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Fuse selection (blow times)

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Gyro:
Yes, good idea. You need to watch out for fuse element fatigue due to cycling over its lifetime. A cycling relay test should hopefully accelerate any potential failures.

Simon:
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.

bdunham7:

--- Quote from: Simon on August 23, 2019, 07:38:52 am ---I am designing a fuse board. A PCB with one connection in and several out individually fused. This is automotive for brushless fans so no large inrush currents. But I am uncertain about the time to blow I should choose.

I seem to have a choice of 5 seconds with 200% current and 60 seconds with 200% current. Unfortunately SMD fuses do not seem to follow any standards in case size so I can't pick one later.

So is 5s too fast? but isn't 60s a bit long? I know that my safer bet is 60s as they will never blow accidentally.

--- End quote ---

Is there any condition that would result in an overcurrent that might correct itself?  Is there no other circuit fuse? I'm thinking of the motor starting against an excessive load, like a water spray, that doesn't persist?  Or is the drive circuitry such that the current is limited unless there is a gross failure?  And how much overload can your supply and  wiring take?  Since it appears that this is a one-time fuse that would not be serviceable, I'm thinking the correct design would be to have other inherent current limiting with a faster fuse.  FWIW, I have many years of automotive experience and have never seen slow-blow fuses in an application like this, and only rarely have I seen fuses blow where there wasn't a definite problem with the fan motor or wiring.  The main exception was a particular design that allowed water, or even ice, to build up in the bottom of the fan housing that would blow the fuse if the air conditioning compressor and condenser fan were activated, which would happen if the truck was started and defrost selected.  The fix was to drill some holes and replace the fuse.

Gyro:

--- 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!

floobydust:
Are you sure this is possible, SMT at 40A?

I worked with hydraulic power units that used several brushed 12V cooling fans.
We had problems with connectors and wiring heating up, as the brushed motors seemed to draw more and more current with age.
We had problems with DIN rail terminal blocks and fusing heating up, due to the high currents. Each fan was around 15A going up past 20A. It really needed to be a 24V system.

Automotive blade fuses have significant resistance and will also make heat, several watts. Example MINI blade fuse 30A generates 1.2W at 20A. This, and the socket make a hot spot on your PCB.
Car stereo amplifiers are using two fuses in parallel, to keep costs and size down.

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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