Exploding lightbulbs

[wptski]

ELA:

Where did you get the 2.7 value from? Yeah, I blew by some of that complex math! I have a digital scope too that gives the True RMS values also but playing with distorted waveforms, the values are the same as a True RMS DMM. The scope gives you a visual peak value no matter what. It's not a perfect waveform, so you can't use .707, so that why I ask where you got the 2.7 value?

 

[ELA]

In post # 76 where I showed the waveform. In the text box it shows that the Crest factor was calculated as a ratio of the peak/RMS values the scope presented.

 

[wptski]

In post # 76 where I showed the waveform. In the text box it shows that the Crest factor was calculated as a ratio of the peak/RMS values the scope presented.

ELA:

So, you assume that the scope can handle that distortion level and Peak and RMS values are correct?

EDIT:
It will be correct "only" if it's within the CF and bandwith specs of the scope. Your using its measurements to calculate CF when you have to know waveform's CF first to determine if the values are correct!

 

[ELA]

No I did not assume.
I know my scope can handle the waveform presented because it has a bandwidth much, much greater than that of the signal in question.

Let me ask you:
Do you trust your scope to give you an accurate representation of a waveforms peak value at 60,180, 300hz?
If not then I would throw it away.

The "true RMS" handheld meters RMS values compared close to the RMS value of the scope giving me confidence in the RMS value. So if we trust the RMS value is roughly correct and we trust the peak value from the scope, then I trust the crest factor calculation.
In addition you can derive an intuitive estimation of what the RMS should be based on the scope waveform (as compared to a true sinewave), just to add some confidence that you are not grossly out of range.


If you do not trust these then you would need to do a complex waveform analysis (curvefit) and then perform the RMS math. Of course once again you would need to have some confidence in your scope to display an accurate representation of a waveform.


Here is another link that provides some more true RMS tests. If you page down about 6 times (near the bottom) there is a paragraph that talks about using the scope.

http://www.tmworld.com/article/CA187601.html

 

[Energy-Miser]

ELA has posted a very good link by Doug Criner, that explains the limitations of handheld meters in calculating the RMS of signals. I repeat that link here, it is worth checking out:
http://www.enginova.com/true_rms_volts.htm
Bottom line, most handheld devices give you the RMS of the ac portion of the signal (another words, they strip the DC or average portion of the signal first) which of course is only an accurate measure of the RMS value of your signal if your waveform werer symetric around zero. A simple way suggested in that article to arrive at the correct value of the RMS is to square the RMS the meter reads, add to this the square of the DC level of your signal and then to take the square root of the result. Most meters will give you your signal's DC level, by pressing the DC button while measuring the voltage.

 

[wptski]

ELA:

I've had that page bookmarked a while back and the reason I mentioned your scope's specs. In the above post where you have a picture of your scope's screen, your list the make/model of the DMMs, all Fluke. What's the make/model of the scope?

I have several scopes but the ones we can talk about here are a Fluke Scopemeter 199B and a 123.

None of those DMMs you listed will read the RMS value of a pulse train correctly. Only ones that have a fighting chance at it are one with a DC AC+DC function like a Fluke 89-IV/187/189/287/289 or Scopemeters.