PWM Investigation - Effective Voltage & RMS

[Noah]

Hello guys!

I'm interested in PWM stuff and I have a simple question: How do I calculate the effective voltage of a PWM and how can I measure it?

I tried different measuring methods: A PWM at 50Hz, 20% and Vpp = 2V. The true RMS Fluke multimeter shows the same AC voltage with a Offset 0V and a Offset of -1V. But this can't be true because the most 'part' of the period is at -1V...? The Agilent Oscillospoe measures also different values...

 I there a formula to calculate this or does anyone have experience in PWM effective voltage?

[mjrandle]

Yes there is, Google "rms square wave duty cycle".

Regards,

Mike

[w2aew]

You may want to view my video on the topic of RMS voltage.  The first part of the video talks about what RMS means and shows some examples of calculating RMS for sinewaves.  Then I talk about square waves (that are balanced around ground), and then I talk about digital pulses, like PWM pulses which have their "low" value equal to ground, and some high value equal to Vp.  The video shows that the RMS value of a PWM signal such as this is Vp * sqrt(duty_cycle).  The discussion of PWM waveforms begins at about 6:45 in the video.

[The Electrician]

The true RMS Fluke multimeter shows the same AC voltage with a Offset 0V and a Offset of -1V. But this can't be true because the most 'part' of the period is at -1V...? The Agilent Oscillospoe measures also different values...

What is the model number of your Fluke?  If you get the same value for offset and no offset, you probably haven't set your Fluke to measure "True RMS AC+DC"

See post #39 of this thread:

https://www.eevblog.com/forum/chat/how-can-i-test-if-a-dmm-is-reading-in-true-rms/msg167774/#msg167774

Also see:

https://www.eevblog.com/forum/testgear/testing-dmms-rms-measuring-capability/msg171733/#msg171733