12 volt fuses

Discussion in 'General Electronics Chat' started by nickmms, Jan 10, 2014.

  1. nickmms

    Thread Starter New Member

    Jan 2, 2014
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    Hi all
    I have been testing some of our equipment and found that the fuses do not blow at correct amps. I am running a 12Volt PSU with an in line fuse on +12V... but the 5 amp fuses do not blow to about 10 amps. I have tried 2 different brands both are glass fast blow rating at 5 amps 250v. I searched on internet and from what I can gather even though it is rated at 250V it still should blow at 5 amps.
    thanks
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. nickmms

    Thread Starter New Member

    Jan 2, 2014
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  4. nickmms

    Thread Starter New Member

    Jan 2, 2014
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  5. KJ6EAD

    Senior Member

    Apr 30, 2011
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    The major fuse manufacturers all publish helpful documents explaining fusing time/current curves for their products. Bertus linked you to one.

    Fuses don't open the instant a load current slightly exceeds their nominal rating. In most cases, to do so would cause nuisance opening. Typical specifications will include a maximum time allowed to open at 200% of nominal rating and it is far from instantaneous!
     
  6. nickmms

    Thread Starter New Member

    Jan 2, 2014
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    Thanks
    I have been going through the stuff Bertus sent and other sites. I thought it might be about the 200% where it actually blows.
     
  7. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    See the "time curve" in the datasheet for each specific fuse.
    That tells you how fast and at what percent of the fuse rating a fuse should open up.
     
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  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If you want a fuse with a very fast sweep through, there is Rectifier fuses, intended principally to protect solid state equipment.
    Apart from the normal slo-blo and fast blow rated types.
    Max.
     
  9. nickmms

    Thread Starter New Member

    Jan 2, 2014
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    thanks Max
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
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    Buried somewhere on one of the On-Semi power rectifier application handbooks is a few pages on fuse ratings - there's a graph plotting % overcurrent vs time for fast, medium & slow-blow fuses.

    Apparently all 3 curves have a very shallow slope at 125% overload and the time axis went upto thousands of seconds.

    I vaguely remember that one of the 3 curves intersected 1 second at 600% overload.
     
  11. k7elp60

    Senior Member

    Nov 4, 2008
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    Fuses also have a voltage rating. The fuses for type ATM and ATO or DC use are rated a 32V. This voltage rating is important as if the fuse is used in a voltage higher than 32V it may not open the circuit when it blows. AGC style are generally 250V, there are some exceptions of the slow-blow that are rated at 32V also. If a 250V fuse is used on a 12V circuit it may not open the circuit when it opens (blow). So it is important to use fuses in the right voltage range.
     
  12. #12

    Expert

    Nov 30, 2010
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    This resembles the fan speed thread in Projects right now. You can not buy a fuse rated for 5 amps that will blow today with 5.0001 amps going through it. Fuses are not intended to be precision current sources. They are designed to protect circuits from overt fault conditions. If you want to limit the circuit to 5.0000 amps you will have to design a circuit to do that.
     
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