EYE on NPI – Bourns MF-LSMF Series High-Power PPTC Resettable Fuses #EyeOnNPI @Bourns @digikey @adafruit

This week’s EYE ON NPI (video) is a fusion of form and functionality – it’s the MF-LSMF Series High-Power PPTC Resettable Fuse collection from Bourns!

These are super-fuses, made of a polymeric material that allows them to be small and thin, but pass large amounts of current, at high bias voltages.

With current ranges up to 6A hold (12A trip!) and 33 VDC max voltage, these fuses are burly enough to be used in large LED or motor-driving projects that normally would need a more expensive PMIC or eFuse or a non-resettable glass/ceramic fuse.

PPTC stands for polymetric positive temperature coefficient. PTC means that as the temperature rises around the resistor, the resistance also rises. There are linear kinds of PTC’s that can be used for temperature measurement (silistors) but these are ones that have a non-linear rise (switching type). The resistance actually starts dipping slowly and then shoots up after reaching a certain, adjustable, temperature called the Curie point.

When current starts flowing through a PTC, the resistance dips a little, and its quite low to start, usually in the milli-ohm range. Once the current increases a certain amount, the wattage through the PTC’s resistance (P = V*I = R*I^2) sets the dissipating heat, the temperature rises, and the Curie point is reached. Then the resistance shoots up to ‘open’ the circuit, essentially disconnecting the two sides of the circuit since the resistance is in the 10’s or 100’s of Kohms. However, now the current has dropped and so the power dissipation goes down to zero. After a few seconds or minutes, the material cools down past the Curie point and the resistance allows current through again. (This is why, if your USB port or random electronics product stops working, it does work to unplug it, wait a few minutes, and then try again.)

Compare this with classic ‘wire’ fuses with ceramic or glass or thermoplastic bodies, and a thin wire inside. When too much current goes through the fuse wire, it gets hotter and hotter and eventually melts, cutting off the load from the source voltage. But it doesn’t self ‘heal’ – it’s a one time thing only! Bourns has adjusted the Curie point on each fuse to match a different voltage and current limit, and they can be trusted to quickly trip when the current goes over the rated limitation.

PPTC’s are inexpensive, easy-to-use, and add quick protection to your products – you may have seen them on many of our own designs such as the Adafruit Metro which as a simple 5V 500mA one. Now with these wide current and voltage ranges from Bourns, there’s no reason not to add one!

A wide range of Bourns MF-LSMF Series High-Power PPTC Resettable Fuses are in stock now at Digi-Key for immediate shipment. Order today and you can keep your high-power design safe and sound by tomorrow afternoon.


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