Electronics > Projects, Designs, and Technical Stuff
Fuse selection (blow times)
Simon:
--- Quote from: floobydust on August 24, 2019, 08:11:06 pm ---
--- 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.
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Lets leave the customer politics out of it. I am replacing an existing item that has already been use with satisfaction although clearly it has not yet had to operate.
--- Quote ---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.
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Correct. So as already stated the fans are very reliable. They will shut down automatically on any overload and do soft starting. I have never seen any fail. The fans are IP68 so water is not an issue in creating any electrical problems. You also cannot simply replace the fuse module, it's too burried. The fans are rated for 40'000 hours at full speed and these are speed controlled down so would potentially last longer.
--- Quote ---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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Yes indeed, currently 2 layers of 70µm are used. I am looking to use 210µm. The problem is that I will also have my MD comparing my new cost to the old cost. I think i will also consider 0.8mm PCB to help move the heat to the second layer and make vias shorter and i am considering having some pins soldered in a pad to take the corrent through.
No electronic fusing is not possible for the same reason the fan mosfets can blow before the fuse does so can electronic protection blow with ease. You have to have mechanical circuit breaking somewhere.
floobydust:
I've learned to look at the whole picture, anything off in the original requirements can make things end badly. I trust the fan's design will never change and they can withstand automotive -ve overvoltage transients without failure...
I think heat will be your largest hassle, as everything wants to make a watt or so of heat. If you need to run in elevated ambients say over 70C then it is very difficult to keep it small and cheap. Slow-blow fuses have high resistance and will operate hot.
People I know in the automotive industry are (low volume) using aluminum Mcore pc boards for heat sinking. High production volumes like ECU's using thin FR-4 bonded to the die-cast metal enclosure. There are many high-tech thermal adhesives. It can be cheaper to spend more bom dollars on lower RDS-on mosfets (=cooler) to save more on the enclosure and assembly labour.
Silicone rubbers I find are terrible for cooling, like wrapping your board in a blanket. You lose all convection cooling and the encapsulant acts as an insulator. Hard epoxies are better at conduction heat transfer. You can also use the product's flying leads as heatsinks out the wires.
Most all fuses are characterized and rated in free-air, and when potted their properties change significantly. The potting compound provides conduction-cooling so the fuse's trip current becomes much higher with longer trip times. The curves are no longer valid after potting. Some fuse end caps/housings have air gaps and encapsulant can seep in to the element and make the fuse never really trip. The engineers never notice, but regulatory approval agencies that are on the ball will refuse. Fuses that are already epoxy encapsulated have an internal air space, potting compounds cannot get inside so their characteristics do not change and they are suitable for potting. Littelfuse engineering is very helpful if you have any questions.
Small SMT fuses usually have like a solder mask over top, so potting does not get inside but does hamper heating of the element, and the trip curves are shifted. You can see construction under a microscope, the element is on ceramic substrate.
Your fuses can only protect the wiring and pcb against fire and melting, so seemingly high trip currents I think are fine. The hold current must not be too high so an unhealthy fan drawing 50% more current is not allowed to keep going.
If you have an MCU you can monitor the output voltages and know if a fuse has tripped and blink an LED or something. It's just a few extra resistors. I found cooling fans are almost mission critical and the system (engine, pumps etc.) overheats so a fault or alarm was helpful.
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