Filtered AC V measurement with multimeter

[Gunther_Guss]

TL;DR Summary

What result does multimeter show when measuring filtered AC V ? is it Vave or Vrms of the voltage ripple ? And whats the formulas to calculate this ? Thanks.

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[Borek]

What does the manual say?

 

[berkeman]

TL;DR Summary: What result does multimeter show when measuring filtered AC V ? is it Vave or Vrms of the voltage ripple ? And whats the formulas to calculate this ? Thanks.
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DMMs will generally show Vrms on their ACV setting, but as @Borek says it's always best to check the Owner's Manual.

 

[Gunther_Guss]

What does the manual say?

TL;DR Summary: What result does multimeter show when measuring filtered AC V ? is it Vave or Vrms of the voltage ripple ? And whats the formulas to calculate this ? Thanks.

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I think it measures Vrms cause when i try to measure it by oscilloscope i get the same result as Vrms, but what i wanted to know is that what result does multimeter show when measuring filtered AC V after rectification cause i want to know whats the Vp/Vmax of its wave, oscilloscope only shows it's ripple voltage, even though first mili second after we input the voltage source it'll show us the whole wave or its Vmax. I want to calculate this by only know what the result from the multimeter, thanks.

 

[Rive]

filtered AC V after rectification

You mean, AC component on the buffer cap?
That's quite a rabbit hole, with plenty of harmonics and load-dependent voltages
In general, that's not really for (cheap) DMMs.

 

[f95toli]

I think it measures Vrms cause when i try to measure it by oscilloscope i get the same result as Vrms, but what i wanted to know is that what result does multimeter show when measuring filtered AC V after rectification cause i want to know whats the Vp/Vmax of its wave, oscilloscope only shows it's ripple voltage, even though first mili second after we input the voltage source it'll show us the whole wave or its Vmax. I want to calculate this by only know what the result from the multimeter, thanks.

Multimeters will by convention always attempt to show the RMS value of a signal. However, most cheaper DMM will simply assume that what you are measuring is a pure sine wave and then calculate the RMS value, meaning the shown value is not going to be accurate for non-sinusoidal signals.
More expensive multimeters will attempt to actually measure the RMS value directly, meaning they should show the correct value even if you are measuring say a triangular wave. Most of these multimeters will have some text on them saying e.g., "TrueRMS". However, all instruments have limitations, meaning even a TrueRMS multimeter is not going to be accurate if you are trying to measure something that is outside its specified capability. For example, if the frequency of the signal is too high or, as could be the case, here it contains high-frequency components (which might be the case for e.g., a square wave)

So, just to repeat what was stated above, you need to check the manual.
Generally speaking, for more complex signals you should always use an oscilloscope and/or a spectrum analyser if you need accurate values.

 

[tech99]

If you have an analogue multimeter, hence slow to respond, when you connect it to DC-with-ripple it will read the average value. This is not quite the same as RMS but in this case where the ripple is small, probably not greatly in error. If you connect the multimeter via a capacitor, that will remove the DC component and you can use the multimeter on its AC scale. This will give an RMS ripple value assuming a sinusoidal ripple. You can find the peak value of the DC-with-ripple waveform if wished by shunting a capacitor across the meter when it is on a DC voltage scale.
If using an electronic multimeter, there is always the annoyance of it being either too rapid to reliably average the waveform being measured or alternatively, using a DC range, the electronics might capture the peak rather than the average.