Fuse question

Discussion in 'General Electronics Chat' started by zander, Mar 28, 2009.

  1. zander

    Thread Starter New Member

    Jul 26, 2008
    Hopefully this should be a simple obvious question!!

    I'm using a length of nichrome wire on a 12v supply to create some heat for my project.
    It draws 800ma at 12v.

    If I place a 1 amp automotive fuse on the hot side of the supply will this protect is from getting to hot if someone then uses a higher voltage (say 24v) by mistake and it draws say 1.5 amps.

    Just trying to thinks safety!!

    The fuse would then blow right, its drawing 1.5amps (1 amp fuse)??? :confused:

  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    Should work fine given there is a sufficient but not too great a difference between your normal 800mA requirement and the 1A fuse rating.

    Fuses are normally "designed" to allow current flow up to and including their nominal rating with some overload expected. Could actually blow at 120% of nominal current. Depends on the type of fuse.

    Caveat: I guess your nichrome wire will draw well over 1.0 Amp at 24V DC - although being a non-linear resistance I'm not sure exactly what value the current will reach. Joule heating (with Rwire being proportional to temp) increases with the square of the current.

    Why not conduct some tests to see if the chosen fuse blows at a sufficiently consistent current value such that it affords reasonable confidence in the protection method.

    Hopefully this is just a project and not something you are thinking of marketing.
  3. zander

    Thread Starter New Member

    Jul 26, 2008
    Thanks for the info, yeh testing is the only way, just trying to get as much advice before getting started. Just a project also.
    Just one more thing, if say a 32mm glass fuse is rated at 240v 1amp, the fuse should still blow at say 1.2 amps 12v just the same?
    Thanks again
  4. italo

    New Member

    Nov 20, 2005
    fuses all fuses blow open yes. the thing that you don't understand is if it blows the flash ionizing air will be contained withing the glass enclosure while the wire will not do that and finaly fuses are not there to protect the circuits but ot prevent fires by disconnecting. read and understand.
  5. Wendy


    Mar 24, 2008
    Actually fuses are to protect electronics, to prevent damage from spreading. They don't always work as planned, but that is the intent.

    The reason for the voltage rating is if you have a low voltage fuse, and it blows in a high voltage application, you can actually have an arc much like italo describes. If the fuse is high voltage and in a low voltage application, no problem.
  6. kubeek


    Sep 20, 2005
    There is a big difference between F-, the fast blow fuses, and the T-, the slow blow fuses.
    In your example, if you used the slow type, it may blow too slow at the 1.2A current to protect the circuit. You should look at the datasheet of the fuse you want to use, or at least of the same type from other manufacturers.
    There usualy are graphs with current vs. blow time, which will tell you a lot.

    You can even think of a light bulb as a fuse, but with blow time of 1000 hours at the rated current :)