What Size resistor to use?

Discussion in 'The Projects Forum' started by Derek Emerson, Mar 16, 2016.

  1. Derek Emerson

    Thread Starter Member

    Mar 16, 2016
    Hi All

    I'm sure this will be an amazingly simple question for someone to answer, however as a hobbyist in electronics I am looking for some clarification.

    I have a 2 pin bi-colour LED (Red/Green) and will be running it from a PP3 battery (9v). I simply want to be able to put the led on the battery one way for one colour and the other way round for the other colour. My question is this "What size resistor sahould I use, the LED I will be using is this one:


    Kind Regards

  2. GopherT

    AAC Fanatic!

    Nov 23, 2012

    To calculate this, you need battery voltage = 9V
    Forward voltage drop of diode = 2V
    Current max of LED = 25mA (so we will use 10 mA or so)

    (9V - 2V) / 0.010A = 7V / 0.010A = 700 ohms.

    If you want 20 mA, you can calculate that easily now. Or for 5mA

    Anything in the range of 390 to 1500 should be ok (adjust to your desired battery life and brightness).
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    Just to add a bit..
    resistance calculation is
    (Vb-Vf)/I = R
    Vb=supply voltage
    Vf=LED forward voltage rating
    I = desired LED current in amps
    R= calculated resistance
    (9-2)/.010 = 700 ohms (using Gophers Vf and I numbers above)

    Then you also need to calculate for the min wattage of the resistor needed.
    So now..
    P = calculated minimum resistor wattage rating..
    2*(7*.010) = .14 watts minimum (I'd use a 1/4W resistor.. or .25W vs a 1/8W that will run HOT)
    Note: the 2* is a "safety factor".. You always want to double or triple the calculated wattage to avoid the resistor running too hot..
    And with wattage its always "minimum" having a higher wattage resistor is totally fine.. It won't effect LED current or anything.. Just takes up more space and run cooler the higher you go..

    To learn something more and allow you to remember these easier.. Check out this..
    Notice our calculations are based on those proven formulas.. (just adjusted some for wattage safety factor and the fact that an LED essentially "eats" some of the voltage dictated on its forward voltage rating..)
    P=E*I = wattage calculation
    R=E/I = resistance calculation
    GopherT likes this.
  4. Derek Emerson

    Thread Starter Member

    Mar 16, 2016
    Thanks for the help.

    I had worked it out at around 340 but as mentioned I just wanted some clarification.

    Thanks again, it is very much appreciated, you have all been most helpful.

    I'll take a look at the link and save it for future reference.
  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    It could be 340 depending on the Vf of the LED.. Or multiple LEDs in series