Electronics > Projects, Designs, and Technical Stuff
Fuse selection (blow times)
Simon:
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.
Simon:
--- Quote from: Gyro on August 23, 2019, 07:16:50 pm ---
--- Quote from: Simon on August 23, 2019, 07:05:05 pm ---that is what i am thinking, I can do the prototype with the 5s fuses and test in the mean time before production starts. If i find a problem i can switch to the 60s ones. The other factor being that i will pot the PCB so i don't know if that will alter the fuse characteristic, it may well do the 5s ones as they are 1206, the slow blow ones are 13mm long I guess to put some thermal inertia in for the delay but I am potentially doing that to my 5s ones anyway.
--- End quote ---
A potted module with fuses inside? :o
That sounds like a long term repeat order money-maker!
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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.
Simon:
--- Quote ---Are you sure this is possible, SMT at 40A?
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Yes you can, in 1206 size for the 5s timeout but the 60s ones are a lot larger.
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You have to size the wire and connectors carefully. If there was significant voltage drop, fan current was higher. A fan motor would eventually short-circuit when the brushes failed.
I would say a brushless fan has inrush current until it is up to speed, but automotive fuse are very very slow blow anyway.
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No brushless fans don't have inrush current other than what charges their capacitors. Brushless fans speed up slowly and don't exceed the maximum current draw. This is one of their selling points. so if there is less than 1000µF in them a 5s fuse may well be fine.
I just investigated a brushed fan inrust current and yes basically 10-20x running current for 200ms. The inrush is due to the motor being stationary with no back EMF being generated so the full supply goes across the resistance of the wires until the rotor starts to spin.
floobydust:
I would insist on the fuse being upstream, as it has to exist anyway for the wiring/circuit feeding your board. That could be the (high current) protective element.
Having the customer do a board swap if there are any problems, fan motor fails (mosfets, water, mech) or wiring, and your product gets the blame and comes in under warranty claim. Especially if potted, it goes in the garbage.
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.
Simon:
There will be a main fuse, but each fan spur needs fusing as the wire guage reduces. It is highly pedantic as the wiring is more than protected but the custemer is always right.
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