Fuse selection (blow times) - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: August 23, 2019, 07:38:52 am
content: 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.

author: OM222O
author_rank: Frequent Contributor
timestamp: August 23, 2019, 08:24:22 am
content: Have you considered using a PTC fuse + fast blow?if the PTC doesn't manage to reduce / stabilize the current within 5s, that means there is definitely something wrong.generally a fast blow on it's own should be good enough though.

author: Simon
author_rank: Unknown
timestamp: August 23, 2019, 08:26:29 am
content: have you ever tried to find 40A SMD PTC? I gave up, they are not popular and expensive. The brushless fans look after themselves so really this is just to satisfy the project requirements.

author: OM222O
author_rank: Frequent Contributor
timestamp: August 23, 2019, 09:32:30 am
content: you didn't mention any current specs at firstQuote from: Simon on August 23, 2019, 07:38:52 ammost car batteries are rated for upto 200A max ... you can't have "several fans" each drawing 40As if you also want the car itself to do anything ... also I'm curious what kind of automotive fan draws 40A? that's more inline with electric jet enginesanyhow, if they have built in protection and you just want to satisfy the project requirements then it should be obvious ... just go for the 60s ones since it doesn't matter?

author: Simon
author_rank: Unknown
timestamp: August 23, 2019, 09:35:30 am
content: Quote from: OM222O on August 23, 2019, 09:32:30 amOh well, I guess I must be crazy using several 30+A fans

author: Whales
author_rank: Super Contributor
timestamp: August 23, 2019, 11:05:31 am
content: If this is a one off: buy both sets of fuses and try, starting with the fast ones.Middle-ground: Profile the fans' current draw. Stall them, stall them after they have been running, heat their driver to 100degC. If it's very well behaved and limited then the 5s blow is probably fine.The proper way to do it: do a statistical analysis weighing up false-positive blows (ie should not have blown) and true-negatives (fire started, fuse didn't blow). You'll need to investigate failure modes for this to make sense.

author: Simon
author_rank: Unknown
timestamp: August 23, 2019, 12:36:54 pm
content: Quote from: Whales on August 23, 2019, 11:05:31 amHm, no this is production. Unfortunately time is limited and i can't easily simulate the faults. I can't just stop a fan at 4'700rpm. Ultimately we are looking at protecting the wiring here.

author: Nauris
author_rank: Regular Contributor
timestamp: August 23, 2019, 02:18:45 pm
content: If it is automotive, what about proper fuse box with standard size fuses like is normally done? Soldered in smd fuses in some custom pcb makes for angry servicemen (and customers).Likehttps://eu.mouser.com/datasheet/2/240/littelfuse-pdm-hwa20-D658-info-sheet-779849.pdf

author: Simon
author_rank: Unknown
timestamp: August 23, 2019, 03:48:33 pm
content: Possibly, space is quite restricted. The fans don't generally fail a fuse so it's really just for compliance.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 23, 2019, 04:17:43 pm
content: In a production automotive environment I would expect the fan to be fused at the main fuse panel with standard automotive fuses. It should only require a separate fuse if you are connecting a low load device to a much higher rated circuit.If you really must use an SMD fuse use one that is well oversized and slow blow. 60 s at 200% rating sounds fine. A real short circuit will be well over 200% current, and the wiring should easily be able to handle that sort of load unless it is criminally undersized.Don't use a PTC in a car, especially not in an engine bay. At least for the moderately high current (5-10 A and above) the temperature derating curves are really steep. You could easily end up with something having a 6:1 ratio of worst case hold to worst case trip current. And the last thing you want is your cooling fans false tripping the PTC on a hot day while idling.

author: Simon
author_rank: Unknown
timestamp: August 23, 2019, 04:48:19 pm
content: Yes, ultimately it's to protect for a dead short circuit. And yes i am going prom a larger supply to multiple fans so refusing undividually.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 23, 2019, 06:26:07 pm
content: But can you just use wiring that can handle the full circuit capacity? Or is the added cost and weight of the copper prohibitive?

