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

25 Leds, 9V battery, which resistors to use?

  1. Feb 15, 2010 #1
    I'm wiring together 25 rgb leds, each take 3V and I was wondering what would be the most efficient way to wire up as many as possible off one power source using resistors. Which resistors should I use? Series or Parallel? 9V or 12V battery?

    This is for a small project I'm working on myself.
    I know this is a noob question but I'm just starting phys 2 in college.

    Thanks for your help
  2. jcsd
  3. Feb 15, 2010 #2


    User Avatar
    Science Advisor

    You will need to find out a few things first.

    RGB Leds.
    So, 3 leds (red, green, blue) in each package? Do these have a common anode or a common cathode?
    If so, then there is no chance of having LEDs in series.

    They would all have to be in parallel.

    Next, do you know how much current each LED is supposed to get? It may be different for the different colours to get similar brightness from them.

    Last question, do you know how you intend to drive them? With a computer?

    I know a lot of questions make you want to run away, but please work through them. They are essential to get a proper answer for you.
  4. Feb 15, 2010 #3
    The red green and blue are not individual leds, they are together in one led, each led changes from red, to blue, to green after a certain amount of time.

    When you say drive, i supposed you mean power? If so, then I will power them with a 9V or 12V better.

    And they each require 3V.

    And questions dont scare me, the faster i answer, the fast i get to start soldering :)
    Last edited: Feb 15, 2010
  5. Feb 15, 2010 #4


    User Avatar
    Science Advisor

    How many wires come out of each LED?

    Do you know how much current the LEDs are supposed to use?

    If you just want to try one, get a 220 ohm resistor and put it in series with one of the LEDs and put the combination across 6 volts. This should give a safe current of about 13 mA.
    If you only have 9 volts try a 390 ohm resistor.

    It will probably only work one way, although I have some LEDs that give a different colour if the polarity is reversed.
  6. Feb 15, 2010 #5
    I have no idea how many mA they can take. I bought them off ebay, and there wasent much information given. Is there other way to find out the max?

    There are two wires coming off each led, annode and the cathode. And about the resistors... ill have a buch of them, but ill check the ohm and get back to you
  7. Feb 15, 2010 #6


    User Avatar
    Science Advisor

    No, I don't think you can work out a maximum current (except by destroying a LED and then not using that much current again !)

    Assume 20 mA is safe.

    So you could work out the resistor value as follows:

    R = (Supply voltage minus 3 volts) divided by 0.02

    eg 12 volts - 3 volts = 9 volts.
    9 volts divided by 0.02 = 450 ohms
    so you would buy a 470 ohm resistor.

    Try this with one resistor and one LED first to test the LEDs and see what they do.

    It may be possible to put 2 or 3 LEDs in series with one resistor.

    If you had two LEDs in series the resistor would be:
    R = (Supply voltage minus 6 volts) divided by 0.02

    If you had three LEDs in series the resistor would be:
    R = (Supply voltage minus 9 volts) divided by 0.02
  8. Feb 15, 2010 #7
    Sounds good. I had a bunch of two types of resistors and i believe one of them was a 470, ill check when I go over to a friends house tonight, we are working on it over there.

    So i would only get 4 leds with one 12V?
  9. Feb 15, 2010 #8


    User Avatar
    Science Advisor

    No, you need some voltage across the resistor. At least 25% of the total voltage. The smaller this resistor is, the more sensitive to supply voltage the LEDs become.

    Maybe 3 LEDs and a 150 ohm resistor with 12 volts.

    Resistors are cheap. They cost a few cents each.

    LEDs will be destroyed instantly if you put them across any source of power (greater than their turn-on voltage) without a resistor. You get a bright flash and then nothing.
  10. Feb 15, 2010 #9
  11. Feb 16, 2010 #10


    User Avatar
    Science Advisor

    There are several online design tools to help with this. Here are a couple:

    Thanks for those.

    The second site seems to do a better job.

    The first one gave 4 LEDs and a 1 ohm resistor in series when I asked for 4 LEDs @ 3 volts each and a 12 volt supply. This is not good design.

    The second one saw the hazard and designed it with two strings of two LEDs and a better choice of resistors.
  12. Feb 20, 2010 #11
    I am a bit confused. How come all LEDs take the same voltage. I thought blue ones always have a higher gap voltage. Are you sure that the resistors are not build in.? Do you have an ampere meter? If you connect 3V and see something between 10mA and 25mA then the resistor is build in. In that case you could just wire them in 3 parallel chains on one 9V battery or four on a 12V (I assume you want to make some colored ambient light and not some changing pattern). But a normal 9V block would be empty quickly. They deliver maybe 2500mAh afaik, so they are empty within 5 hours. Or a third of that if you make white light.
  13. Feb 20, 2010 #12
    He's right. Not all LED's have the same internal bias.

    Check out this http://www.sparkfun.com/commerce/product_info.php?products_id=9264".

    The wiring diagram should be useful.

    By the way, a little internet sleuthing should turn up a data sheet. Do the arrays have any numbers written on them? Better yet, do you know who made 'em?
    Last edited by a moderator: Apr 24, 2017
  14. Feb 20, 2010 #13


    User Avatar
    Science Advisor

    These are supposed to be RGB LEDs but with only two wires. I am waiting to see how they do this.

    I have some that are parallel red and green LEDs that conduct in opposite directions.
    So, you can get Red with one polarity and Green with the other or Yellow with AC.

    But I haven't seen any that can do RGB with only two wires.
  15. Feb 21, 2010 #14
    You must have missed post #3...

    It's a little misleading to call them "RGB" LEDs -- maybe "color changing" LEDs.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook