Author Topic: Fuse selection (blow times)  (Read 2897 times)

0 Members and 1 Guest are viewing this topic.

Offline SimonTopic starter

  • Global Moderator
  • *****
  • Posts: 18118
  • Country: gb
  • Did that just blow up? No? might work after all !!
    • Simon's Electronics
Fuse selection (blow times)
« 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.
 

Offline OM222O

  • Frequent Contributor
  • **
  • Posts: 768
  • Country: gb
Re: Fuse selection (blow times)
« Reply #1 on: August 23, 2019, 08:24:22 am »
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.
 

Offline SimonTopic starter

  • Global Moderator
  • *****
  • Posts: 18118
  • Country: gb
  • Did that just blow up? No? might work after all !!
    • Simon's Electronics
Re: Fuse selection (blow times)
« Reply #2 on: August 23, 2019, 08:26:29 am »
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.
 

Offline OM222O

  • Frequent Contributor
  • **
  • Posts: 768
  • Country: gb
Re: Fuse selection (blow times)
« Reply #3 on: August 23, 2019, 09:32:30 am »
you didn't mention any current specs  at first :-/O

I am designing a fuse board. A PCB with one connection in and several out individually fused. This is automotive for brushless fans

most 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 engines  :-DD

anyhow, 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?
« Last Edit: August 23, 2019, 09:34:33 am by OM222O »
 

Offline SimonTopic starter

  • Global Moderator
  • *****
  • Posts: 18118
  • Country: gb
  • Did that just blow up? No? might work after all !!
    • Simon's Electronics
Re: Fuse selection (blow times)
« Reply #4 on: August 23, 2019, 09:35:30 am »
you didn't mention any current specs  at first :-/O

most 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 engines  :-DD


Oh well, I guess I must be crazy using several 30+A fans :palm:
« Last Edit: August 23, 2019, 12:37:19 pm by Simon »
 

Offline Whales

  • Super Contributor
  • ***
  • Posts: 2121
  • Country: au
    • Halestrom
Re: Fuse selection (blow times)
« Reply #5 on: August 23, 2019, 11:05:31 am »
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.




Offline SimonTopic starter

  • Global Moderator
  • *****
  • Posts: 18118
  • Country: gb
  • Did that just blow up? No? might work after all !!
    • Simon's Electronics
Re: Fuse selection (blow times)
« Reply #6 on: August 23, 2019, 12:36:54 pm »
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.


Hm, 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.
 

Offline Nauris

  • Regular Contributor
  • *
  • Posts: 188
  • Country: fi
Re: Fuse selection (blow times)
« Reply #7 on: August 23, 2019, 02:18:45 pm »
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).
Like
https://eu.mouser.com/datasheet/2/240/littelfuse-pdm-hwa20-D658-info-sheet-779849.pdf
 

Offline SimonTopic starter

  • Global Moderator
  • *****
  • Posts: 18118
  • Country: gb
  • Did that just blow up? No? might work after all !!
    • Simon's Electronics
Re: Fuse selection (blow times)
« Reply #8 on: August 23, 2019, 03:48:33 pm »
Possibly, space is quite restricted. The fans don't generally fail a fuse so it's really just for compliance.
 

Offline ejeffrey

  • Super Contributor
  • ***
  • Posts: 4033
  • Country: us
Re: Fuse selection (blow times)
« Reply #9 on: August 23, 2019, 04:17:43 pm »
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.

 

Offline SimonTopic starter

  • Global Moderator
  • *****
  • Posts: 18118
  • Country: gb
  • Did that just blow up? No? might work after all !!
    • Simon's Electronics
Re: Fuse selection (blow times)
« Reply #10 on: August 23, 2019, 04:48:19 pm »
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.
 

Offline ejeffrey

  • Super Contributor
  • ***
  • Posts: 4033
  • Country: us
Re: Fuse selection (blow times)
« Reply #11 on: August 23, 2019, 06:26:07 pm »
But can you just use wiring that can handle the full circuit capacity?  Or is the added cost and weight of the copper prohibitive?
 

