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| Polyfuse 8MILLI-ohms post-trip? am I missing something? |
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| SiliconWizard:
--- Quote from: james_s on September 28, 2019, 10:36:09 pm ---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? --- End quote --- Because that's how they are supposed to be used. Don't you prefer a component that has minimal series resistance in normal conditions, and degrade a bit when shit happens (which should be a rare occurence unless your design is fucked up). Besides, they will not really stabilize to a given ESR. Every time they trip, they will tend to degrade a bit more. |
| Siwastaja:
--- Quote from: SiliconWizard on September 29, 2019, 04:09:47 pm ---Besides, they will not really stabilize to a given ESR. Every time they trip, they will tend to degrade a bit more. --- End quote --- This. If they did stabilize after one or two trips, pre-conditioning them would make sense. Use polyfuses only if you expect handful (tens, not thousands) of trips, and design for a large leeway in resistance (much more than R1). If you can't make it work, then don't use a polyfuse; design an active electronic efuse circuit (ICs do exist), protected by a classical single blow fuse (which only blows if the efuse circuit fails). |
| T3sl4co1l:
Yeah, that's the other thing about polyfuses: the self-resetting is a convenience, not a guarantee. They are good for ~hundreds of cycles and are supposed to fail (permanently) open. Tim |
| ejeffrey:
One more thing: surface mount ones are considerably worse than through hole in most parameters. I haven't paid much attention to the R R1 spread, but the hold to trip ratio in particular is much worse for surface mount devices. |
| T3sl4co1l:
THT also more consistent on current rating, since less dependence on trace/plane heatsinking. But way slower, since they're usually much bigger (and amps+). Tim |
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