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Thread: Resistor for led on pos or neg terminal?

  1. #1

    Resistor for led on pos or neg terminal?

    Hi, I just got a 3 watt 80 luman led that I hooked up which works great but the only possible problem I just noticed is that when using a online led calculator some show that the resistor goes on the positive line and some show that it goes on the negative side. I hooked it up on the negative side as that was the first diagram I came across when doing this, wasn't till later that I noticed both ways are shown. It works now but I just don't want to burn it up if should be on the pos side.

    Which is correct? the pos or the neg side?


    Led Specs:
    3 watt
    80 lm
    3.4 - 3.6 v
    700 ma


    Voltage source:
    5 volt


    Resistor:
    2 watt 2.2 ohm


    Thanks

  2. #2
    The Main Man Dewayne's Avatar
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    Resistor for led on pos or neg terminal?

    Positive lead. From what I understand it will reduce the voltage before it gets the LED.

    Our LED calc shows it that way too.

    http://www.modders-inc.com/modules.php?name=Tools&op=LEDResistorCalculator

  3. #3

    Resistor for led on pos or neg terminal?

    Thanks AF. After reading about it the pos side does make more sense which is what I would expect it to be. At this point it is working on the neg side but I will switch them later and see what happens.

    Still weird how I am getting a 50 50 ratio for the pos and neg connections from online led calculators.

    In any case I will switch it and let you know how it turned out.

  4. #4
    The Main Man Dewayne's Avatar
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    Resistor for led on pos or neg terminal?

    Knock on fusionfun's door as he is a electronics expert around here. But truthfully I am not too sure if it really matters as it is DC voltage and you are dropping the voltage on the entire circuit either way. But it does make more sense to me to drop it for before it gets to the device. It might make it last longer that way.

  5. #5

    Resistor for led on pos or neg terminal?

    I just switched from neg to pos side and it seems to be no difference but I am going to leave it on the pos side as that makes more sense.

    On a side note, led's this strong require a heat sink and even using an old video card circle heat sink everything still gets very warm almost hot to the touch. These are some strong sonna beeches.

  6. #6
    Clear Coat
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    Resistor for led on pos or neg terminal?

    It it doesn't matter which side you hook it up on. Just be consistent so you don't make a mistake and leave one out.

  7. #7
    Super Moderator tech-daddy's Avatar
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    Resistor for led on pos or neg terminal?

    Agreed to all, it does not matter. I am, however, on the same side of the argumetn that "it just makes more sense"... so I default to my survival instincts on this!

    Positive side.

    EE types are probably giggling at my superstitions...

  8. #8

    Resistor for led on pos or neg terminal?

    Thanks for the feedback guys.

    Just be consistent so you don't make a mistake and leave one out.
    It is just a single led that I hooked up in my computer on a switch located in the back so when I am doing something in the case I don't need to grab a flashlight. I have it spliced into a usb cable for power that I keep plugged into the back of the case. I have the usb cable running out of the back of the case through the water cooling hose holes so it is not seen.

    Although after doing this and using it a couple of times I find it extremely helpful and am not sure why they don't build cases with a switched light inside them.

  9. #9
    Clear Coat
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    Resistor for led on pos or neg terminal?

    EE types are probably giggling at my superstitions...
    Are you KIDDING? I hook up resistors in the same DIRECTION! Though that may be more OCD than superstition...

    Although after doing this and using it a couple of times I find it extremely helpful and am not sure why they don't build cases with a switched light inside them.
    Sounds like a great idea, I usually just prefer grabbing one of the many flashlights I have instead though. (flashlight hoarder) It makes me feel slightly valid in having 4-5 flashlights within easy reach of my desk.

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