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Thoughts on fusing power injection lines

Discussion in 'Computers, Cabling & Other Miscellaneous Hardware' started by nutz4lights, Nov 11, 2013.

  1. nutz4lights

    nutz4lights Full Time Elf

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    So, I was sitting around thinking about this today and realized that the fuse value on the power injection lines is not exactly a cut and dry element. Here is my setup:

    350W power supply > P12R/S > 12 outputs with 100 WS2811 pixels on each connected to pixel one
    <same ^^> > two separate wires > Bussman in-line fuse > each wire split to 6 power injection lines that are connected at pixel 100

    Let me know if that doesn't make any sense.

    By my measurements on these pixel strings, each of the 100 pixel outputs will draw 2.8A using white light (only around 2.0A with any of the colors) with 1/2 of that provided by the direct output from the P12R/S and the other 1/2 coming from the power injection line connected to each string at pixel 100. Since I wire-nutted six of the power injection lines to one Bussman in-line fuse, that should mean that the maximum current draw on each of the two power injection lines coming out of the power supply will draw around 8.4A, right?

    Now the question... what fuse would YOU use in that in-line fuse holder. It came with a 20A fuse, I picked up some 10A fuses... part of me feels like the 10A would be cutting it too close... So, what say ye?

    If I look at the P12R/S, they have each output protected with a 7.5A blade fuse and each group of six outputs protected by a 30A blade fuse. If I go by that logic, even the 20A fuse that came with the fuse holder wouldn't be high enough...

    Thanks for the discussion...
     
  2. ԆцряєсϮ

    ԆцряєсϮ Senior Elf

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    Easiest way to work out your fusesThe fuse rating only needs to be lower than the current carrying capacity of the cable its protecting.
    This way if a fault occurs on the cable, the fuse blows before the cable melts and prevents magic smoke.
     
  3. OP
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    nutz4lights

    nutz4lights Full Time Elf

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    That is a good point... From that perspective, the wire from the power supply is 12AWG stranded (same as the wire gauge that the fuse holder came with) and then it is wire-nutted to the extension cord wire-sets that go to the lights, which are 20AWG. Those are all in parallel coming off of the 12AWG wire though... if there is an issue there, there will also be an issue in the string which is only 20AWG.

    Thanks!
     
  4. DanJ

    DanJ Full Time Elf

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    Nutz--why dont' you just measure the current draw directly and choose a fuse (I'm a poet--choose a fuse.. but I digress) that is just a bit higher capacity than that current draw? That should give you enough "room" to avoid nuisance fuse blows while providing protection.
     
  5. OP
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    nutz4lights

    nutz4lights Full Time Elf

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    That's what I'm getting at is why is the appropriate value above the current draw that makes sense.
     
  6. DanJ

    DanJ Full Time Elf

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    I will ask one of my electrical engineering buddies tomorrow for some rules of thumb on electrical protection. I thought about this today, but then got working on stuff and forgot about it. I will remember tomorrow!!!
     

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