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  1. #1
    Senior_Member2 diff_lock2's Avatar
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    3watt led Tail light.

    I just ordered a red 3watt led from my local electronics store (what would you call it). It should be here next week. Well since i know so little i asked the guy about how i should power this thing.

    He said that current limiting is more important that voltage... WHAT?

    Anywho he sugested i use this:


    I was thinking i could use a 3 way or 2 way switch and have like a high/low setting thing going on. Like 300ma and then like 600ma or something. the led can take up to 1a i think. Once i get the led i will see how bright i want it.


    Questions: If i use a 4.8v batt pack rated at like 2500mah and set the regulator (using that resistor) at 250mah, will i get like 5hours run time, and will the led light at all? I really don't get how you can make 4.8v work on a 3.5v led, with that contraption. the guy said it should work fine.

    Is he right?

    EDIT: i have no idea how to read that simple formula but i think that Vref/Iout=R1 am i correct?

  2. #2
    Senior Member Zero_Enigma's Avatar
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    Umm.. here is a suggestin seeing as I can't read the formulas. -.-; http://www.taskled.com and look at the Nflex or Bflex regulators. From what I've read before resistors are not that effeicent and build up heat. A Nflex is $30 and gives you 350/500/700/1000mA modes. It also has a built in User Interface Bike mode (UIB) which you can set it. The unit also has a programmable low voltage warning all built into one small unit.

    This page can serve as a guide to your build even tho you're building a rear taillight. I believe the nflex or bflex has a min. voltage of 4v. They are buck regulators meaning you give it more voltage and it chokes it down to the needed voltage for the LED if I am explaining it right.
    Zero_Enigma

  3. #3
    Senior_Member2 diff_lock2's Avatar
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    I think my circuit does the same thing, its just 2$... I will see. I got a text message from the Store telling me they can not get 3w reds. So i made an offer of getting 3 1w reds...

  4. #4
    Zoom zoom zoom zoom bonk znomit's Avatar
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    You just need a resistive drive, nothing fancy.
    Find out the Vf (forward voltage) of the LED. Probably about 3V. Subtract from the max battery voltage 4.8v.
    So after your LED has taken 3V you have 1.8V left over. You need to select a resistor that will allow 600mA to flow are 1.8V.
    V=IxR so V/I=R... 1.8/0.6 = 3ohm.
    Power = IxIxR in this case 1watt. Go but a 3ohm 1watt resistor, then buy another and a couple of switches.

    Simply circuit:
    Batt---------------LED-----------------------R-------------
    |............................................................................| Switch
    --------------------------------------------------------------

    For diming you want to include the second R that you can short out with a switch. Include this between your circuit and battery. This switch puts the second resistor into the circuit doubling the resistive load and halving the current.

    -------------
    .....|..........|
    .....R........Switch
    .....|..........|
    ------------

    Ignor the dots!
    Last edited by znomit; 08-23-07 at 05:15 PM. Reason: edited to clean up ascii circuits

  5. #5
    Senior Member Zero_Enigma's Avatar
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    Zero_Enigma

  6. #6
    Senior_Member2 diff_lock2's Avatar
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    Well the update is, i can't find 3wat reds. So i got 2 1w reds and 1 lens for 13eu (ordered, will be here mon.) And i found a simple circut that was chip, the place had a deal on these 1.5A chips (more than enough) 40 cents a piece. At first i tried to make a HIGH/LOW switch and ended up with a busted fuse in my multi meter. Then i revised wtf i just built. It was a total mess. SO i started simple and make a fixed regultor, 250ma. That is 100ma less than what the LED's max is rated at. So i am guessing that i will save batt (extend run time) and keep the led's cooler.


    So the black one alone is the DC in and the other two rca's are the 250 regulated out puts. i have not wired up the negative terminals yet (ground?).






  7. #7
    Senior_Member2 diff_lock2's Avatar
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  8. #8
    Recumbent Evangelist
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    It would be easier, and probably just as efficient, to use a single resistor for current limiting. First, take a look at the LED's datasheet (I assume it's a Luxeon I): http://www.luxeon.com/pdfs/DS25.pdf

    You can see on page 6 that at 350mA, the LED will drop 2.95 V (typical). If you're running it from a 4.8V battery (4 NiMH rechargeables, right?), that means the resistor has to take 4.8V-2.95V = 1.85V. At 350mA, that means you will need a 5.2 ohm resistor (1.85V / 0.35A = 5.28 ohms). It will have to be at least a 1W resistor (1.85V x 0.35A = 0.65W). That's a lot of wasted power, so I suggest you either decrease your voltage to 3V, or increase it to 6V and run both LEDs in series.

    At 3V, you could probably get away with no resistor at all, since the difference is only 0.05V. But to be on the safe side, a 1 ohm, 0.25W resistor would be enough.

    The connection is simple, just wire up the resistor in series with the LED.
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  9. #9
    Senior_Member2 diff_lock2's Avatar
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    Well the point of those curent limiters was to make the batt last longer. I have no idea how they work, i just assumed that this way would be better. I guess not. Initially i wanted to run 3 batts and a low ohm resistor all in series. I will see how long my lights will run at 300mA each (total of 600) with these limiters.

  10. #10
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    what will kill an LED is overvoltage. for instance if a 12v battery really puts out 13.1-12.1 volts for the first 10 minutes, you have a 12v LED, that LED will not last 100,000 hours, more like 10,000 hours or less. you gotta have a way to regulate any overvoltages to exactly, or less, than 12v. that is about the only thing to get worked out with LED lights. so just make sure to build a circuit that limits the volts and you'll get long life out of the LED. also don't forget to heat sink high wattage LED's. high heat will kill them too.

  11. #11
    Senior Member intrepidbiker's Avatar
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    I just want to make sure that I'm not screwing up my lights with my wiring. I've got a 10 watt bulb and a 20 watt bulb in parallel and a 3 watt, 12 volt LED taillight set up to turn on when either of the headlights is turned on.

    I'm using a 12volt, 5ah SLA battery. I don't need any kind of special regulator do I?

  12. #12
    Recumbent Evangelist
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    Quote Originally Posted by intrepidbiker View Post
    I just want to make sure that I'm not screwing up my lights with my wiring. I've got a 10 watt bulb and a 20 watt bulb in parallel and a 3 watt, 12 volt LED taillight set up to turn on when either of the headlights is turned on.

    I'm using a 12volt, 5ah SLA battery. I don't need any kind of special regulator do I?
    It all depends on how you have them hooked up. Do you have pictures or a diagram of your design?
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  13. #13
    Senior Member intrepidbiker's Avatar
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    Well, no. Sorry. I'll try and make one and upload it. I had a guy from radio shack look at it and he said it was okay... but I have gotten bad advice from Radio Shacks before.


    Verbal description.
    Positive battery terminal has a wire with a 5 amp car fuse attached. The positive wire travels to a 3 way switch with 6 prongs. A wire connects a "on" prong to a headlight (two headlights total). The negative wires on the headlights are connected to one another and return to the battery via one negative wire.

    My taillight's positive wire goes to the switch and is connected to two prongs (the ones opposite to the headlights). The taillight's negative wire goes out and meets the negative wire from the headlights and returns to the battery.

    That make sense?

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