# Measuring PWM power

#### GLEMU

Joined Jan 18, 2012
1
Hi,
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?
/GLEMU

#### Wendy

Joined Mar 24, 2008
22,164
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.