PWM.. an AC voltage or DC??????

[elex_enthusiast]

i am a bit confused if a pwm an AC volt or a DC?... and i am wondering if it is posible to measure its true rms value by an AC function of my DMM..please explain....

[w2aew]

i am a bit confused if a pwm an AC volt or a DC?... and i am wondering if it is posible to measure its true rms value by an AC function of my DMM..please explain....

It's neither, really.  AC means that the voltage/current changes polarity.  A PWM signal typically is a logic signal that turns on and off.  So, a PWM signal is more accurately described as a pulsed DC signal.

Most True-RMS reading DMMs will AC couple the signal when set to AC, so they won't give you the right reading because the DC component is taken away.  If you put the meter in DC mode, it may read the average voltage properly (depending on the meter, and the frequency of the PWM signal). But, the average voltage of a PWM signal is not the same as the RMS value.

Some True-RMS DMMs may specifically state that they measure true-rms AC+DC.  If so, then they should read the RMS value correctly, as long as the frequency of the signal is within the capability of the meter.

I just did a video on this. 
https://www.eevblog.com/forum/beginners/back-to-basics-video-what-is-rms-voltage/

[c4757p]

You can compute the RMS value.

avg = duty * peak
rms = sqrt(duty*peak^2) = sqrt(avg*peak)

So measure the average (filter it with an RC filter if the meter is confused), and you should know the peak (what voltage is the circuit running at?), then find sqrt(avg*peak).

[elex_enthusiast]

i am a bit confused if a pwm an AC volt or a DC?... and i am wondering if it is posible to measure its true rms value by an AC function of my DMM..please explain....

It's neither, really.  AC means that the voltage/current changes polarity.  A PWM signal typically is a logic signal that turns on and off.  So, a PWM signal is more accurately described as a pulsed DC signal.

Most True-RMS reading DMMs will AC couple the signal when set to AC, so they won't give you the right reading because the DC component is taken away.  If you put the meter in DC mode, it may read the average voltage properly (depending on the meter, and the frequency of the PWM signal). But, the average voltage of a PWM signal is not the same as the RMS value.

Some True-RMS DMMs may specifically state that they measure true-rms AC+DC.  If so, then they should read the RMS value correctly, as long as the frequency of the signal is within the capability of the meter.

I just did a video on this. 
https://www.eevblog.com/forum/beginners/back-to-basics-video-what-is-rms-voltage/

so does it mean that the ac+dc function of my fluke 287 can determine the rms vlaue of a pwm signal?...i thought it was only good for mixed signals

[c4757p]

so does it mean that the ac+dc function of my fluke 287 can determine the two rms vlaue of a pwm signal?...i thought it was only good for mixed signals

Yes, it can, as long as the frequency isn't too high. Looks like the 287 has a 100 kHz bandwidth - try to stay below 50 kHz or so. It's a square wave, so it has high frequency content that will be lost.

What do you think "mixed signals" means??

[NiHaoMike]

It's both. Usually, only the DC component is what ultimately matters, but it's possible to make real use of the AC component. For example, if you have a piezo attached to the PWM output of a microcontroller, you could use low duty cycles to reduce the volume.