Resistors for changing RGB LED array

Discussion in 'The Projects Forum' started by syarb002, Jan 6, 2009.

  1. syarb002

    Thread Starter New Member

    Jan 6, 2009
    Hello all,

    I am new here, looking for some help on a project I am working on. I am building an array of 16 color changing LED's.

    Specs on the LED's:

    The forward voltage is listed as 3.8V-4.5V. I'm assuming that this varies with the color produced? Anyway, I am trying to determine a safe voltage value to use in calculating what resistors I am going to need. I'm either going to use one resistor for each LED, or one resistor per 2 LED's. My design current is 20mA, and input voltage will be 6V from 4 "AAA" batteries. The circuit is parallel.

    Can anyone help me? Thanks...
  2. bertus


    Apr 5, 2008
  3. SgtWookie


    Jul 17, 2007
  4. syarb002

    Thread Starter New Member

    Jan 6, 2009

    For some reason I thought I had downloaded that and looked through it...must have been thinking of something else.

    I am not at all an electronics expert, but if I was to still use a 6V supply voltage, setting up in parallel with one resistor per LED, I would say:

    (6V-4.5V) / 0.09A = ~ 16.6 Ohms


    Or...using a 9V supply, connect two LEDs in series with one another with no resistor?

    (9V - 2(4.5V)) = 0
  5. SgtWookie


    Jul 17, 2007
    The specs call for a 4.5v supply. If you want maximum life from your LEDs, you should stick with that.

    As far as running them in series, see the schematics in the datasheet. If one burns out, you'll still want the other one to continue running, right? The suggested circuits will do that for you.
  6. syarb002

    Thread Starter New Member

    Jan 6, 2009

    Thank you for the advice. I will stick with 4.5V and use a 3 "AAA" battery input. If I do that and set up the LEDs in parallel, that would mean no resistors needed as there is no additional voltage to "resist", right?
  7. Alberto

    Active Member

    Nov 7, 2008
    The answer is yes, but bear in mind that two leds in parallel will double the current needed. so when you parallel you must multiply 90 milliamps times the numbers of leds you are paralleling and than check how much current your battery can supply. You must not exceed this value and this is the limit to the paralleling.

  8. eblc1388

    AAC Fanatic!

    Nov 28, 2008
    Don't think of it as a normal LED.

    It is a custom MCU built specifically for switching three color LEDs in sequence. The 4.5V is supply voltage for the MCU.
  9. Audioguru

    New Member

    Dec 20, 2007
    Three AAA alkaline cells are 4.5V only when they are brand new. The voltage quickly drops to 3.6V when they are used. Then the LEDs will quickly become very dim.

    Use four 1.25V Ni-MH rechareable cells that are 1.25V x 4= 5V for most of a charge and add a diode in series to drop the voltage to 4.3V.