Different colour LEDs series & parallel resistance amount?

Discussion in 'General Electronics Chat' started by jackofalltrades, Oct 21, 2015.

  1. jackofalltrades

    Thread Starter New Member

    Oct 21, 2015
    Hi, I'm just getting into electronics and I'm attempting to make a small LED lamp powered by a 300mA 9v DC power plug - and I've run into a little problem.
    I've been following some instructions, and they basically stated that their LEDs are all the same colour/voltage, just with a different colour coating (i.e. white LED with red, blue plastic bit.)

    However, I'm using actual different colour LEDs with a clear plastic. They have different forward voltages, I'm not sure how to do the resistance because of the red - blue - red LEDs in series?

    Forward Voltages max
    Red 2.0
    Blue 3.2
    UV 3.2

    I've attached a picture of how the schematic looks in the tutorial:


    Thanks for any help you can give, I really appreciate it!
  2. Wendy


    Mar 24, 2008
  3. ericgibbs

    Senior Member

    Jan 29, 2010
    You add up the LED colour voltages in the series network, then subtract that value from 9V.
    This gives the voltage that you must drop across the series resistor.
    Divide that voltage by the LED current required by the LEDs.

    Typically this LED current is approx 15mA to 20mA, for regular LED's, choose 15mA.

    So for your top chain, Rd,Bl, Rd thats 2.0 + 3.2 +2.0 = 7.2v
    Subtract 9v - 7.2v = 1.8v
    Resistor value 1.8v/0.015A =120 Ohms.


    BTW: you MUST have a series resistor with an LED in order to limit the LED current, else it will fail.!

  4. ScottWang


    Aug 23, 2012
    I am assuming that the LED are all rating for 20mA, and just using 80% of 20mA, that is 16mA.
    You could try to calculate the R values.

    mcgyvr likes this.
  5. jackofalltrades

    Thread Starter New Member

    Oct 21, 2015
    I can't thank you all enough, the penny has dropped! I understand exactly what to do now, and should (hopefully haha) be able to do it in the future.

    Thanks again