To be deleted

[Void]

Looks good Itsu.

My Siglent SDS1204X-E scope can do the same sort of math calculations
that you are using to compute the power consumption, so I tried a power measurement
comparison test with two test circuits using the exact same type of white LEDs in both test
circuits. One test circuit is DC, so the power measurement is straight forward. The other
test circuit is using a 10% duty cycle pulse train to power the LED. The LEDs in both circuits
were adjusted to be approximately the same brightness, but that is very hard to do by eye,
so their brightness is probably not a really close match, but they were in the same ballpark of
brightness anyway. The frequency of the pulse waveform was set to 10 kHz.

See the attached test circuit diagrams which I used in this test.
The LED power consumption in the DC test circuit came out to: 2.71V x 5.2 mA = 14.1 mW
The LED power consumption in my AC test circuit was measured using a product math function
and then taking the mean of the resulting waveform (White trace).
(mean of the product of the voltage and current waveforms), and is: 1.67W /100 = 16.7 mW

The difference between the two power measurements may be largely due to my eye not being able
to accurately compare the brighness of the white LEDs in the two test circuits. There may be other factors
in there that are affecting the power measurement a bit as well. The 100 Ohm CSR resistor value is close
enough to 100 Ohms, but off by a few Ohms. So using the scope product Math function and taking the
mean of the resulting waveform seems to give a reasonable result, in this test setup anyway.

Notice that in the attached Scope screenshot that the measured scope RMS values for the LED voltage and
LED current are not useful in doing any power measurements when measuring asymmetrical AC waveforms.
The current is being measured across a 100 Ohm CSR, so current must be divided by 100.

Ch 1 - Yellow trace - LED Voltage waveform
Ch 2 - Pink trace - LED current x100 (Set to Inverted)
White trace - Power waveform - Product of Ch1 x Ch2, with the Mean of this waveform indicated.

P.S. No jokes about Channel 2 having a pink trace. That's the way the scope is configured, and I don't
see any way to change the trace color. Why they chose pink instead of orange or red is beyond me.

[itsu]

Very nice Void,

yes, eyeballing a led (pulsed or steady) is almost impossible, you need a black box / lux meter to measure properly, but still you came very close.

Indeed, it again shows that the measured scope RMS values for the LED voltage and LED current are not useful in doing any power measurements
when measuring asymmetrical AC waveforms.


 
Strange colors indeed, most scopes i know have fixed colors, so nothing to do about that i guess.


Itsu

[Void]

Hi Itsu. A few things I don't really understand with my scope is how the scope
is computing those RMS values. I would have thought the RMS values would come out
a lot lower considering I was only using a 10% duty cycle. My scope indicated 9.54 V RMS
for the voltage waveform and 12.8 mA RMS (1.28 / 100) for the current waveform. Not sure
how my scope is arriving at those values. The actual RMS voltage across the LED should be
around 2.7 V to 2.8 V, and the actual RMS LED current should be around 6 mA. 
All my scope's manual says for the RMS measurement is: "RMS: Root mean square of all data values."

Another thing that I think is odd is both Channel 1 and 2 were set to have the traces centered
vertically in the screen, but channel 1 has the waveform sunk down to its average DC value,
which I think is normal when displaying this kind of waveform on a scope using DC coupling, but
the waveform for channel 2 is displaying with the lower part of the waveform in the center of the screen.
Channel 2 is also set to DC coupling. I am not sure why channel 1 displays as sunk down to the average
while channel 2 is not. Can anyone explain why the 2 channels are displaying differently?

The scope is a fairly new scope, so it could have bugs I suppose. Not sure if the above mentioned results
are what should be expected, but they seem odd to me anyway.

[itsu]

Void,

i have the same (down offset) yellow voltage trace when using an AC pulse.
When using a DC pulse its like normal.

Also, do you use a grounded Signal Generator?  If so then the (grounded) black lead will short out
the 100 Ohm  resistor as very probably the both scope ground leads are also grounded.

Itsu

[Void]

Hi Itsu. Yes, I guess because the current is rectified by the LED,
the current waveform shows differently than the voltage waveform, which was AC with 0V DC offset. 

I was using a portable signal generator that was powered from my bench power supply.
I used my DMM and measured the resistance between the scope ground and the signal
generator ground across the 100 Ohm resistor, and it measures as 103 Ohms, so the
signal generator ground was isolated from the scope ground. If it was a short,
I wouldn't have measured any current across the 100 Ohm resistor.

Also, I think I may know why the scope is calculating the RMS values the way it is, by I am
still reviewing that. I will post something here if I figure it out.