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Discussion board help and admin topics => Problems and Solutions for Accurate Measurements => Topic started by: ayeaye on October 31, 2016, 12:58:52 AM

Title: Measuring AC signal with a digital multimeter
Post by: ayeaye on October 31, 2016, 12:58:52 AM
My multimeter is MAS830B, but if you have any other cheap multimeter that has 830 in its type, then it most likely has the same integrated circuit, which means that it functions the same. It is not a bad integrated circuit.

All that is written about measuring AC in the specifications, is that it can measure voltage in its AC ranges up to 400 Hz, and that it measures average. In the AC ranges it is the average of the absolute values, which is a root mean square.

But digital multimeter uses an analog to digital converter to measure values, after that it evidently calculates an average of the measured values. Thus it should also be capable of measuring some AC signals in its DC voltage ranges.

So i tested it. I measured a square wave signal with 50% duty cycle, generated by Arduino.

It appeared that in the DC range, from 10 Hz to 2.5 MHz, it measured 2.43 V, which is an exact average of the TTL signal generated by Arduino, with an amplitude of 4.86 V. And the reading was stable. With more than 2.5 MHz, the measurements became less than that. With the same frequencies it measured 4.8 V in its AC range.

I don't really believe the specification, which says that the precision is +/- 3 (of the displayed value, like with 2.43 it's 2.40 to 2.46), but it's true that the reading can be 1 less (of the displayed value) than the real value, because it does not do rounding.

I did this simple experiment just to have some idea of how it measures. And the speed with which its analog to digital converter works. Use DC ranges to measure AC signal only when your signal is always above zero, or below zero, otherwise the result will be inaccurate. Otherwise, the accuracy of the measurement depends on your signal. You may roughly estimate though, that when your signal rapidly changes back and forth during less than 1 us, then the measurement will not be accurate, as the digital to analog converter doesn't measure that fast.

Ok, don't believe me, test it yourself. I just told about one experiment i did, to have a bit more idea about it. Hope it was useful for someone for some purpose.
Title: Re: Measuring AC signal with a digital multimeter
Post by: verpies on October 31, 2016, 02:15:57 AM
it measures average. In the AC ranges it is the average of the absolute values, which is a root mean square.
Wrong
Title: Re: Measuring AC signal with a digital multimeter
Post by: ayeaye on October 31, 2016, 03:46:26 AM
Wrong
Right. But then i any more cannot well explain what it does in the AC range. It should then do something like, measures peak, and when the signal resembles sine, it divides it by sqrt(2), when it doesn't, shows the value of the peak. Which is weird. And wrong, because peak is not the root mean square of the positive square wave.

It is said in its instruction manual about AC range exactly the following:
Frequency range: 40 Hz to 400 Hz
Display: Average (effective value of sinusoid)

In the DC range it looks like that it clearly calculates average. But i have not tried with other signals than square.
Title: Re: Measuring AC signal with a digital multimeter
Post by: verpies on October 31, 2016, 01:02:02 PM
IMO the meter rectifies and then applies that signal to an RC integrator and that is applied to a voltage divider, that always applies the same correction quotient assuming that the input signal is a sine wave.
Title: Re: Measuring AC signal with a digital multimeter
Post by: AlienGrey on October 31, 2016, 06:12:47 PM
IMO the meter rectifies and then applies that signal to an RC integrator and that is applied to a voltage divider, that always applies the same correction quotient assuming that the input signal is a sine wave.
It's now a RMS chip in the device it's only accurate around the 50 / 60 hz range depending on the timing as you said anything else use a scope and HF in a candescent bulb will vibrate the filament and lite the bulb that way so you might not necessarily have more power at all. (Rout Means Sqr) my advice is look up the chips and see what they do and how they work.
Title: Re: Measuring AC signal with a digital multimeter
Post by: verpies on October 31, 2016, 06:42:11 PM
If the meter's manual states "averaging" then it is not very likely that the marketing people would write that instead of boasting about the TrueRMS feature which requires an expensive multiplier such as the Gilbert cell.
Title: Re: Measuring AC signal with a digital multimeter
Post by: ayeaye on November 01, 2016, 01:37:00 AM
I maybe figured out how it measures in the AC range. I think it has to be some simple algorithm, it doesn't do anything fancy. But it doesn't bother to measure values below zero, it only measures values above zero, assuming that values below zero are a mirror image, like in a perfect sine.

I also measured square waves with different duty cycles. In the DC range they were perfectly the average with the given duty cycle. But in the AC range it appeared to be the peak value, multiplied by how much the duty cycle was greater than 50%. That is, when it was 25%, the values were 0.5 times the peak value, and when it was 75%, it was 1.5 times peak value.

By that i guess what it does in the AC range is, it calculates the squares of the values above zero, multiplies each square by two, then adds them all and divides by the number of all measured values, then takes square root of it. By testing it with more complex signals, it could be found out whether this is indeed what it does. Who does it may post the results here.