Offline SimonTopic starter

  • Global Moderator
  • *****
  • Posts: 18118
  • Country: gb
  • Did that just blow up? No? might work after all !!
    • Simon's Electronics
Re: Fuse selection (blow times)
« Reply #12 on: August 23, 2019, 06:39:13 pm »
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.
 

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 22436
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: Fuse selection (blow times)
« Reply #13 on: August 23, 2019, 06:52:38 pm »
Slow blow is mainly if you need to protect the wiring while allowing for loads with large inrush.

Use the regular fast fuse.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline SimonTopic starter

  • Global Moderator
  • *****
  • Posts: 18118
  • Country: gb
  • Did that just blow up? No? might work after all !!
    • Simon's Electronics
Re: Fuse selection (blow times)
« Reply #14 on: August 23, 2019, 06:56:01 pm »
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.
 

Online Gyro

  • Super Contributor
  • ***
  • Posts: 10173
  • Country: gb
Re: Fuse selection (blow times)
« Reply #15 on: August 23, 2019, 07:01:22 pm »
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.
Best Regards, Chris
 

Offline SimonTopic starter

  • Global Moderator
  • *****
  • Posts: 18118
  • Country: gb
  • Did that just blow up? No? might work after all !!
    • Simon's Electronics
Re: Fuse selection (blow times)
« Reply #16 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.
 

Offline bdunham7

  • Super Contributor
  • ***
  • Posts: 8175
  • Country: us
Re: Fuse selection (blow times)
« Reply #17 on: August 23, 2019, 07:11:02 pm »
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.

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.
« Last Edit: August 23, 2019, 07:12:49 pm by bdunham7 »
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Online Gyro

  • Super Contributor
  • ***
  • Posts: 10173
  • Country: gb
Re: Fuse selection (blow times)
« Reply #18 on: August 23, 2019, 07:16:50 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.

A potted module with fuses inside?  :o

That sounds like a long term repeat order money-maker!
« Last Edit: August 23, 2019, 07:18:28 pm by Gyro »
Best Regards, Chris
 

Offline floobydust

  • Super Contributor
  • ***
  • Posts: 7678
  • Country: ca
Re: Fuse selection (blow times)
« Reply #19 on: August 23, 2019, 07:18:53 pm »
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.
 

Offline SimonTopic starter

  • Global Moderator
  • *****
  • Posts: 18118
  • Country: gb
  • Did that just blow up? No? might work after all !!
    • Simon's Electronics
Re: Fuse selection (blow times)
« Reply #20 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.
 

Offline SimonTopic starter

  • Global Moderator
  • *****
  • Posts: 18118
  • Country: gb
  • Did that just blow up? No? might work after all !!
    • Simon's Electronics
Re: Fuse selection (blow times)
« Reply #21 on: August 23, 2019, 07:26:23 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.

A potted module with fuses inside?  :o

That sounds like a long term repeat order money-maker!

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.
 

Offline SimonTopic starter

  • Global Moderator
  • *****
  • Posts: 18118
  • Country: gb
  • Did that just blow up? No? might work after all !!
    • Simon's Electronics
Re: Fuse selection (blow times)
« Reply #22 on: August 23, 2019, 07:36:43 pm »
Quote
Are you sure this is possible, SMT at 40A?


Yes you can, in 1206 size for the 5s timeout but the 60s ones are a lot larger.

Quote

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.

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.
 

Offline floobydust

  • Super Contributor
  • ***
  • Posts: 7678
  • Country: ca
Re: Fuse selection (blow times)
« Reply #23 on: August 23, 2019, 07:59:10 pm »
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.

 

Offline SimonTopic starter

  • Global Moderator
  • *****
  • Posts: 18118
  • Country: gb
  • Did that just blow up? No? might work after all !!
    • Simon's Electronics
Re: Fuse selection (blow times)
« Reply #24 on: August 23, 2019, 08:01:49 pm »
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.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf