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Thread: Running extra LEDs in the boot?

  
  1. #1
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    nry's Avatar
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    Running extra LEDs in the boot?

    If I wanted some extra LEDs in the boot (say I was going to re-do my amp mounting position and was debating some perspex with LED lighting behind...) could I simply wire some into the boot light so they come on when the boot is opened?

    Any need for resistors etc if I can do this and how many/what type of LED/resistor would be suitable? I've got loads of both left from the dash LED conversion, a mix of 3mm and 5mm LEDs.

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    Sam01's Avatar
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    I'm also interested in the answer to this so free bump

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    Yes resistors will be needed. The same type as you used when doing the dash, as the dash bulbs + the boot light will both be 12v

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    In my last car, i ran 8 LED's off the interior courtesy light with no resistor, just wired the LED's in a circuit... they worked fine :S

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    I forgot I asked this question lol!

    Sounds easy enough then and I'm not off the mark by thinking of wiring them to the courtesy boot light

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    Hi nry
    All you need to know - coz it`s obvious you`r not familiar with Ohm law - is the max Current that you LED`s gonna take
    Every LED has max allowed current before he blow/fail

    So it`s easy :

    Voltage is 12 V = U
    If You Max Current is for example 0.05 A = I
    You resistor need to be R = U / I = 12 / 0.05 = 240 Ohm

    Resistors have parameter called Watts P = U * I = 12 * 50 mA = 600 mW

    So in this very simple equation/example you can Connect 1 LED with 1 Resistor 240 Ohm/ 1 W ( coz you need min 0.6 W ) and when you connect you circuit to the battery ( 12 V ) it will shine and will not blow

    The other method is to connect in sequence for example 10 LEDs
    You must keep in mind that LED`s have polarity !

    In that case you may not need to use Resistor BUT you need HIGH AMP/Watt LED`s and you need to connect more then 5 LED`s (for example)

    Of course no one can tell this for sure - we do not know what LED`s you gonna use but you CAN`T connect 1 , 2 , 3 , or maybe 4 using this method

    If you wire them in sequence !!!
    Decrease numbers LED`s -> high SHINE -> high chance to blow them
    increase numbers LED`s -> LOW SHINE -> LOW chance to blow them

    After all LED`s are not expensive so you can buy 20 and start TEST
    Wire them all 20 in sequence and begin to remove 1 by one , when you left with 4 to say , and they blow that will be you min number of LED`s

    Then if you like you can add 1 more and you have for example 5 LED`s in sequence , you can wire then you car with as much you wanna in groups of sequence of 5 LED`s and connect the groups in parallel :)

    I hope i`v helped :)
    "There is a many ways to complete the job, usually you pick the hardest one ... " -= Niksavoy =-
    Astra MK4 - 98 - CDX - Hatchback 4 drs - 1.6/16V ECOTEC

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    SQ Man's Avatar
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    Or you could do it the easy way http://www.led-calculator.com/
    Don't forget, typical voltage in a car with engine running is 13.8v.

    red - 2.0v, orange - 2.0v, yellow - 2.1v, green - 2.2v, blue - 3.3v, white - 3.3v
    2003 Sapphire Black 3Dr DTi Astra
    Custom Irmscher kit, Interior Leather/Alcantara retrim & Alpine/Focal Install
    Install progress (----------)

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    Oddjob's Avatar
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    Quote Originally Posted by SQ Man View Post
    Or you could do it the easy way http://www.led-calculator.com/
    Don't forget, typical voltage in a car with engine running is 13.8v.

    red - 2.0v, orange - 2.0v, yellow - 2.1v, green - 2.2v, blue - 3.3v, white - 3.3v
    Green LEDs are grouped along with blues and whites in that they are normally 3-3.3v LEDs not 2.2v as stated above. Not a lot of difference if I'm honest but could be crucial.

    Most LEDs I buy are rated at 30ma max continuous forward current. However that figure is a max and it's best to select a resistor with a 20ma LED in mind as I find that the Astra tends to push out a far higher voltage etc than you'd imagine. This tends to fry the LEDs early as you've not allowed for that. For instance Niksavoys 240ohm resistor used with a standard 3v would blow almost instantly, 240ohm is FAR to low a resistor, 470ohm is the minimum I'd use and in fact I tend to use at least 560ohm to prevent early failure.
    My rule of thumb is to look at the LED and see what catargory it falls into, 2v or 3v depending on colour. Next think of the max voltage you think it's going to encounter, in the Astras case it's approx 14.4v, whilst the Astra does sit around 13.8v it does run much higher on occasions so err on the side of safety. Subtract the LED voltage from the car voltage, so say 14.4v minus 3v. That leaves 11.4v, remove the decimal point and add a 0, now we have 1140, divide by 2 and that's the resistor you need, in that case 570, now select the closest resistor to that figure which is 560 and that's it. I can do it in my head now it's so easy. Adding say 2 more LEDs to the string would mean 14.4v minus 9v = 5.4v add a 0 so 540 divide by 2 = 270. Pretty simple once you get your head round it
    52 Astra G 1.8, 18" Mille Miglia alloys, 325mm Brembo 4 pot conversion, Coupe leather interior, Raid strut brace, Whiteline rear ARB, Irmscher bits (skirts, Carbon steering wheel, Bonnet extension, M3 mirrors, boot & roof spoiler), Reiger Rear Diffuser & Front Bumper, Koni Adj. dampers, 35/30 Vogtland springs, Duplex quad exit ASCAR stainless exhaust.

  9. #9
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    nry's Avatar
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    Blimey heck...I did two years of a Physics Degree but I didn't think I'd need it...

    At the mo I'll not be doing this anyhow, but for reasons of time and keeping my wife happy (i.e. not spending too much time on the car!) over anything else but cheers for the help!

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