Hooking up LED lights to 12V battery

Discussion in 'OnBoard Electronics & Controls' started by Nickpontoon, Oct 5, 2012.

  1. Nickpontoon
    Joined: Oct 2012
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    Nickpontoon New Member

    Hi all,

    I'm new to the forum, I'm trying to help my dad with some underwater LED lights for his pontoon boat. He recently purchased 2XLED lights which have 4 small LED's in each fitting. The lights were bought off Ebay and I'm unable to find a wiring diagram. My dad installed the lights when anti fouling the boat recently and I extended the length of the wires using figure 8 cable around approx. .75mm which was a heavier gauge cable than the tails hanging out of the lights. I soldered the cables, taped seperately then covered with a resin/epoxy heat shrink. the boat is now back in the water. I've since installed the wiring neatly back to a switch and out of the switch ran a figure 8 back to the main battery on the boat. I tested the lights on a small portable battery on installation and they worked fine and they also work off the battery now. The problem that I'm having is that when I add a fuse to the circuit (10 amp) it is blowing. So, here's my questions;
    1.What size fuse do I need for this application?
    2. Do LED lights require fusing?
    3. Is it OK to lug .75mm cable onto battery main terminals of a boat and finally could the resin join simply have moisture in it and if so, why do the lights work unfused?

    Any help would be greatly appreciated.

    Regards,

    Nick
     
  2. hoytedow
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    hoytedow Wood Butcher

    You may have a short. It sounds like you are drawing over 10 amps which should not be the case. Check your waterproofing as 8 LED's should draw less than 5 amps, depending on size.
     
  3. frasco
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    frasco Junior Member

  4. CDK
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    CDK retired engineer

    You definitely have a short circuit. A 4 LED array draws 0.035 Amps if small LEDs are used, 0.125 Amps if they are power LEDs. There must also be some provision to limit the current, a built-in resistor or electronic circuit.
    With the proper wiring the lights will not blow a 0.5 Amps fuse.
     
  5. hoytedow
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    hoytedow Wood Butcher

    Thanks for the good link. One interesting point it illuminated is that in one configuration LEDs in string utilized their own natural resistance.

    "The other configuration is to run the sum of the supply voltage at approximately 75 – 85% of the combined LED voltages. This uses the LEDs' combined inherent resistance to stop thermal run-away."
     
  6. hoytedow
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    hoytedow Wood Butcher

    I guess I lost a decimal somewhere. :eek:
     
  7. Nickpontoon
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    Nickpontoon New Member

    Thanks for the help everyone. Back to the drawing board I guess, or perhaps the duckboard!
     
  8. hoytedow
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    hoytedow Wood Butcher

    Welcome to the forum, Nickpontoon.
     
  9. rxcomposite
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    rxcomposite Senior Member

    Nick,

    If it is pure LED's with no electronic circuit or resistor, it will blow the fuse or burn the LED. Since an LED needs about 1.2 volts to become active, a small flashlight battery of about 1.5 volts will light it but a 12 volt batterry will burn it. LED's are a constant current device and the most practical way of limiting the current is by inserting a resistor in series. To find the resistance of the resistor, you need to do a little math.

    R=E/I, where R in ohms, E in volts, I in amperes. Now the voltage will be divided from the source to the resistor and LED in series. That is 12 volts(source)=Voltage across resistor+Voltage across LED. If the LED needs 1.2 volts, then 12-1.2= 10.8 volts, the voltage the resistor needs to be dropped. So plugging the numbers, with the data given by CDK

    Resistance= 10.8volts/0.035 Amperes
    = 308.6 ohms

    This is just rough approximations but should give you enough to start with. Battery voltage changes from 10.5 V to 13.8 V fully charged and LED current requirement varies from type to type. LED voltage drop also changes a little when fully bright.

    Another thing. You cannot connect LED's in parrallel, It has to be in series. LED's are constant current device, meaning it needs to be fed with a constant current to remain actively bright over a changing voltage source. To do that, an Integrated Circuit (IC)does a better job. There are simple 3 legged IC current regulator in the market that cost only a few cents. Try downloading "How to" articles in the net in order to learn more about doing this. If you can't find it, I will search the article later for you.
     
  10. gonzo
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    gonzo Senior Member

    Are you sure the polarity is right? They are diodes and will allow current to flow in one direction.
     
  11. Don H
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    Don H Junior Member

    Hi Nick, yes you should fuse LEDS or any other piece of gear attached to the battery. The purpose of the fuse is to protect the wiring not the attachment. As a general rule of thumb the current rating of wire is about 10 times the area. In your case .75mm is good for about 7.5 amps max. I generally like to have at least 100% extra rating so a 3 amp fuse max is what i would use. This should be plenty for 4 leds. If you have working leds but blow a 10A fuse i think your wiring needs checking and you will probably find your fuse is directly across the battery.
    These ebay LED lights often have a regulator built in and do not need a resistor or any external limiting. If you have been connecting your lights up so far you can be sure there is something in the light. If there wasn't they would have been destroyed as soon as you connected them.
    RX , leds can and are connected in parrallel and it is often more desirable for a parrallel system rather than a series system. In a series wiring if a led fails all leds are off,Luckily leds generally fail to an open circuit condition. If a series led fails to short circuit the over voltage applied to the remaining leds will shorten their lifespan .
    4 leds in series will require around 4.8v and say 20 Ma ( voltage and current will vary depending on the led colour) 4 leds in parrallel will require around 1.6v and 80ma. If one of the parrallel leds fails the other 3 still run unaffected.
    Often system such as led signage use a combination of series and parrallel to stay withing reasonable voltage and current requirements..

    Thanks Don
     
  12. michael pierzga
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    michael pierzga Senior Member

    High power underwater type LEDs draw substantial power. Even more if the LED device has a built in power supply. Typical Marine led packages accept 12 to 36volt input.

    I Have A 900 lumen high intensity LED flashlight. A maglight type flashlight with metal case . Its defect is that after10 minutes of use the flashlights chassis become so hot you can no longer touch it. takes alot of energy to create that heat

    Consult your produce manual for power requirements.. perhaps you need a bigger fuse.
     
  13. Don H
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    Don H Junior Member

    Hi Michael, i think you would be surprised at how little power they do consume. To blow a 10 amp fuse Nick would need at least 120wattts of lighting. Even the real high powered leds of LUMILED, CREE or Osrams Dragon have a max of 5w each. I havent seen exactly what Nick is useing but i suspect being ebay leds they are standard superbright leds of up to maybe 10,000mcd.
    Out of curiosity i had a look at the Aqualuma range of underwater leds and they quote a current draw of less than 2 amps for their FF12 underwater leds. These lights are putting out 2750 lumens. Heat on your torch isnt a very good indictor of energy being used, most people would be hard pressed to hold 1 watt of heat energy in their fingers , try holding a small low voltage light bulb of a couple of watts in your hand for a while and you will see what i mean.
    Thanks Don
     
  14. michael pierzga
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    michael pierzga Senior Member

    Dont really know...LEDs output ratings confuse me. Im my hand I have a bright exterior LED and fitting. The document says 1.4 amp at 12v. Its a bright LED. If you had a cluster of them perhaps the fuse would blow.

    Id say consult the documentation. And what does LUMEN mean ? in relation to watts or candlepower ie...a 24v 20 watt incandecent equals how many lumen ?
     

  15. michael pierzga
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    michael pierzga Senior Member

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