Measuring PWM power

Discussion in 'General Electronics Chat' started by GLEMU, Jan 18, 2012.

  1. GLEMU

    Thread Starter New Member

    Jan 18, 2012
    In a PWM LED controller system, I need to calculate the power dissipation in the LED's. It should be straight forward to calculate power as the product of current and voltage, but I have become unsure, which method is the right one
    Currently Duty cycle (D) is 66%.

    1. With a digital multimeter, I made a DC current measurement. Voltage is also measured in DC with the meter. Both values should be average values. Power is calculated as:
    Pavg = Uavg * Iavg

    2. In an alternative setup, I measured the peak voltage with an oscilloscope. Peak current is measured as the voltage drop over a small series resistor with the oscilloscope.
    I calculate Peak power by :
    Ppeak = Upeak * Ipeak
    then, I multiply by dutycycle:
    Pavg = Ppeak * D

    In method 2, I only get 66% of method 1.
    I assume my assumptions; Uavg = Upeak * D, Iavg = Ipeak*D and Pavg = Ppeak * D are wrong

    Which approach is the right?
    I'd appreciate some advise
  2. Wendy


    Mar 24, 2008
    Your math is fundamentally correct, where your assumptions go wrong is with using meters to measure with. They have their own set of assumptions, none of them based on PWM.

    Look to method 2, if I read you correct. Percentage of duty cycle is percentage of power.

    Bill's Index

    Pulse Width Modulation (something I wrote for PWM)

    LEDs, 555s, Flashers, and Light Chasers

    Chapter 5 - The 555 and PWM

    Since this was based on applied technology and less on math I moved it.