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Thread: Help with LED resistor

  1. #1
    Unenlightened
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    Jul 2019
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    Default Help with LED resistor

    Thanks in advance for any replies. I have searched here and the internet over and over again and have not found an answer I understand. I am trying to power a single 3w, 2.2FV, 700ma cob LED with a car battery system. So I imagine 13.8 volts when running. I have used the resistor calculator and it says use a 18 ohm 7.4v resistor. Is this true and do they make such a thing? I cant find anything close. I initially used a 330 ohm resistor to test because I read that somewhere and the resistor burned up. Please help me if you can.

  2. #2
    Flashaholic* snakebite's Avatar
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    Default Re: Help with LED resistor

    you need a buck converter.
    a resistor will waste lots of power as heat.

  3. #3

    Default Re: Help with LED resistor

    Quote Originally Posted by auotnoz View Post
    Thanks in advance for any replies. I have searched here and the internet over and over again and have not found an answer I understand. I am trying to power a single 3w, 2.2FV, 700ma cob LED with a car battery system. So I imagine 13.8 volts when running. I have used the resistor calculator and it says use a 18 ohm 7.4v resistor. Is this true and do they make such a thing? I cant find anything close. I initially used a 330 ohm resistor to test because I read that somewhere and the resistor burned up. Please help me if you can.
    You could make your own wire wound resistor using normal wire. You just need a meter to measure its resistance once you get it wound. This would most likely be the most cost-effective way of doing it since higher wattage resistors can be quite pricey. The downside to this would probably be a very long coil of wire. You might be able to use a smaller gauge wire and thus a shorter length as long as its not so small that it becomes a fusible link.

    I would agree that using a driver board would be a better solution but only so that you have a better power supply to your LED rather than direct driving it where a sudden spike or reverse polarity would likely burn it up in seconds.

    Also I'm not sure where you are located but that might be part of the problem. I found an 18 ohm resistor easily in my Google search.
    Last edited by lightfooted; 07-07-2019 at 01:45 AM.

  4. #4
    Enlightened Genes's Avatar
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    Default Re: Help with LED resistor

    If you are dead set with using a resistor, you will need an approximately 17 ohm, 8 1/4 watt; both values are odd and not readily available. Resistors don't generally have voltage ratings. Your best choice would be a 20 ohm, 10 watt resistor, and it will get very hot. It will melt plastic or insulation that it comes in contact with. Your very best choice would be a buck converter to drop the voltage from about 14 volts to 2.5 volts or so. Your cob probably has a maximum rating of 3 watts, but 2.2 volts at 700 mils works out to be a little over 1.5 watts.

  5. #5
    Flashaholic
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    Default Re: Help with LED resistor

    Using a dropping resistor in high power LED circuit might be OK in a limited set of cases but this is not one. Voltage drop required and power
    loss is just to large; you might as well use an incan. light.

    Best to use switching down-converter preferably with settable constant-current output. You can build your own starting with suitable parts,
    or buy a pre-made module; many options out there. Just one part is PT4115 driver IC, common in automotive (auxiliary) LED lighting.

    Dave

  6. #6
    Flashaholic
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    Default Re: Help with LED resistor

    An experiment I did was to modify a low-cost 12v to USB (5v) lighter plug charger to drive two 1W LEDs in series (about 300mA) in constant-current mode. It might have worked with three LEDs.

    Original circuit output 5v at 1A (which I sort of doubt by how hot the converter IC was getting with less than 0.5A load).

    The chip 1.25v feedback was shunted with a 4-ohm 1W resistor to ground, which resulted in constant current of about 0.31A. I had two 1W LEDs in series working. The efficiency is not too bad, way better than a dropping resistor or linear regulator.

    Something easy might be to bring 5v output from 1A or 2.1A USB charger, then use a dropping resistor in series with the LED. Overall efficiency will still be only about 50%, but much better than dropping 14v to ~3v which is about 20% efficient. For LED at ~700mA I would try nominally a 3.3 ohm, 2W or higher resistor, or 2.7 ohms.

    Dave

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