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Led & resistors

  1. Sep 17, 2008 #1
    hi everyone, im new to this forum, hope im posting a thread
    in the right place, if not could any MOD please place my thread
    correctly... thanx

    guys i need to know how to connect my resistor to a LED
    i have 12 white leds 5mm that is 3.6v each @ 30mA
    im using a 12V power supply for it, i have 12 resistors
    for them that are 390ohms and 0.25watts its band colour is of
    1. orange
    2. white
    3. brown
    4. gold

    the assistant at the electrical shop told me these are the best for my leds, but i forgot to ask him how should it be connected, i have read that you connect the resistor to the positive pin of the LED, but i have read on the net its best to connect all of them
    in series,but i have not seen any diagrams, could any one please help me with a diagram or even verbal explanation on how i could connect all the led with a resistor each in a series manner...

    thanx guys... ;)
  2. jcsd
  3. Sep 17, 2008 #2


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    I moved your question here, as this is an electrical engineering thing.

    The resistor serves to limit the current in the LED. A LED doesn't follow Ohm's law, so if you connect a 12 V source to a LED, the current would be too high, and you'd burn it.

    You need to connect the LED in series with the resistor, but a priori it doesn't matter whether you do this on the anode or the cathode side. A LED is a kind of diode, so it has an almost fixed voltage drop. A normal silicon diode has a voltage drop of about 0.7V (give or take), while for a LED, this can be of the order of 2V. (yours seem to be 3.6V)
    If you connect the LED + resistor in series to a 12 V supply, then 3.6 V will be taken up by the LED,and the remaining 8.4V will be the voltage drop over the resistor. If it is a 390 ohm resistor, then the current through the resistor will be 8.4V / 390 Ohm = ~22 mA and this will (series!) also be the current through the LED. You should look at the specs of your LED, but this looks like a good LED current - it is a bit below the rated current of 30 mA (in other words, the guy who sold them to you was right).

    So you should cable each led in series with its own resistor, and these series should be put all in parallel, each connected to the 12 V supply.
  4. Sep 17, 2008 #3
    thanx so much, so would i connect it as such

    all 12 of the led with resistors?, see my attachment ;)

    Attached Files:

    • 1.bmp
      File size:
      163 KB
  5. Sep 17, 2008 #4
    connect all your LED in series . and ech LED should have each resistors.ie:- connect the resistors across LED depending on the range ..Quite a few of the answers were along the lines of "Wire each one with a resistor, you'll get better results." I've read claims that without a resistor, voltage spikes will burn out the diodes. It seems that a resistor for each LED is by far the more preferred method. Why?

    What I don't understand is that if the correct number of LEDs are strung together in series to produce the desired voltage drop (ie. resistance) for a given suppy voltage, then they will act the same to voltage spikes as a single LED and resistor, correct? At least that's how I understand Ohms Law. In both cases, a sufficient voltage spike will burn out the diodes in both circuits.
  6. Sep 17, 2008 #5
    ok i do understand that sha_virgo
    as mentioned you said connect in series, i have posted an attachment as a sample
    of what i am speaking off, so would i be correct in doing this as illustrated by my diagram?
    would this be the correct connection ?
  7. Sep 17, 2008 #6
    like this ?

    Attached Files:

    • 3.bmp
      File size:
      118.3 KB
  8. Sep 17, 2008 #7
    Yes, that is the correct connection.
  9. Sep 17, 2008 #8
    thanx bolts

    thanx guys, i will continue with my circuit
  10. Sep 17, 2008 #9


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    The reason is that if you, say, double the voltage on a led with a resistor, you will more or less double the current, as the dominant voltage drop is over the resistor, and the resistor follows Ohm's law. However, if you have a voltage spike over a chain of diodes (leds), then the current will rise exponentially (as the voltage-current law in a diode is not Ohm's law - a proportional relationship between voltage and current - but rather an exponential). So you can have, say, 20 times the current upon doubling the voltage. This is why it is always a good idea to limit the current in a diode (a led) with something like a resistor.
  11. Sep 17, 2008 #10


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    You generally only connect LEDs in series if they are relatively well matched (ie, sorted) in their brightness versus current characteristic. If you buy them as a set meant to be connected in series, then you can connect several in series and use a constant current power supply, or use a single resistor and a voltage source.

    But if you are buying jellybean LEDs from the store that are not sold as a matched set, then you will generally need one resister per LED, with them connected in parallel to the voltage supply as in your 3.BMP figure. You wouldn't generally do that with a 12V power supply, however, since that wastes a lot of power.
  12. Sep 18, 2008 #11
    would connecting lets say 4 leds in series with a 3v each led rating to a 12v power supply work?
    as the 4 led with 3v each gives you = 12v
    would this work without the use of a resistor?


    ive seen this site and it shows it can be done, is this right and safe to do?
  13. Sep 18, 2008 #12


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    See posts 9 and 10. The question is: what determines the current in the LEDs now ? Answer, it is the steep exponential V-I relationship of the diodes. Now, of course, steep doesn't mean, vertical, so there will be an operating point which might have an acceptable current. But you are very sensitive to the overall voltage: the current will rise exponentially with voltage, and the a priori current for a given voltage will depend strongly on the V-I characteristics of the diodes, which might have larger tolerances than those on resistors. In other words, it is not clear from the start whether you will get 15 mA or 78 mA when you design such a thing.
  14. Sep 18, 2008 #13
    which would be better to connect? i have 13 leds to connect across a 12v supply
    would it be best to connect them in series or parellel? i read that parellel allows you to add more leds on a single voltage like if you have 3 leds of same spec each requires 3v each and you have a 3v power supply then connecting parrellel would allow each led
    3v each but would drain the power supply quicker,but this wont be a problem as the power supply is constant and not a battery? which would you suggest to be better for me to connect?
  15. Sep 18, 2008 #14

    i understand, ok so i have blue and white leds , both of the same specs only the colours differ, does white require more power to burn than blue?
  16. Oct 10, 2008 #15
    guys to which wire do we connect a switch?

    the positive or negative?
    i have the resistors connected to the positive...

    please let me know .... thanx
  17. Oct 10, 2008 #16


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    You may connect the switch to either the positive or negative side of the power supply.
  18. Oct 10, 2008 #17
    thank you so much
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