How to find ON current for a LED ???

Discussion in 'General Electronics Chat' started by 0by1, May 11, 2011.

  1. 0by1

    Thread Starter New Member

    May 10, 2011
    Hi all,

    For a LED having Vf=2V, If=20mA,

    >Whats the ON current for this LED??
    >Is that necessary to limit current to 20mA?? so much current !!!!

    Thanks in advance.
  2. #12


    Nov 30, 2010
    The MAXIMUM led current is "If", amps forward,20 ma.
    I run them at 10 ma because that's plenty to see them as an indicator light and probably increases their life span. They get pretty weak at 5 ma, but now you know how to treat them.
  3. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    Some have 10mA, others are 20mA, and still others are 25 or 30mA.
    High powered ones can be 100mA or 350mA and up to 1000mA.
    Plus lots of ones in between.

    The DATASHEET for the specific LED will give you the forward current (If)

    It is often recommended to aim slightly lower in designing the power supply circuit.

    Say 18mA, for LED's that have a 20mA forward current rating. This can increase lifespan of the device quite substantially.

    You MUST provide some means for limiting current. Series resistors are the usual method. Constant current configuration of voltage regulators is another.
  4. 0by1

    Thread Starter New Member

    May 10, 2011
    Thanks for reply,
    Now I am final stage....

    @ #12 - according to you 10mA is good for 20mA LED
    @ Kermit2 - according to you 18mA is good for 20mA LED

    How to decide that??
  5. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    Because 18 is LESS than 20. It really is that simple.

    Someone else may decide 15mA is what they want, and I choose 18mA. All part of the wonderful world of circuit design.

    YOU decide what to do.
    0by1 likes this.
  6. #12


    Nov 30, 2010
    Fire it up and look at it. If it's bright enough at 10 ma, use that. If it's bright enough at 18 ma, use that. Just don't go over 20ma.
    0by1 likes this.
  7. Wendy


    Mar 24, 2008
    There are many times I don't worry about at all, I just throw a 1KΩ resistor because I have a ton of them, and know it will be under the rated current. LEDs are pretty rugged devices (other than connecting them backwards), it is easy to overthink them.

    Many cases, where you are using them for illumination, you want the max, Given they have 10's to 100's of thousands of hours of life if you cut their lifespan by a few thousand hours by overdriving them slightly you would never notice.

    They tend to announce clearly when you are abusing them. Changes is color is a clue.
    0by1 likes this.
  8. Audioguru


    Dec 20, 2007
    An LED that is rated at 20mA has a maximum allowed current of 30mA or 40mA. If you operate it at "only" 20mA then it will last for hundreds of thousands of hours if is is name-brand.

    If you want the LED to be at its rated brightness then use 20mA. If you want it to be dimmed a little then use 10mA.
  9. creakndale

    Active Member

    Mar 13, 2009
    That's why I use "low current" 2mA LEDs.

  10. radiohead

    Distinguished Member

    May 28, 2009
    Of course a current-limiting resistor will be needed.

    Subtract the LED operating voltage from your working voltage to get the amount of voltage the resistor has to drop. Then use Ohm's Law calculate the value of the resistor. After that, use Joule's Law _P_ to calculate the wattage of the resistor.
    I E
  11. iONic

    AAC Fanatic!

    Nov 16, 2007
    How much you choose to limit the current depends on the use for the LED. If it's for a simple indicator, then a lower current may suffice (10mA - 15mA), but if it main function is
    to provide light for seeing things, then 18mA - 20mA might be better. For longevity, do not go over 20mA(If). Bottom line, I am agreeing with AudioGuru's assessment of (If).
  12. nbw


    May 8, 2011
    This is true! If your red LED suddenly goes a bit orangey... then yellow... look out!