For the current waveform in my AC test, it is a DC pulse waveform with a 10% duty cycle,

so I believe the correct RMS formula for that type of waveform is: Vp x sqrt (duty cycle ratio).

Based on the current waveform in my scope screen shot, this would give:

4 x sqrt (0.1) = 1.26

The scope calculated the current RMS as 1.28, so that is pretty close.

1.28 /100 = 12.8 mA

However, the actual RMS current for the LED should be around 6 mA (estimated), so we can't

use that calculated RMS value by the scope.

For the voltage waveform, we have an AC pulse waveform which the scope is offsetting

with a negative offset. The RMS voltage value calculated by the scope is also not correct for our circuit,

because the LED is only conducting for 10% of the time, but the scope is calculating the RMS value for

the entire waveform cycle. I am still fiddling to see if I can figure out how the scope calculates that 9.54V RMS

value for the voltage waveform.

The actual RMS voltage across the LED was probably about 2.75 V (estimated).

At any rate, the RMS values calculated by my scope obviously can't be used to calculate the power consumption

of the LED with a pulse type waveform being applied to the LED.

P.S.

After thinking about it, I guess the way the scope calculates RMS values is to square all the measured sample

values taken over a period of time, then calculate the mean of those squared values, and then take the square root

of the calculated mean. I am feeling too lazy right now to save waveform samples to a CSV file and compute it

all out using Excel, but I might try it at some point to confirm if that is how the scope calculates the RMS values.

P.P.S.

Since the voltage and current waveforms are pretty close to being a nice rectangular shape in my test setup,

another way to do a sanity check on the measurement of the power consumption of the LED in my AC test circuit

is to use the actual scope measured values (as shown in the scope screen shot I posted above).

If we do it this way we get (I think this is right, but maybe I am overlooking something

):

We are only concerned about the voltage waveform when the LED is conducting,

so, V measured = 0.65 divisions x 5 volts per division = 3.25V

I measured = ( 4 x 1 A per division) / 100 = 40 mA

Duty cycle is 10%, so the duty cycle ratio is 0.1

**3.25V x 40 mA x 0.1 = 13 mW**The scope's measured/calculated value for the LED power consumption (using the Math function) was

**16.7 mW**.

Ballpark, but not exactly really close. Maybe this method of estimating the power consumption of the

LED is not right, or maybe it is not so accurate. Or maybe the power calculated using the Math function was off a bit.

I think this only works out (roughly) because the waveforms are quite clean rectangular waveforms.

However, thinking about it, that should actually be what the LED is doing. According to the scope measurements,

the LED has 3.25V across it and conducts 40 mA during its On time, which is 10% of the time.

That very possibly may not be so good for the LED however, if left running for a long time.

This is why it is way easier to try to self-loop a circuit that you think might be OU, and see

if it will self-sustain itself. No measurements or calculations required, and much less chance of

making an error somewhere.