12V DC LED Help

Discussion in 'The Projects Forum' started by JMMooney515, Nov 21, 2007.

  1. JMMooney515

    Thread Starter New Member

    Nov 21, 2007
    2
    0
    Hello. I am working on replacing some of my interior lights in my car. I have been reading up and came up with a schematic. I am going to be running 6 LEDs in parallel. I used a resistor calculator and it said to use a 82ohm, 1W resistor. The LED specs: forward voltage is 3.2V and the LED current is 20mA. I know I can buy lights but I like doing it myself. I have read that the LM7317 is used because of DC's regulating voltage. Let me know if I have anything wrong and how I can fix it please. Here is a link to the picture, since I couldn't scale it down, of the schematic: http://moonjob.googlepages.com/12vMapLED.JPG

    -Mooney
     
  2. GS3

    Senior Member

    Sep 21, 2007
    408
    35
    You do not want to run LEDs in parallel because they are not exactly alike and some will take more current than others. You can run them in series or you can provide each one with its own resistor to ensure current limitation.
     
  3. JMMooney515

    Thread Starter New Member

    Nov 21, 2007
    2
    0
    Ok. So I will be able to run 2 sets of 3 of these LEDs in series on a 12V source with a 120ohm resistor (1/8W)? Is the voltage regulator (LM317) still a good idea to use? Is that the correct wiring for it in my schematic? This picture for 6 series LEDs:
    [​IMG]
     
  4. GS3

    Senior Member

    Sep 21, 2007
    408
    35
    That is correct for 12 V but you need to take into account that an automobile voltage will go as far up as 14 so you need to take that into account. here are your options:

    You can rise the value of the resistor but this solution means a substantial variation in brightness depending on voltage.

    You can group the LEDs in twos or single and size the resistors accordingly. The variation in brightness will be less but the efficiency will also be less as more power is lost in the resistors.

    You can provide a circuit wich regulates the current automatically but this is a bit more complex.
     
  5. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
    http://led.linear1.org/1led.wiz

    Use 12.6v as the voltage and put a capacitor just prior to the LEDs to smooth out the crap the system sends. Use one resistor per LED, sucks but it works much better and they are cheap.
     
  6. GS3

    Senior Member

    Sep 21, 2007
    408
    35
    Better use 14 V as a worse case calculation.
    Absolutely not necessary.
    This has the advantage of simplicity and most stable light output but you have 6 resistors and you are wasting most of the energy as heat in the resistors. Grouping the LEDs in three groups of two would be a good compromise. And they could be grouped in threes and use a regulated current source.
     
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