Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

NOOB QUESTION: What kind of resistor do I need?

  1. Jun 3, 2012 #1
    First off.. this is my first attempt playing with this kind of stuff.. please have a bit of patience with me :)


    I'm looking to make a costume out of LEDs for burningman.. bought a 1000 of them off ebay and when I connect them the 9V battery i'm guessing is too strong for it and burns the LED out.

    is it possible to run say 500 LEDs off of 1 9V battery?

    These are the specs of my LEDs (which is another language to me)



    WLD : 465-470 NM

    IV: 8000-10000 MCD

    VF: 3.4 -3.6V



    please advise. thanks :)
     
  2. jcsd
  3. Jun 3, 2012 #2

    Bobbywhy

    User Avatar
    Gold Member

    MikieCGY, Welcome to Physics Forums!

    It appears you have the blue LEDs made by China Young Sun LED Technical Co., LTD. Their model (part) number is YSL-R1042B5C-D13. The specification sheet is available at:
    http://www.sparkfun.com/datasheets/Components/LED/Blue-10mm.pdf

    The LED’s output wavelength, intensity, and forward voltage you mentioned were measured with a current of 80 milliamperes flowing through it. The suggested operating current is 65-75 mA, which would consume 180 milliwatts of power.

    Using your 9 Volt battery, to energize one LED you would use a 120 ohm resistor in series with the LED (between the battery and the LED) to limit the current to around 70 mA.

    No, you cannot run 500 LEDs off one 9 battery! A typical alkaline 9 V battery has the capacity of 565 mAhr, meaning it can supply 565 milliamps for one hour. So if you connected eight circuits like the one described above in parallel they would operate for about one hour. You will have to either use bigger batteries or fewer LEDs, or both. Have a good time!

    p.s. I went to Burningman in 1982 and had a fantastic experience! I wore a ball cap with a solar cell powering a tiny dc motor with a plastic propeller on top.
     
  4. Jun 8, 2012 #3
    Thanks for the quick reply Bobby! Also going to a hobby shop today in hopes of buying some resistors instead of buying them online..

    does each LED need its own resistor?

    How many LEDs can one 9V battery power?

    Can you suggest a different power source?


    Looking forward to the burn.. its definitely grown since 1982 ... been wanting to go for a fe years now and finally going this year

    I'm pretty crafty but my electronic knowledge is just god awful :(


    thanks!
     
  5. Jun 9, 2012 #4

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    You could run LEDs in pairs, connected in series with a 30Ω resistor from 9V, that would be more efficient on your power. You would get twice the bang per buck? Each pair would take about 70mA.
    250 pairs would need 17A. That represents 17X9 = 150W+, which would be totally daft and would fry you, even if you could carry all the batteries needed! You wouldn't need anything like as many as that. You could get a good show with twenty pairs, which would draw 1.4A (12w).

    Actually, you would be better to experiment with just how little current you could get the LEDs to light up at all then use more. That would depend upon the exact behaviour (rather than the published spec) of the LEDs.
    AS a power source, six D type cells would be better than 9V batteries and you can get holders for them. You would be well advised to use a fuse in the circuit - just next to one end of the battery pack. D type cells hold a lot of energy and you don't want to be dressed up in a potential crematorium if you get a short circuit somewhere!

    To save on current drain it may be worth while pulsing the diodes and accepting the reduced brightness so that you can use more light sources all over you. But you'd need an oscillator / switching circuit, which may not be straightforward for you.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook