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Stupid electrical question

Discussion in 'Cruising Sailors' started by WadeBoggsCarpetWorld, May 9, 2019. Add this thread to a FAQ

  1. walt

    walt

    Joined Jun 1, 2007
    3,261 posts, 402 likes
    Macgregor 26S Hobie TI, Capri Coronado 15
    US Denver, Colorado
    Consumer electronics like this which are sold where the end user installs often have some sort of method to tolerate little mishaps because returns are very expensive. In this case, its a LED light fixture and those can be designed to tolerate reverse hookup. And sometimes even work fine hooked up either way. More tolerant adds cost to the design.

    A very simple method sometime used in designs is to put a diode across the input power. If you hook up the DC power correctly, the diode is reverse biased and doesnt do anything. If you hook up the power backwards, the diode is forward biased and looks somewhat like a short. This is meant to work in conjunction with a fuse where when hooked up backwards, the input diode gets forward biased and "lots" of current flows and causes the fuse (easy to replace) to blow. The fuse needs to blow before the diode does - hence the current rating of the fuse.

    We really dont know why the manufacture said to put a 1 amp fuse in that line and without looking at the schematic, wont know. Will the OP have any issue ignoring the 1 amp? Most likely the worst case is just burning out the device and then returning it for another. The manufacture is trying to avoid this but you probably have that option. Is there any safety risk by ignoring the manufactures fuse recommendation? All most for sure not since you have any case of wire short covered with the 5 amp fuse.

    The OP is not getting bad advice here to blow off the 1 amp fuse. Im just pointing out (as a retired consumer electronics design engineer) one possible reason the 1 amp fuse is specified by the manufacture.
     


  2. Project_Mayhem

    Project_Mayhem

    Joined Sep 24, 2018
    373 posts, 70 likes
    O'Day 25
    US Waukegan
    Wouldnt it make more sense to put the diode in series rather than parallel with the device it's supposed to protect? No risk of frying a diode or blowing a fuse. There is of course a voltage drop
     


  3. walt

    walt

    Joined Jun 1, 2007
    3,261 posts, 402 likes
    Macgregor 26S Hobie TI, Capri Coronado 15
    US Denver, Colorado
    Definitely that would work. We are completely second guessing the manufacture here on the recommendation but..

    A guess on my part.. LED light fixtures for boats are partly about lowering power requirements. You can add a diode in series internal to the device that would just have the voltage drop when hooked up correctly, reverse biased when hooked up backwards. Some of the LED light fixtures have a bridge diode setup on the input so you can hook them up either way but now you have two diode drops.

    If efficiency was the design concern (dont know), adding an inexpensive diode in series would result in the input power to the device needing to be about 6 percent higher in order to get the same light output from the LED circuit (I looked at one case with about a 3 watt LED light - if you want the details, let me know).

    Is 6 percent extra power needed for the same light output important enough to require the external fuse.. Dont know. I also dont know exactly why the extra fuse was specified. But I have seen cases and know its a design tool to use a diode and fuse to protect for reverse bias without the additional power loss.
     


  4. WadeBoggsCarpetWorld

    WadeBoggsCarpetWorld

    Joined Apr 1, 2019
    17 posts, 2 likes
    Freedom 38
    Sunset Spy US Boston, MA
    I think you're overthinking this; the fuse is just to protect the two circuits, in case you aren't going to use a breaker. Take a look at the manual, it seems pretty clear.
     


  5. walt

    walt

    Joined Jun 1, 2007
    3,261 posts, 402 likes
    Macgregor 26S Hobie TI, Capri Coronado 15
    US Denver, Colorado
    Not likely to read the manual yet. I was just going by what you said in the original post. Since this is your problem, maybe you can find the details in the manual and post them, it would be interesting.
     


  6. WadeBoggsCarpetWorld

    WadeBoggsCarpetWorld

    Joined Apr 1, 2019
    17 posts, 2 likes
    Freedom 38
    Sunset Spy US Boston, MA
    Here's the entirety of what it says about wiring:
    "Protect circuits with 1 amp. fuses or circuit breakers to both red wires.
    Red wire 1 = battery positive (+) = masthead (225 degrees) light.
    Red wire 2 = battery positive (+) = Deck Light
    Black wires = battery negative (-)."
     


  7. walt

    walt

    Joined Jun 1, 2007
    3,261 posts, 402 likes
    Macgregor 26S Hobie TI, Capri Coronado 15
    US Denver, Colorado
    Aha.. maybe I see how you are interpreting this. If I understand, you interpret this as you can use a 1 amp fuse... OR.. some unspecified circuit breaker.

    I interpret that as they are recommending either a 1 amp fuse or a 1 amp breaker on both red power wires.

    Anyhow.. I dont think this matters much at all especially if you have it hooked up already and its working fine.
     


  8. planoron

    planoron

    Joined May 17, 2014
    101 posts, 12 likes
    hunter 380
    US Plano, TX
    Sorry for the late reply. Looks like a wealth of information. Yes the 1a fuse will protect the light and it circuits since it's 10-30vdc. The 5a breaker will cover any fire/wire issues because that is what breakers are for. The fuse is there to protect the light at the top of your mast. I rewired my mast with new led's and even added some bad ass spreader lights that flood out 10'from the beam. They are great pulling in at night or alongside
     


  9. WadeBoggsCarpetWorld

    WadeBoggsCarpetWorld

    Joined Apr 1, 2019
    17 posts, 2 likes
    Freedom 38
    Sunset Spy US Boston, MA
    That sounds cool, and very useful. The problem is, I ain’t got any spreaders! :(
     




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