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12VDC to 1.7VDC and 2.4VDC

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  • 12VDC to 1.7VDC and 2.4VDC

    I am wireing LEDS to a 12VDC source..The leds are rated 1.7 to 2.4 VDC..anyway to convert this? There will be 50 LEDS wired together...I need help
    -Dan

  • #2
    LEDs can run fine off 12V. The voltage ratings are the forward voltage required for the diode to conduct. The only issue is selecting the right current limiting resistor(this must be wired in series with the LED) for the input voltage. I'll dig up the formula for calculating it for you(I can't remember every formula in my head. We can't all be that good damnit).

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    • #3
      wire a few in series, and WHAT do you need 50 for???

      IE:

      + -0-0-0-0-0-0- - == 12v stable

      Else if you want to wire them like this;

      + -0- -
      + -0- -
      + etc -

      you would need to cut the voltage line down to 2v. Try using a few diff resistors with a volt meter till you find one that works and cuts it down..

      BTW, I said this before, but WHAT do you need 50 for???

      Scott--
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      • #4
        Here is the formula. You will need to know what drive current the leds will require(20mA is a safe bet usually).

        R(Ohms)= (Vin-Vforward) / Current in Amps

        for for an LED with a forward voltage of say 2.2 Volts and a drive current of 20mA running of 12 Volts it would be:

        R= (12-2.2) / 0.02
        R= 9.8 / 0.02
        R= 490 Ohms

        So you could use a 470 Ohm resistor (490 Ohm resistors don't exist. You may need to use the next highest value resistor as using a lower value will increase the current. It's unlikely a slight increase in current will damage the LED). The circuit is like this:

        ->|- is the LED
        -/\/\/\- is the resistor

        +12V -------/\/\/\------->|---------GND

        I hope this helps

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        • #5
          I wouldn't recommend connecting LEDs in series straight to 12V. If you do it this way the current through the LEDs will increase greatly when the engine is running (battery up to 13.8V or higher). The recommended way to connect LEDs in series is the add up the forward voltages of all the LEDs and use the same formula I gave earlier (with the total forward voltage) to get a value for a series resistor. It is recommended when using LEDs in series that the total forward voltage be a maximum of 80% of the supply voltage as a general rule. This means you will only probably get away with 4 or 5 LEDs in each series.

          so for 5 20ma LEDs each with a forward voltage of 2.0 Volts it would be.

          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

          This would be wired like this:

          12V----/\/\/\--->|--->|--->|--->|--->|----GND

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          • #6
            Originally posted by DarkWolf:
            <STRONG>wire a few in series, and WHAT do you need 50 for??? </STRONG>
            maybe building a new spoiler taillight assembly....?

            Project - GAME OVER :(

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            • #7
              Maybe we will never know...
              I'm sure we will though...

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              • #8
                They are just for some tailights on a blazer.. I will run the LEDs in a series with one resistor per 5 but one more question....If i give the Led more Milla amps will the leds get brighter? Cause i gotta have them get brighter when i hit on the brakes

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                • #9
                  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.
                  Athlon 500, 128 MB, ATI Xpert 4MB AGP TV-Out, Sound Blaster Live! Value, 8.4 GB, 2x20 Backlit LCD, Keypower 250 Watt ATX DC-DC, DSchmitd Startup/Shutdown Controller, IRMan + Backlit IR Remote, Custom Keypad, 5.6" NTSC LCD, In-Dash Slot Loading DVD, all installed in an '87 Honda Accord LX-i

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                  • #10
                    Yes I know...On my car there are 3 wires...ground, postive light, brake light. So...correct me if i am wrong...but ((connect 5 leds with one 100 resitor)*10) and connect a diode before the resistor and then connect it to the postive light wire...Then I would do the same thing but put a lower resistor like 50...and hook that up to the same 5 leds like this... Here is a pic to explain what i am saying and what i want to do...

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                    • #11
                      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

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                      • #12
                        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.

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

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                          • #14
                            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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                            • #15
                              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 ]

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