What am I missing about fuses?

Discussion in 'General Electronics Chat' started by Trevor Rymell, Sep 29, 2014.

  1. Trevor Rymell

    Thread Starter New Member

    Sep 27, 2014
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    What exactly is the role of fuses in electronic circuits? Sorry if this is a naive question from a beginner.

    I ask this because over the many years I've had electronic gadgets and equipment stop working or start smoking or otherwise fail and I've lifted the lid to look inside, I've never yet found a single instance of a repair that involved simply replacing a fuse.

    I've always followed the standard advice: check the fuse first but without exception what I've found is one of two things:

    1. One or more components on the PCB AND the onboard fuse has blown.
    2. One or more components on the PCB have blown BUT the onboard fuse is undamaged.

    Apart from these, I can't remember ever finding the fuse in the mains plug blown. It's possible to imagine something like an AC mains electric motor with no electronics involved, blowing a fuse if say, the motor is overloaded or overheated. Replace the fuse and everything will probably be fine.

    My assumption has always been that the fuse is a deliberate weak point designed to fail first before current overloads cause damage to the rest of the circuit or something like that. But clearly this can't be right because something must have go wrong within the circuit in the first place which then either causes the fuse to blow afterwards or not.

    So in short, what is the fuse supposed to protect?

    Thanks for reading.

    Trevor:confused:
     
  2. ISB123

    Well-Known Member

    May 21, 2014
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    The point of fuse is to save your house,saving the circuit is just a bonus.
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    In theory, a circuit component fails causing the circuit to draw excessive current. Without the fuse, this could potentially result in a fire.

    As ISB says, the fuse is to save your house.

    When the fuse blows, one needs to find the underlying reason for it to blow.
    Sometimes, the fault was temporary and every thing is fine after replacing the fuse.
     
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  4. Trevor Rymell

    Thread Starter New Member

    Sep 27, 2014
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    You might be right if it was the fuse that's blowing every time but my point is that it isn't. So what's the point of fuses?
     
  5. MrChips

    Moderator

    Oct 2, 2009
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    In those cases the fault does not result in excessive current. Hence the fuse remains intact.

    The fuse is still essential for the cases I described.
     
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  6. Trevor Rymell

    Thread Starter New Member

    Sep 27, 2014
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    Most computer problems can be attributed to a simple problem - the guy in front of the keyboard.
     
  7. ISB123

    Well-Known Member

    May 21, 2014
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    Maybe fuse isn't rated properly.
     
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  8. alfacliff

    Well-Known Member

    Dec 13, 2013
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    just because a fuse hasnt blown dosnt mean it is not essential, are firemen not essential just because you havnt had a fire? you should learn how to id the dauxe of blown fuses, and what the fuses look like afterwords. like a gloass fuse with the inside blackened and metal on the inside, ususally means a shorted preimary side of the power supply.
     
  9. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Actually there is much more to protection devices than this.

    As with firemen (I note the example above) the prime function is to save life.

    All property comes after that.

    As noted above fuses normally blow for a reason (they do have a finite life like all other components and may fail of their own accord for no outside reason but their service life is a long one.)

    You have noted there is a fuse in your mains plug.
    Not all countries require these.
    Further these are usually of large rating than the equipment internal fuse, so one would expect the internal fuse to fail in preference.

    The first and foremost point of the mains fuse is so that if there is an electrical fault condition leading to exposed metalwork becoming live and therefore potentially lethal, a fault current of suffient magnitude will be generated to disconnect the equipment from the supply.
    This is eminently sensible and practical, but is not generally a requirement.

    Does this help?

    This is why the official term is 'disconnection device'

    Subsidiary fusing may protect the equipment, the mains supply or the wiring from faults that do not present a potentially lethal condition.
     
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  10. Trevor Rymell

    Thread Starter New Member

    Sep 27, 2014
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    Thanks a lot. Yes, it helps.

     
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