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Thread: 12VDC to 1.7VDC and 2.4VDC

  1. #11
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    Connecting LEDs in parallel is a bad idea, because of the differences in Vf (LED forward voltage) will cause differing currents to flow through each one, causing different light intensities. The best way is a series/parallel configurations. Use the 'typical' Vf spec, and figure out the number you can wire in series before you run out of head room. For example. With a Vf of 2.4V, wire (13.5V - 3.5V (head room)) / 2.4V (led Vf) = 4.17 = 4. So wire them four in series.

    The ballast resistor calculation comes next. Say you want 50mA through each one (real bright). That is the 3.5V (head room) / 50mA = 70 ohm = 68 ohm. So, for each series combination, you have a 68 ohm, 1/4W resistor.

    Wire these series LEDs including the series resistor in parallel combinations until you have them all done (50 (total LEDs) / 4 (LEDs in each series string) = 12.5 strings. What that means, is that one string will only have 2 LEDs, and the resistor will have to be changed for this. It will be 3.5V + (2.4V (LED) * 2 (LEDs missing)) / 50mA = 166 ohm = 170ohm.

    Use the diodes as mentioned, but eliminate the resistor for the stop light, and just put one in for the park light. I've done it just like this before, looks awesome! Don't forget the Nichia White LEDs for the backup lights. Agilent has some kick-*** LEDs too for red and green. Check out www.luxeon.com too!

    Hope this helps.

    Presslab

  2. #12
    Constant Bitrate
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    you can't "give" the leds more current. they will pull the most current that they need when you have no resistors attached. now my knowledge of electronics is limited, and i dont know if this is sounding right at all...but you would have to set up some kind of switch so that when you brake the leds switch to a power source without a resistor, enabling them to be their full brightness. you could accomplish this with a relay of some type.
    Sorry, but that is totally incorrect. You must limit the current flowing through LEDs. Manufacturers generally have a maximum current(typ. 15-40mA) allowable in the LEDs datasheet. LEDs (light emitting diodes) are as the name suggests diodes. If you connect them without a series resistor they will try to pass as much current as is available and be destroyed allmost instantly(at 12V). If you want several levels of brightness then run the with two different series resistors. Also it is fine to connect multiple series of 5 LEDs across 12V in parallel as long as each series has its own current limiting resistor. I'll give you an example of multiple chains of series LEDs being run at multiple brightness levels using two resistors per chain of 5 to change brightness levels. In the example the LEDs will be normally off, dull when the headlighs are on and bright when the breaks are on.

    To make the dull current 10 mA use the formula from my previous post.
    R= (Vin - V(total)forward) / Current
    R= (12-(5*2.0)) / 0.01
    R= (12-10) / 0.01
    R= 2 / 0.01
    R= 200 Ohms

    and a brignt current of 20 mA.
    R= (Vin - V(total)forward) / Current
    R= (12-(5*2.0)) / 0.02
    R= (12-10) / 0.02
    R= 2 / 0.02
    R= 100 Ohms

    So for half bright we need 200 Ohms and for full we need 100 Ohms per series of 5 LEDs.

    I'll edit this post and add a circuit diagram as soon as I have drawn it in crappy paint. This will show more clearly how to wire this baby up.

  3. #13
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    You COULD use an adjustable regulator??
    Thats what I would do...

  4. #14
    Retired Admin Aaron Cake's Avatar
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    Originally posted by SkinnyBoy:
    <STRONG>You COULD use an adjustable regulator??
    Thats what I would do...</STRONG>
    That would be a bad idea. The 78xx series of regulators (which is what I assume you are referring to) can deliver 1.5A of current, enough to easily fry a bunch of LEDs. LEDs need a current limiting resistor to prevent them from drawing excess current, overheating, and exploding. While a voltage regulator can be configured to function as a constant current source, resistors are easier, cheaper, and much simpler.
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  5. #15
    Constant Bitrate
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    Thanks Aaron. It seems difficult to get this point across for some reason


    It helps if you spell the moderators name right.

    [ 08-10-2001: Message edited by: Technics ]

  6. #16
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    easier, yes, cheaper, maybe, I would personally use the regulator, because as the cars battery voltage rises and falls, so would the briteness of the leds...

  7. #17
    FLAC
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    Originally posted by SkinnyBoy:
    <STRONG>easier, yes, cheaper, maybe, I would personally use the regulator, because as the cars battery voltage rises and falls, so would the briteness of the leds...
    </STRONG>
    you probably would have the same scenario even using a regulator. A resistor is much cheaper, much more efficent and easier for most people

  8. #18
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    you probably would have the same scenario even using a regulator
    Not so...

  9. #19
    Constant Bitrate
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    You would need a constant current source not a regulator for driving LEDs anyway. LEDs don't require a fixed voltage they require a fixed current.

  10. #20
    Retired Admin Aaron Cake's Avatar
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    In a car, I would be very worried if the voltage dropped enough to significantly effect the brightness of resistor-driven LEDs. Would look like a serious problem with the alternator, forcing you to pull over anyway. I bet if you look at the cars produced with LED taillights today, they simply use resistors.
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