Electronics > Projects, Designs, and Technical Stuff

Fuse selection (blow times)

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Simon:

--- Quote from: floobydust on August 23, 2019, 07:59:10 pm ---For a 1206 package, I'm skeptical it's practical as the contact surface area isn't there for steady 20-40A. You need heavy copper PCB but a tiny 1206 pad is just too small for long term high currents and over 1W dissipated.



--- End quote ---

I'm thinking 210µm, it's only a dollar more than anything else.

bdunham7:

--- Quote from: Simon on August 23, 2019, 07:25:16 pm ---The motor drive looks after itself. If you put your hand on the hub even at low speed (you don't do that at 4'700rpm) it will cut out. The only thing i have had so far which is the reason for rethinking fuse strategy is that i had fans running with one fan unplugged for some testing. that unplugged fan was being driven backwards. When i plugeed it in it blew up but did not take the fuse out. But the last fuse was PCB track. i think i will bow my head to the fuse makers and admit they know more than me.

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Would overcurrent protection have prevented the fan from blowing up?  If so, in this event, whether an intermittent connection or an unplugging during service, which would be preferable--a blown up fan or a irreparably damaged power distribution board?

If, as you say elsewhere, the wiring is adequately protected otherwise, and you don't really want to protect anything or do anything except meet your customer's demand, I suppose the answer is the least likely fuse to ever blow, in this case the slo-blo model.

Simon:
why I did was not representative of real use and could never happen. If the fan was damaged it would have been blown mosfets, not sure any fuse can deal with that. My issue with too slow a blow is that the epoxy starts to melt....

bdunham7:

--- Quote from: Simon on August 23, 2019, 09:52:54 pm ---why I did was not representative of real use and could never happen. If the fan was damaged it would have been blown mosfets, not sure any fuse can deal with that. My issue with too slow a blow is that the epoxy starts to melt....

--- End quote ---

That's good though--it shows that it didn't give up without trying!  :)

floobydust:

--- Quote from: Simon on August 23, 2019, 07:05:05 pm ---As i already said it is to satisfy compliance and the loads are unlikely to blow a fuse but the customer wants one. So a fuse they get that will never be needed but it's there.

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No, "the customer is always right" does not 100% apply to engineering. The customer does not know the safety standards, or have experience doing product development. Blindly following what they want can be a noose around the engineer's neck. You have to explain and negotiate the product's requirements with a customer, sales/marketing, upper management etc. Sometimes you have to firmly say "no" to their demands because otherwise you end up making something unsafe or unreliable.

If you use non-replaceable fuses, your product is only as good as the fans.
A lemon fan- mosfets that short, bearings failing, water getting in, plastic blades cracking and flying off- mean overcurrent and your fuse pops. A guy puts a new module in and it also pops a fuse. Hmmm. I have seen this, and by time they figure the (fan) load is the problem, there is a box of dud modules needing costly repairs at no fault of the module.
I would also question say 4 of 20A fans and 80A feed for a board, it's not practical due to the heat generated by connections, pcb traces, and components. I had a hard time designing for high ambient temperatures with high currents and potting. I don't know your details but be careful you don't make a silly product.  Something that fails a lot can sink a company.
Possibly an electronic fuse by monitoring load current, and MCU with trouble codes would be better.

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