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Thread: Do I need a relay and if so why?

  1. #1
    Unenlightened
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    Default Do I need a relay and if so why?

    Okay so long story short a few years ago I had an 01 Dakota had bad wiring from previous owner I bought two led fog/driving lights. I went to use my turn signal one day after I had stopped using the lights for quite awhile and all of a sudden it wouldn't work. I had to take it to a mechanic they were able to find my turn signal wire was burnt into he told me the reason was I needed a relay on my fog lamps. I am confused because they were fused and I don't get why I need a relay I have looked online but none of it makes any sense to me. Because if I have it fused it should blow the fuse before it burns my wire into.


    So now I bought theses lights on amazon to put on my boat everyone tells me I buy a wiring harness. Well that's nice but not practical in my situation because I already have everything i need except for these relays. They will wired to a rocker switch and be fused believe me, but I also want it done right and can't afford to burn the boat up or burn wires into.

    Here is the link to the big bar light (https://www.аmazon.com/gp/product/B0...?ie=UTF8&psc=1) I have tested the amps personally it draws 3.33 amps on a 12 volt battery. Do I need a relay for this light and if so why?

    Also I bought a pair (two) of these on amazon as well (https://www.amаzon.com/gp/product/B0...?ie=UTF8&psc=1) I have tested one of the two lights, and one the amp draw on a 12 volt battery is 1.73 amps do I need a relay and if so why?

    The bar light will be on the same rocker switch as the two smaller lights but I can wire them to separate rocker switches if I need to.

    Example:
    Rocker Switch 1
    Bar Light & a pair (two) spot lights wired together
    Total of 6.79 amps

    OR

    Example 2
    Rocker Switch 1
    Bar Light total of 3.33 amps

    Rocker Switch 2
    A pair (Two) spot light wired together total of 3.46 amps


    Thanks,
    Blake
    Last edited by Alaric Darconville; 07-10-2018 at 09:20 AM.

  2. #2

    Default Re: Do I need a relay and if so why?

    Hard to follow your post dakota/boat? I'll assume your talking about a boat. A switch for each lighting system would be ideal. Then you would have more control over the amount of light and beam pattern of light you're putting out there vs just an on off wall of light. If your amp draw is accurate a quality switch will handle it just fine without relays

  3. #3

    Default Re: Do I need a relay and if so why?

    You do not need a relay to control your lighting if:
    .. You have a fused circuit capable of 20 amps and
    .. Your fused circuit is supplied directly from the b+ terminal of the fuse panel or from the battery and
    .. You have a rocker switch rated at about 30 amps and
    .. You have no problem with these lights staying on when your key is switched off.
    .. Use 12 g wire for a distance up to about 10 or 12 feet.

    Even though your lights may be pulling about 7 amps [at 14.2 volts operational that's about 100 Watts], you still need to accommodate in-rush current and voltage drop across the wires. You also want a 30 amp switch so that it does not provide a high resistance point in your circuit.

    If you want your fogs to only come on when your headlights are on low beam [like the are supposed to] then you need a relay that is activated by the low beam circuit that provides power to your fog lamps. That relay should be wired normally open and be rated at 30 amps [which is quite common].

    You can also activate your truck fog lights with the park lamp circuit.

    You can do something similar with your boat. I would use one 20 A supply wire and at least two 30 A rocker switches like Jasonok suggested. The different spots/types of lights get different rockers. Just use the same gauge wire throughout to keep it simple. Otherwise, you will need additional fuses to match the different wire gauges.
    Last edited by adnj; 07-09-2018 at 11:13 PM.
    I got nothing else to say...

  4. #4
    Moderator Alaric Darconville's Avatar
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    Default Re: Do I need a relay and if so why?

    Quote Originally Posted by blgreene1989 View Post
    Okay so long story short a few years ago I had an 01 Dakota had bad wiring from previous owner I bought two led fog/driving lights. I went to use my turn signal one day after I had stopped using the lights for quite awhile and all of a sudden it wouldn't work. I had to take it to a mechanic they were able to find my turn signal wire was burnt into he told me the reason was I needed a relay on my fog lamps. I am confused because they were fused and I don't get why I need a relay I have looked online but none of it makes any sense to me. Because if I have it fused it should blow the fuse before it burns my wire into.
    Those "fog/driving" lights can't be both. Fog lamps and auxiliary high beams are two very distinctly different kinds of lamps.

    The turn signal wire was burned because the fuse didn't do the job it was designed to do. There is no earthly reason, however, why a fog lamp or auxiliary high beam would need any connection to the turn signal circuit-- there is ONE reason there could be a tie-in with your turn signals and fog lamps, and that's if the fog lamp also performs a cornering function, such as the lamp on the side of the turn you are signaling comes on so long as you are signaling a turn.

    Someone modified the electrical system in some bizarre and dangerous ways.

    So now I bought theses lights on amazon to put on my boat everyone tells me I buy a wiring harness. Well that's nice but not practical in my situation because I already have everything i need except for these relays. They will wired to a rocker switch and be fused believe me, but I also want it done right and can't afford to burn the boat up or burn wires into.
    The usual switches out there aren't really designed for constant high current loads. Use switches only for controlling relays. Fuse the switch itself (use a very low-amp fuse for it, since it only needs to send maybe 100mA at most to the relay itself. Fuse the high current portions of the relay.

    Here is the link to the big bar light (https://www.amаzon.com/gp/product/B0...?ie=UTF8&psc=1) I have tested the amps personally it draws 3.33 amps on a 12 volt battery. Do I need a relay for this light and if so why?
    You probably need not to use that light bar except for very low speeds like when rock crawling or fording a stream. I doubt it's engineered well, it will have no compliance with any road-going lamp requirements and so shouldn't be used on the road-- it'll make things worse for you. And that "Ultimate Luminous Effeciency (sic)" -- I think they mean "luminous efficacy"; "luminous efficiency" is not the right phrase (nor could they spell).


    Also I bought a pair (two) of these on amazon as well (https://www.аmаzon.com/gp/product/B0...?ie=UTF8&psc=1) I have tested one of the two lights, and one the amp draw on a 12 volt battery is 1.73 amps do I need a relay and if so why?
    Using a relay still keeps the current used within the cabin much lower. Perhaps it may be unnecessary, but again, it helps protect your switch from overheating. The quality of many off-the-shelf switches is pretty low.

    Again, those lamps aren't going to be useful, safe, or legal on the road.

  5. #5
    Moderator Alaric Darconville's Avatar
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    Default Re: Do I need a relay and if so why?

    Quote Originally Posted by JasonOk View Post
    Hard to follow your post dakota/boat? I'll assume your talking about a boat. A switch for each lighting system would be ideal. Then you would have more control over the amount of light and beam pattern of light you're putting out there vs just an on off wall of light. If your amp draw is accurate a quality switch will handle it just fine without relays
    I wasn't aware boats had turn signals; maybe "boat" is just a pet name for it (but usually that'd be used to describe something like a '74 Lincoln Continental).

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