Running fuses in parallel before an AC/DC converter

Discussion in 'Power Electronics' started by Non-Sequitur, Dec 10, 2016.

  1. Non-Sequitur

    Thread Starter Member

    Oct 27, 2014
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    I have a TDK-Lambda KMS30A-12 AC/DC power supply that takes in 120VAC and spits out a nice clean 12VDC. Just before the AC/DC device (AC side) is a 3.5A Littelfuse, as per the vendor's recommendation. This little setup is in some pretty extreme environments. So far we've not hit a case where the fuse has blown (thank goodness). However, if the fuse did blow, we definitely do NOT want the device stop working if there is any way to keep going. My question is, what if I put another Littelfuse 3.5A in parallel to the first?

    I realize that thee is a reason why the first one blew, but I don't care at that point. I would rather have the device keep working, hopefully. A second chance at it working rather than just stopping due to a single fuse would be preferable. In the end, I don't care if I get it back in any working condition. I just want to extend the possibility life of the device.

    Can someone explain the physics involved in having two fuses? Is there twice the amperage capable of getting through with two fuses in parallel? Or does one just sort of assume the duties as long as it is good, and the other takes over if the first one dies? Please edumakate me.

    Thanks!
     
  2. tcmtech

    Distinguished Member

    Nov 4, 2013
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    Add the amperages and that's the equivalent fuse you have.

    If resetability is a concern replace the fuse with a circuit breaker of same rating.
     
    Non-Sequitur likes this.
  3. Non-Sequitur

    Thread Starter Member

    Oct 27, 2014
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    Once the device is deployed, there is no way to retrieve it. So being able to reset it is not part of the equation. Which is really the reason for my post. I deploy the device and something happens, its gone. An additional fuse might (or might not) add life to the device. As it is now, one fuse dead, one device dead.
     
  4. Sensacell

    Moderator

    Jun 19, 2012
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    Fuses in parallel = bad idea.

    Where have you EVER seen this done in the real world? - nowhere.

    Fuse blown = Problem = Device not working

    Not much you can do to change that.
     
  5. Non-Sequitur

    Thread Starter Member

    Oct 27, 2014
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    Sometimes you have to do things that aren't USUALLY done, whatever real world is. I think I'll go with the first idea; two fuses together produces the sum of their amperages. I may even drop both to 3.15A to give just over 6. This will still provide enough juice if one blows. And I suspect that the AC/DC device will survive a 6.3A surge. Will be easy enough to test.
     
  6. #12

    Expert

    Nov 30, 2010
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    Have you ever had a nuisance failure (fuse blows, nothing is wrong)?
    No...imaginary problem
    Yes...increase the rating of the fuse.

    What if there is a real failure?
    In that case, you can't retrieve the object. It is dead. Just short out the fuse holder and let it burn. You're never going to see it again anyway.
     
    Joshua Axford likes this.
  7. Non-Sequitur

    Thread Starter Member

    Oct 27, 2014
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    If an additional fuse in parallel MIGHT add a little extra life, rather than the one fuse one life scenario, why not give it a shot. What I didn't know was what the amperage using two would be.
     
  8. Sensacell

    Moderator

    Jun 19, 2012
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    If you want a higher amperage fuse - replace the main one with a higher rating.

    If you parallel two fuses, the chances of them sharing the current evenly is low, leading to a totally unpredictable trip current that will be lower than you expect.
     
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  9. Non-Sequitur

    Thread Starter Member

    Oct 27, 2014
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    Not worried if they share current evenly, as long as it at least is as great as a single fuse. If one blows, a chance at a longer life. If both blow, no worse off than with a single fuse.
     
  10. #12

    Expert

    Nov 30, 2010
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    Nobody does it because it doesn't work. Fuses don't take turns. They both conduct as one, therefore you only need one fuse. Put in a higher rated fuse.
     
    Joshua Axford likes this.
  11. Non-Sequitur

    Thread Starter Member

    Oct 27, 2014
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    Thanks for your input, I'm going to give it a try anyway and test it out.
     
  12. #12

    Expert

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    We're glad you came here to get top quality advice so you could ignore it.:rolleyes:
     
    elephunk, ErnieM and hexreader like this.
  13. Non-Sequitur

    Thread Starter Member

    Oct 27, 2014
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    I've saved a lot of lives ignoring top quality advice... if you're lucky you won't be one of those.
     
  14. tcmtech

    Distinguished Member

    Nov 4, 2013
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    Put a self resetting one in. :rolleyes:

    Use some logic here and you will see why it won't work even if they both acted independently.

    Fuse A and B are both rated for 3.5 amps and a condition develops where the circuit pulls 4 amps blowing out fuse A thus instantaneously transferring that same 4 amp load to fuse B which then also blows.

    Zero gain in that theory of operation.

    We have our doubts regarding both your life saving and the true quality of the advice. More than likely both are highly exaggerated. :oops:
     
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  15. Non-Sequitur

    Thread Starter Member

    Oct 27, 2014
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  16. Non-Sequitur

    Thread Starter Member

    Oct 27, 2014
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    As far exaggeration... Since you have no idea who I am or what my background is, a statement like that only serves to convince me you've really never done anything relevant with your life; and therefore cannot imagine anyone who has speaking truthfully about anything. But it's ok. Everyone bleeds the same color, there are no atheists in foxholes, and sometimes it sucks to have to put on your big boy pants and leave the warm comforting couch in your mother's basement,
     
  17. tcmtech

    Distinguished Member

    Nov 4, 2013
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    Ah, I see a clinton snowflake didn't feel special after my reply so it's time to throw a fit.

    The one paragraph summation of my life for you,
    For starters I have a technical degree plus electrical engineering, don't actually have to work for a living if I don't want too, own more equipment and tools than the majority of most general contractor's, am building a second new house myself on the same property I cleared and built my first home and shop from scratch on 17 years ago, everything I own even the land I am on is and has been paid for for a very long time, the last time I filed taxes I paid in more in taxes alone than I used to make for gross annual pay 10 years ago and I am only 42 years old. And technically I haven't worked a normal job in near two years now because, well let's face it. I really don't have too! :p

    As for the rest of you comments they sound like the typical empty posturing drivel real mamas basement couch dwelling nobodies trying to pretend they are former military say. Real military people who've been through that sort of action don't say that sort of stuff to justify they are someone or something. Especially as a unsolicited reply to a basic internet forum post as mildly put as I said.

    And you do what for a living and yet had to come here to ask a junior high school electronics question then argued that the people here who have been electrical engineers, technicians and system designers for life and know more how electrical circuits work in detail than average person can begin to comprehend are wrong about it? Dumbass butt hurt pride much becaseu common sense failed you in a bonehead simple logic problem? :rolleyes:

    BTW, we love to tear ass hats like yo apart here until you go over the edge and get kicked out so have fun basement baby wanna be somebody some day troll boy. :D
     
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  18. #12

    Expert

    Nov 30, 2010
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    Thanks for saving me the time to type that, @tcmtech :p

    Yeah, I'm just sitting here with a college education, a State license, a paid off home, and 50 years of experience, so I wouldn't know how to use fuses.:rolleyes::D
     
  19. tcmtech

    Distinguished Member

    Nov 4, 2013
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    And I wipe my ass with a phone book. :rolleyes:
     
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