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How-To Wire up LEDs to a 12volt system

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  • How-To Wire up LEDs to a 12volt system

    Since it came up in another thread i thought i would do a quick how-to for wiring up LEDs to run off a 12volt system.

    it is called a 12volt system but i set mine up to run off of 14volts considering that is closer to what the alternator puts out. i am not sure the difference would matter much at all, but i'd rather add an LED to compensate for the minimal lack in intensity rather than reduce the life of my LEDs.

    ok...step #1: buy your LEDs (the rest of the how-to depends on what you get ). i usually just jump on ebay and see what i can find that fits my design. for this write up i will use the example of what i had in my truck....they were designed to run on 3.3volts at 25mA (milliamps). this stuff doesn't much matter but for reference they were 5mm LEDs, blue in color, 8000MCD in brightness.

    step #2: calculate what resistor you need to drop your 14volt electrical system down to the voltage and current your LEDs are designed for.

    *as a side note, it is fun to take spare LEDs and just hook them up to 14volts...they fry, make sizzling noises, turn brown, and smell bad.

    anyway...we take ohms law (flipped for our purposes): resistance = voltage / current. we have 14 volts and want to get that down to 3.3volts. so we have a working voltage of 10.7 (14-3.3) and a current of 0.025amps ( that is the 25milliamps converted to amps). 10.7 / .025 = 428ohms.

    now 428 ohms is kinda oddball, so just swing for 430ohm or 450ohm or whatever is closest. it was calculated to a 14 volt system so even 420ohm or slightly less would not hurt.

    step #3: solder resistors to LEDs. you want to solder the resistor to the positive (+) side of the LED. you can use this pic to determine which is + and which is - as you will need to wire them up correctly after the resistor is soldered on anyway:



    this isn't a soldering how-to so i am not going to go into detail but feel free to ask if you want a couple pointers (here's a freebee...heat the wire from below and apply the solder from above).

    step #4: wire them up correctly!! this entire how-to was written based on wiring the LEDs in parallel, NOT series. basically, all the LEDs should share a common power source and a common ground but they should NOT be wired one after the other. the power source should branch out to each LED and they should all join back together to go to a common ground. this is the best pic i could find to try and visualize what i am trying to write:



    you want the bottom setup, not the top. do not wire them one after the other, wire them each individually but using the same power and ground sources. hopefully this makes sense cause it's important .

    that being said...make sure your power source is at least 12volts. i find most "12 volt sources" to run anywhere from 12.5 to 13.5 volts with the car on...thus the 14volt consideration at the beginning of the project.

    i tried to make this as basic as possible and explain everything the best i can but if you have questions please post them. if i can't help i'm sure someone else on here can!

    and this how-to will pretty much work for anything you want to wire into your car that is not designed to run of a 12volt system. the laws of electricity don't change, so just adapt the numbers to fit your situation and you should be all set!

  • #2
    nice write up Bristol,
    here's a web site that is really helpfull for this stuff.
    http://metku.net/index.html?sect=view&n ... /index_eng

    oneOone
    1996 Jeep Grand Cherokee V8
    2005 Pontiac G6 GT....Wife (Liquid Silver)
    1983 Jeep CJ7.....for the 8 year old

    SSSS member #5

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    • #3
      Originally posted by oneOone
      here's a web site that is really helpfull for this stuff.
      The only problem I have with that web site's parallel LED setup is they use just one resistor. If an LED burns out, breaks, or otherwise fails, then the circuit changes such that all the other LEDs start getting more current, so they burn brighter and/or just burn out.

      Bristol's setup with a resistor attached to every LED would be preferred.
      (Rev) Greg :: 2006 Fusion Orange GTP Coupe auto - 35% tint - MPD hood - Razzi wing- Dr. Speed CAB - GTR exhaust - GM STB - caliper paint/decals - mesh lower grilles - more to come

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      • #4
        Some of my input on this:

        Car's voltages--unless there's something seriously wrong with the Voltage regulator, usually peak at 13.8v. If you design from this voltage with some wiggle room in planning for the max forward cuyrrent of the LED, you should be fine.

        On parallel LED's. Don't do them. Strings LED's wired in series, parallel connected to each other; all balanced out with same LED's, same Resistors (hopefully from the same batch) is the best way to go for running any more than, say, 6 LED's on a car's voltage system.

        Unless you're planing on making whatever into a "wire" with the LED hanging off light a Chritmas light--you're better off using a circuit board. I.E. if you're running LED's within your vents, then ye, solder the resistor to the terminal of the LED--which one really donesn't matter, but be consistant, if one,s to the anode (+), then make all to the anode. However, if you're making a custom LED setup for the maplights--go with making a circuit board. Usually your pre-padded "grid" ones are the easist to do--just bridge the pads you need to and add jumper wires, but for the most compact results, you can trace and etch your own.

        Good link to have for making your own PCB's:
        "Cross creviced chasms vast, and endless plains of unshaven ass"
        ~~GWAR's description of Hell
        Skaði: 2008 Pontiac G6 GXP Street Edition

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        • #5
          Re: How-To Wire up LEDs to a 12volt system

          Hmm I wired mine parallel like you mentioned not to I put the resistor before all of the LED's and I used one that resists enough for 4 LED's but will still put enough resistance for 3 LED's so will this cause any problems?
          Blue Man Group

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          • #6
            Re: How-To Wire up LEDs to a 12volt system

            Originally posted by Wicked
            Hmm I wired mine parallel like you mentioned not to I put the resistor before all of the LED's and I used one that resists enough for 4 LED's but will still put enough resistance for 3 LED's so will this cause any problems?

            they would shine brighter running at the correct voltage but it isn't really bad to underpower them.

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            • #7
              Re: How-To Wire up LEDs to a 12volt system

              The problem with wiring up LED's in parallel like that is that the currnet will naturally seek the easiest path, and after some "burn-in" time, unless all of the LED's have the EXACT same characteristics, came from the same batch, etc. If not, and one develops more resistance, not only will it get dim, but it will shunt more current to the oher ones, causing a better chance of failure.

              The best solution is to wire in series, and then parallel of the series: like so:
              Code:
                   |-----/\/\/\-----|>|----|>|----|>|----|
              -    |                                     |    +
              o----+-----/\/\/\-----|>|----|>|----|>|----+----o
                   |                                     |     
                   |-----/\/\/\-----|>|----|>|----|>|----|
              Sorry for the ASCII art...

              Anyhow, if you do it like you suggested, 1 current-limiting resistor powering, say 5 parallel LED's, and one LED develops high resistance, the current will naturally select the other 4 LEDs over the higher resitance one. Thus, instead of the total current running through the resistor divided by 5, you're now dividing it by 4, and possibly overdriving the LED's for their max current rating.

              Again, this goies against the OP, but it's my preference on it. If you wire series, or parallel series, you get more uniform light from the entire circuit for the long run, and if something does die, the rest of the circuit won't be affected.

              This page is an example of that:
              "Cross creviced chasms vast, and endless plains of unshaven ass"
              ~~GWAR's description of Hell
              Skaði: 2008 Pontiac G6 GXP Street Edition

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