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Discussion in 'General Electronics Discussion' started by doliver, Jul 25, 2014.

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  1. doliver

    doliver

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    Jul 25, 2014
    I am building a circuit using two lithium batteries in series that pulls up to 18 amps and don't want to change the fuse every time it blows. I tried two 9 amp ptc resettable fuses in parallel the issue I'm having is the 5 seconds it takes to trip heats up the battery contacts red hot and starts melting the plastic. I cant find any ptc fuses that trip in less that 5 seconds. Does anyone know where to find such a fuse or of another solution.
     
  2. Gryd3

    Gryd3

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    Jun 25, 2014
    Do you have any indication of the amount of current that is being drawn that causes your contacts to turn red hot?
     
  3. doliver

    doliver

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    Jul 25, 2014
    I shorted the fuses across a spare battery to test it. the batteries are rated for a max discharge of 30 amps not shure how much it can actually discharge.
     
  4. doliver

    doliver

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    Jul 25, 2014
    Should have mentioned the fuse hold current is 9 amp the trip current is 15.3 and the max current is 100 a it's a 16v fuse and it's rated trip time is 5.5 seconds which seems fairly acurate
     
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Hi and welcome to Electronics Point :)

    Probably one or more big-a$$ MOSFETs to operate as a fuse. Try this search: http://www.google.com/search?q=electronic+fuse+mosfet&tbm=isch

    MOSFETs are available with current sensing in the source terminal, or you could just monitor the voltage across it. N-channel MOSFETs have the best performance; this means either you need to intercept the negative from the battery, or you need a circuit to generate the gate bias.

    Have you worked with MOSFETs before?
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    If battery terminals glow red hot in normal use then you have a problem. At the very least you're wasting energy. What are you actually trying to do?
     
  7. doliver

    doliver

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    Jul 25, 2014
    I have only worked with mos fets a small amount getting a part number off a schematic. I would like the fuse to be on the negative side in case there is a short between the two batteries and the case wich is hooked to negative . The termanils do not get hot in normal use only when I had a short and the fuse I was using took a full 5 seconds to trip. I appreciate the help and quick responses
     
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    What are the minimum, maximum, and typical voltages of the battery stack?
     
  9. doliver

    doliver

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    Jul 25, 2014
    They are 3.6 volt lithium fully charged 8.4 no more than 8.5 I drain them down to minimum of 6.6 so I believe typical is 7.2. They directly power a step down regulator the negative output of my load goes to the negative of the battery my minimum regulator output Is 2.8v and max is 5.6v.
     
  10. doliver

    doliver

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    Jul 25, 2014
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    That is designed to protect itself rather than the battery, but I guess it might work. Note that many of these protected devices do not have an exact current limit.

    However, the main thing you need to do is to stop your circuit demanding too much current. I'm still not sure why it does. If it blows fuses on a regular basis then it is time to fix the fault, not find something easier than replacing fuses.
     
  12. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    I'm working on a circuit. May be a few days - I have some RL commitments ATM
     
  13. doliver

    doliver

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    Jul 25, 2014
    I agree with fixing the cause of the problem and have done my best to do so by replacing the battery tray with a lower profile one less likely to make contact with the lid. I would still like it to reset itself non the less accidents do happen and once all the wiring is covered I'd prefer to never have to tear into it again. Thanks again for the help
     
  14. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    If that's the only problem, just put some insulation on the inside of the lid. A sheet of plastic from a shirt box, a piece of fibreglass sheet like those used in microwave ovens, or something like that. That would be a lot quicker, simpler and cheaper than building up a whole circuit!
     
  15. doliver

    doliver

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    Jul 25, 2014
    Sounds good in theory but the lid has a lip that fits snugly in the case any added thichness would stop it from fitting properly. I also need the added protection in case I overload the regulator. Although unlikely the thaught of flaming batteries makes me uncomfortabland I coudl use the peace of mind.
     
  16. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    If the lid is making contact with the battery terminals, the only way to protect the battery is to put the fuse INSIDE it! I think you're approaching this problem from the wrong direction.
     
  17. doliver

    doliver

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    Jul 25, 2014
    I'm sorry but your misunderstanding me the lid made contact with the battery terminal one time. At this time I replaced the battery trey with a low profile one and the lid no longer makes contact. The smoking battery from the one time it did make contact did however scare me so I baught resettable fuses. I tested them outside the case with the old battery tray and intentionally shorted the fuse across the battery in which it took 5 seconds made the springs glow and the battery very hot. There is no reacouring issue but a fear of something going wrong from the incident. Not to mention attaching different sized loads to the output leaves the possibility of an overdraw I don'twant to pull more than 20 amps for fear of overloading batteries. Want to be safe as possible without having to rip the thing apart if I were to have a short or overdraw. I hope this clears things up
     
  18. BruceS

    BruceS

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    Jun 25, 2014
  19. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    "Resettable fuses" are fine for low power stuff like USB ports that you don't want to damage if someone accidentally shorts them. They're less of a good idea when you have higher power electronics as they can be called on to dissipate a lot of heat.

    A steam radio approach might be to have a bimetal strip that heats up and breaks the connection :)
     
  20. doliver

    doliver

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    Jul 25, 2014
    I still prefer a resettable circuit. I'm hoping for something I can adapt in the future for other applications I'm learning and fascinated by the concept.
     
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