How to calculate LED average power???

Discussion in 'General Electronics Chat' started by RATPL, Oct 20, 2015.

  1. RATPL

    Thread Starter New Member

    May 7, 2015
    Hi everyone,

    I am using LED Vf=3v and If=700mA. This LEDs used some light array project as flashing leds simultanously.

    i will turn on 15 LEDs at a time. 5x3 (5rows and 3 coloum) matrix.

    Duty cycle is 667ms

    ON time=165mS
    OFF time=502mS

    LES are 2W rated .

    please help to find Average power of LED.
  2. StayatHomeElectronics

    AAC Fanatic!

    Sep 25, 2008
    Instantaneous power of the individual LED is equal to the voltage across the LED multiplied by the current through the LED. When the signal is being turned on and off you can multiply the power by the duty cycle to get the average power. The duty cycle is the time the device is on divided by the complete cycle time.
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    If the LEDs are on for 165mS then you also need to know the instantaneous power they consume, to ensure you're not overloading the power supply. Things can get pretty hot in 165mS !
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    Ok, so you have 15 LEDs each drawing .7 A so that is .7x15=10.5 amps instantaneous current. Your supply should be rated for at least that, and more is better.

    The instantaneous power is just V x I or 3V x 10.5A = 31.5 watts.

    To account for the duty cycle you just need to make a ratio of on time to total time, or 165 / 667 which is just about 25 percent. So the average power is just .25 x 31.5 = 7.8 watts.
  5. Bordodynov

    Well-Known Member

    May 20, 2015
    One LED Power=Vf*If*(ON time)/(Duty cycle)=3V*0.7A*(165/677)=0.512W=512mW
    15 Led Power15=0.512W*15=7.68W
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
    Or let LTSpice do it for you...


    I doubt that V(f) = 3.000V
    Last edited: Oct 20, 2015
    spinnaker likes this.
  7. spinnaker

    AAC Fanatic!

    Oct 29, 2009
    Not to hijack the OP's thread but could you show how this was done?

    I tried and it is running forever for me. If you can show me what I did wrong then I will add a little tutorial on how to do it. It might help us LTSpice newbies.

    I added a current source with the following values.


    I added two spice directives

    .param Ton=165m
    .param Toff=502m

    In the Simulation Command Dialog I added the following


    But it runs forever never displaying the graph