| Electronics > Projects, Designs, and Technical Stuff |
| Polyfuse 8MILLI-ohms post-trip? am I missing something? |
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| pilotplater:
I'm working on an extremely simple PCB to just add some logic-level shifting to an ESP8266 board before sending the signal out to a strip of 5v 'nixie' style LEDs (I know not always required, but I want to cover all bases) I thought maybe I could be smart to add a polyfuse on the main power input, never know when you'll come a gutsa with a short or power supply fault etc. My question has to do with this digikey listing: https://www.digikey.ca/product-detail/en/littelfuse-inc/NANOSMD500LR-2/NANOSMD500LR-2CT-ND/7321653 it seems to suggest that the pre-trip resistance is 2 milli-ohms, and the post-trip resistance is 8......milli-ohms? mega-ohms? I can't imagine Digikey would be so vauge in their listing to have milli-ohms and mega-ohms noted as the same text, but also why would a polyfuse be designed to 'trip' to 0.008 ohms? Seems like a too crazy niche application... Not sure what I'm missing, is this field even relevant? Cheers Andy |
| maginnovision:
What that means is after it trips the inherent resistance goes up. So it starts at least 2 milliohm across the fuse, if it trips it'll be up to 8 milliohms. |
| RoGeorge:
A polifuse "cuts" the current when an over-current situation occurs, then after the situation has restore to normal, the continuity of the fuse is restored. After a trip, the circuit restore its continuity by itself, it does not remain open like a normal fuse. When new, the polifuse has 2 miliohms, when an over-current occurs it almost break the circuit, then, if the over-current situation disappears the polifuse "regenerates" itself, but not as good as brand new, 8 miliohm instead of 2 miliohms. Mega is always written with capital 'M' mili is always written with lower 'm' |
| Zero999:
Yes, as mentioned above, after the first time it's tripped, the resistance is higher than before. This also means it will be more sensitive to over current and will trip at a slightly lower current, then next time. Polyfuses or PTC (Positive Temperate Coefficient) resistors to use the correct term, don't have an infinite life. After multiple trips there may come a point when the normal operating current will cause it to trip, at which point it will need replacing. This is why it's important to not use too lower tripping current rating. |
| james_s:
That's interesting, I never realized that. Why don't they pre-trip them prior to shipment so that they're consistent throughout their useful life? |
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