author: Simon
author_rank: Unknown
timestamp: August 23, 2019, 06:39:13 pm
content: No, it's feeling like an OCD thing. the wire that supplies all fans will be too big to go to any one fan. In I tend to use more individual wires than one bit one. So that is where a PCB helps because the main power comes right in on a power stud on the PCB and then the PCB splits it to the pins of a connector that supplies each fan.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 23, 2019, 06:52:38 pm
content: Slow blow is mainly if you need to protect the wiring while allowing for loads with large inrush.Use the regular fast fuse.Tim

author: Simon
author_rank: Unknown
timestamp: August 23, 2019, 06:56:01 pm
content: well I guess if any capacitance charges in under 5 seconds without too much inrush a 5s time will be enough. I guess i can do that test. Just set up a relay on a battery to cycle.

author: Gyro
author_rank: Super Contributor
timestamp: August 23, 2019, 07:01:22 pm
content: 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.

author: Simon
author_rank: Unknown
timestamp: August 23, 2019, 07:05:05 pm
content: 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.

author: bdunham7
author_rank: Super Contributor
timestamp: August 23, 2019, 07:11:02 pm
content: Quote from: Simon on August 23, 2019, 07:38:52 amIs 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.

author: Gyro
author_rank: Super Contributor
timestamp: August 23, 2019, 07:16:50 pm
content: Quote from: Simon on August 23, 2019, 07:05:05 pmA potted module with fuses inside?That sounds like a long term repeat order money-maker!

author: floobydust
author_rank: Super Contributor
timestamp: August 23, 2019, 07:18:53 pm
content: Are you sure this is possible, SMT at 40A?I worked with hydraulic power units that used severalbrushed12V 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 fuseshave significant resistance and will also make heat, several watts. ExampleMINI blade fuse 30Agenerates 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.

author: Simon
author_rank: Unknown
timestamp: August 23, 2019, 07:25:16 pm
content: 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.

author: Simon
author_rank: Unknown
timestamp: August 23, 2019, 07:26:23 pm
content: Quote from: Gyro on August 23, 2019, 07:16:50 pmAs 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.

author: Simon
author_rank: Unknown
timestamp: August 23, 2019, 07:36:43 pm
content: QuoteYes you can, in 1206 size for the 5s timeout but the 60s ones are a lot larger.QuoteNo 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.

author: floobydust
author_rank: Super Contributor
timestamp: August 23, 2019, 07:59:10 pm
content: 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.

author: Simon
author_rank: Unknown
timestamp: August 23, 2019, 08:01:49 pm
content: 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.

author: Simon
author_rank: Unknown
timestamp: August 23, 2019, 08:16:15 pm
content: Quote from: floobydust on August 23, 2019, 07:59:10 pmI'm thinking 210µm, it's only a dollar more than anything else.

author: bdunham7
author_rank: Super Contributor
timestamp: August 23, 2019, 08:52:29 pm
content: Quote from: Simon on August 23, 2019, 07:25:16 pmWould 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.

author: Simon
author_rank: Unknown
timestamp: August 23, 2019, 09:52:54 pm
content: 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....

author: bdunham7
author_rank: Super Contributor
timestamp: August 24, 2019, 04:26:02 pm
content: Quote from: Simon on August 23, 2019, 09:52:54 pmThat's good though--it shows that it didn't give up without trying!

author: floobydust
author_rank: Super Contributor
timestamp: August 24, 2019, 08:11:06 pm
content: Quote from: Simon on August 23, 2019, 07:05:05 pmNo, "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.

author: Simon
author_rank: Unknown
timestamp: August 25, 2019, 08:26:56 am
content: Quote from: floobydust on August 24, 2019, 08:11:06 pmLets 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.QuoteCorrect. 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.QuoteYes 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.

author: floobydust
author_rank: Super Contributor
timestamp: August 25, 2019, 04:08:50 pm
content: 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.