# Topic | Vdc reading of rectified AC ~= Vrms

Home Forums Mooshimeter Support Vdc reading of rectified AC ~= Vrms

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lynette7530
Participant

App 1.0.22 (1820) + FW: 1462564395

Anyways, I have input from a 24VAC (24Vrms * √２ ~= 33.94V) to a simple full-wave rectifier.

Ch2 was set to “Voltage DC” and autorange + autosample.
Ch1 was not used (except for bleeding a cap later on w/ a 1.5kΩ resistor)

My trusty old Fluke reads 31.6VDC to 32.19VDC (it does pick up the full-waves and lists frequency as 119.97-120.00Hz)

I tried playing around with the sampling and range options but it seemed “stable” at reading ~24V

Once I put a filter cap (47０µF), it would then read ~35VDC.
My guess is that the rms calculation algorithm is kicking for some reason.

Keymaster

Hi MoFoQ,

Can you transition the Mooshimeter to graphing mode and capture a buffer so we know exactly what it’s seeing?

Thanks
~James

lynette7530
Participant

Anonymous
Guest

MoFoQ,

So, the numbers all appear to be within reason and the mooshimeter matches my UT61E very well.

Now, for your measurements, it appears based on the 31.6V from your Fluke, you chose to display Vp instead of VRMS. Is your main concern that the Mooshimeter should measure the Full Wave Rectifier Output in Vpeak instead of VRMS? If so, then should the mooshimeter measure the original sine wave in Vpp or VRMS? I mean I can see value in both, but in general, don’t people pay extra money for meters capable of doing RMS well? I’m just trying to learn something here.

—- Duane

Anonymous
Guest

Since I had everything set up, I measured again using a triangle waveform instead of sine. Maybe the info will be of value to somebody.

Not too bad. The two meters were almost identical, but were lower than theory would predict for the rectified output, unfiltered. The 4.4 values of both meters look closer to the theoretical VAVG of 4.35 (half of 8.7peak) instead of the theoretical VRMS of 5.02. Maybe I did something wrong. Alas, I don’t have a calibrated precision meter to serve as the ultimate judge.
—Duane

lynette7530
Participant

sadly, I don’t have a function generator with me at the moment to try other waveforms.

Also, my trusty old Fluke is on “full auto” (for automatic range, trigger, and time-base) and it gives me Vp instead of Vrms for DC Volt measurements (AC and mV DC/Hi-Z modes give me a rms or avg option)

Keymaster

Thanks Duane and MoFoQ, I learned something too. So the Fluke deliberately gives you the peak value when measuring in DC mode? That’s quite strange.

Also, MoFoQ, sorry I wasn’t clear earlier about what I meant by buffer mode. What I was thinking was that you could turn the meter up to 4000Hz, 256 samples, then take a screen capture of buffer mode of the rectified output you’re trying to figure out. This way you could see the full waveform and see if the meter is introducing some kind of error.

lynette7530
Participant

Like this?

Keymaster

Not quite –

Your settings are correct, but I’m asking you to hit the option key from the graph mode, then select “Buffer Mode”

lynette7530
Participant

I don’t see that option key

Keymaster

I believe it’s the square in the lower right of your display

lynette7530
Participant

One with “>” seems to “pause” the graphing option w/o toggling the “pause/play” button status.

The square next to the home softkey only tiles all running tabs/apps

Anonymous
Guest

RMS of AC = RMS of rectified AC. The Mooshimeter is giving the correct result. (Imagine the square of an AC signal. Now imagine the square of a rectified AC signal. They are the same.)

Only when you add the capacitor (with no load) does the voltage become DC, with a value equal to the peak AC voltage. Again the Mooshimeter is giving the correct result.

lynette7530
Participant

I disagree with this; mathematically, sure it is “correct” that it is giving the RMS value.

However, the app was set to DC mode and thus should not silently use RMS (currently there’s no indicator nor option to use RMS in DC or V-avg or Vp, etc.)

Think of a black-box scenario.
The resulting plotted graph isn’t enough to indicate that what is being measured is a rectified AC let alone that the resulting units are in VDCrms. Without some sort of indicator, there’s no way to tell.
It is essentially saying that VDCrms = VDC

Also “rectified AC” is technically DC (it’s no longer alternating)

Keymaster

Hi guys,

Sorry for delay.

MoFoQ – I realize now your phone is one of the ones that does not provide an options menu button by default. Some Android phones no longer provide it, so I’m adjusting the app to stop relying on it. I believe you can still reach the menu I’m referring to for “buffer mode” by long-pressing the square button. The reason I’m eager to see a buffer capture is that it will show you the exact waveform that the Mooshimeter is seeing, so we can verify that the meter is seeing the peak and RMS voltage you describe.

With the other descriptions you’ve given, I think the Mooshimeter is functioning properly. I think giving the mean value as the DC value is reasonable. If it presented the peak value as DC value, I would see that as misleading in situations where it’s measuring a very spiky signal.

lynette7530
Participant

Sadly, long-pressing doesn’t seem to do it either. :(

It would be nice to have an “indicator” stating that it is being calculated as Vrms though (especially in the logs as currently, there’s no way to distinguish between 24VDC(rms) and 24VDC(non-rms).

Of course, a Peak/min hold feature would be nice too but now I’m just adding to the feature request :D

Anonymous
Guest

That is a snapshot of my graph in buffer mode from the mooshimeter android beta app. The peak to peak voltages appear to match my display readings. The buffer mode really is a nice feature. When measuring a signal such as a unfiltered full rectified wave, neither Vp nor Vavg nor Vrms tell the whole story like a Oscope does. And this buffer mode graph is certainly useful as a psudo-Oscope for low frequency signals.

However (why is there always a however?), I ran into the same issues that MoFoQ did when trying to access buffer mode. I have 4 cheapie pre-paid phones which I never activate and just use as dedicated devices for projects such as this. The two phones with android 4.4.2 can access buffer mode with no problem. The two phones with android 5.01 or 5.1 cannot access buffer mode. Long pressing the square button does not do it. I tried many combinations on both phones and could not get it. I searched the internet and sadly the old “menu” button is just not available on some android 5 phones. Gotta love Android!
—Duane

lynette7530
Participant

ooo…that’s purdy! (re: the buffer mode graph screenshot)

yea, I have 6.0.1 (Nexus 5X) and can’t access the buffer mode graph

Anonymous
Guest
Keymaster

Thanks Paul,

The issue with the graphing activity is that I want to leave as much of the screen as possible available for the graph. I think I’ve worked around the issue though, and the new interface should be more intuitive. I separate the touch areas of the left and right axes, so you can pan and pinch-zoom the left and right axes seperately just by manipulating the two sides of the chart. Buffer mode and XY mode are hidden in a pop-up menu controlled by an onscreen button.

Will push to beta later this afternoon

Thanks

Anonymous
Guest

That sounds very intuitive. Thanks