Author Topic: Measuring AC Power with an oscilloscope? (DS1054z)  (Read 1494 times)

0 Members and 1 Guest are viewing this topic.

Offline Hobgoblin612

  • Contributor
  • Posts: 47
  • Country: au
    • Youtube Channel
Measuring AC Power with an oscilloscope? (DS1054z)
« on: September 28, 2016, 05:14:59 am »
Hi all,

I am trying to test an audio power amp with a dummy load and am having some trouble getting my scope to calculate the power correctly.

Here is my test setup:

(the 1R resistor is a 1% type)

I have the units of Ch 3 set to Amps and the math set to Ch3 x Ch4

What measurement should I set on the math channel to properly calculate the power? Is there a way to do this?
As you can see 4.76A x 41.9V =  199W which none of Avg, Vmid or Rms correctly calculates. I understand why Rms isn't the correct calculation for this but I thought that Avg or Vmid would be correct.

Of course I can just use a calculator and multiply the two rms values together but i'd like the scope to do it in real time so can focus on other things like watching for magic smoke.

Thanks, Timothy.

ooooohh... what does this button do???

Offline danadak

  • Super Contributor
  • ***
  • Posts: 1875
  • Country: us
  • Reactor Operator SSN-583, Retired EE
Re: Measuring AC Power with an oscilloscope? (DS1054z)
« Reply #1 on: September 28, 2016, 01:09:56 pm »
The scope equation shown onscreen lower left shows CH3 + CH4,
not CH3 * CH4 ?

Although if I take the peak of power I get 7.5A x 60 V = 450W instantaneous power which
the math trace ~ shows.

Regards, Dana.
« Last Edit: September 28, 2016, 01:26:14 pm by danadak »
Love Cypress PSOC, ATTiny, Bit Slice, OpAmps, Oscilloscopes, and Analog Gurus like Pease, Miller, Widlar, Dobkin, obsessed with being an engineer

Online WaveyDipole

  • Frequent Contributor
  • **
  • Posts: 366
  • Country: gb
Re: Measuring AC Power with an oscilloscope? (DS1054z)
« Reply #2 on: September 28, 2016, 03:25:09 pm »
I think it is because in the Avg column you are being shown the power based on time average voltage (Vavg) rather than RMS voltage (Vrms).

To convert Vrms into Vavg:  Vrms ~ Vavg * 1.11
To convert Vavg to Vrms:  Vavg ~ Vrms x 0.9:

41.9Vrms x 0.9 ~ 37.71Vavg

We then multiply this by the current to get Wavg:
37.71Vavg x 4.76A ~ 179.50Wavg

Of course if you multiply the average power result by 1.11 you get the expected DC equivalent power:

179.50Wavg * 1.11 ~ 199.25Wdc(equiv), which, allowing for minor rounding errors is the same result as given by 4.76A x 41.9Vrms.

Since I'm not sure how Vmid is calculated, I can't comment on that reading.

« Last Edit: September 28, 2016, 05:13:43 pm by WaveyDipole »

Offline 2N3055

  • Super Contributor
  • ***
  • Posts: 1897
  • Country: hr
Re: Measuring AC Power with an oscilloscope? (DS1054z)
« Reply #3 on: September 28, 2016, 05:41:34 pm »
First don't mix RMS and average.  RMS is effective value (thermal equivalent of dc power that would heat the resistor the same )..
RMS is calculated regardless of waveforms..

So, P(rms)=U(rms)*I(rms) cos (?? ? ?i)

Yes, it is a vector operation... Lets assume both are in phase here... so cosinus nonsense we can disregard..

From that you can derive : P = I 2rms R = V 2rms/R

Which makes me think:

 you have 8 \$\Omega\$(7 \$\Omega\$+1 \$\Omega\$) and 4,78A and 42V,

so  P=4,78A2*8 \$\Omega\$=182,78W RMS,

and P=42V2/8 \$\Omega\$=220,5W RMS

That is not equal, not by a long shot, so something is wrong with the resistors value...

You should measure them and than  we'll see...


Offline alsetalokin4017

  • Super Contributor
  • ***
  • Posts: 1873
  • Country: us
Re: Measuring AC Power with an oscilloscope? (DS1054z)
« Reply #4 on: September 29, 2016, 12:42:41 pm »

-1: Yes, danadak, the screen is showing A * B for the math trace, not A + B.

0: The Rigol DS1054z has a known issue with RMS calculations if one is using certain older firmware revisions. This has (probably) been fixed in the latest firmware revision. So we need to make sure that Hobgoblin is using the current firmware in the scope. This is

1: The Rigol's Measurements are calculated from the Screen Display data, so partial periods shown on screen can influence the results. To get the best accuracy from the left menu Measurements, one should display many full periods on the screen (say, 10 or so rather than 1 or 2) and display them at the greatest height (lowest V/div settings) possible while still keeping them completely on-screen without vertical clipping.

2. The correct desired measurement for the power here is probably Average Power. However in the Audio field, "RMS Power" is an often-cited measurement, even though it isn't really meaningful.  See
for an excellent explanation of this issue.

3. As the above linked document shows, Vrms * Arms = Pavg. 

4. So.... let's first confirm that we are running the latest firmware that calculates RMS approximately correctly. Then let's display a larger number of wave periods on the screen, with the waveforms displayed at a V/div setting that maximizes the height on screen. Since we have sine waves, the number we get from simply multiplying the Vrms x Arms values shown should converge on the value we see in the Math Average measurement window. Maybe. Hopefully.

The easiest person to fool is yourself. -- Richard Feynman

Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo