductive fillers.
The traditional axial leaded straps are available in the MF-SVS, MF-VS, MF-LR, MF-LS, and MF-S families. These are used in a wide range of battery applications such as mobile devices, motors, power tools and electric vehicles. They range in operating currents up to 9 A and operating voltages as high as 24 V. The MF-D series resettable fuses are customized disk PPTCs. They are made from a PTC plaque material and can be customized in shape, dimension, resistance and operating current for specific battery cell applications.
Bourns’ newest addition to battery pack applications is the MF-LL Series. This family is unique as it is the first in the Bourns® PTC range not to use carbon as the conductive element. By using metallic conductive fillers, the device can be made smaller, holds higher current levels and most importantly, offers resistance values in the single digit milliohm range.
Click here to download the Bourns Battery Pack White Paper which addresses Li-ion battery pack and battery cell reliability threats.
Anglia are offering customers a FREE Multifuse® sample kit from Bourns, please fill in the form below to register your interest now.
FREE samples for UK and Ireland customers only, subject to availability.
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