Electronics > Beginners

How to see effect of True RMS

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Zero999:

--- Quote from: spec on November 23, 2018, 09:41:15 am ---
--- Quote from: Hero999 on November 23, 2018, 09:03:50 am ---I'm being picky here, but an averaging meter will just display zero, when given an AC signal with no DC component and is what a meter does when set to read DC.

There are different types of non-true RMS meters:

An averaging non-true RMS AC meter will high pass filter the signal first and rectify it, before taking the average value, which will be less than the RMS value of a pure sine wave, so it will multiply it by a correction factor.

A peak detector will high pass filter and rectify the signal, before taking the peak value, which will be more than the value of a pure sine wave, so it will be divided by √2.

In other words, in a true sine wave meter, the measured waveform is assumed to be sinusoidal and peak or average value is scaled to reflect this.

--- End quote ---

My understanding is that a true RMS meter, or chip, will give the RMS value of any signal, including DC. So, for example, if you applied 1VRMS or 1VDC, the reading would be 1V RMS. If you applied a 1V p/p square wave (equal mark and space) the reading would be 500mV and so on. RMS gives the total area (integral ) under the waveform curve, both positive and negative (intregal). It only has magnitude, not direction. In fact it is often said that RMS is the heating effect, as implied in replies #2  and #9 above.

--- End quote ---
That's true but in practise most true RMS meters are AC coupled, therefore only measure AC. Some meters have a DC+AC setting which will give the RMS reading of any signal, DC included, up to the bandwidth limit of the RMS chip of course.

The Electrician:

--- Quote from: mwb1100 on November 22, 2018, 05:14:56 pm ---I've got a meter that doesn't do True RMS and a meter that does.  I've rad a bunch of stuff about how important TRMS is and seen some youtubes about it.  but I'd like to experiment a little myself.

Is there some item that I'm likely to already have at home (or can buy cheaply) that will show me the effect that TRMS has on meters?  It would be especially nice if the thing had a knob where I could change the magnitude of the effect. 

--- End quote ---

Have a look at this thread: https://www.eevblog.com/forum/testgear/testing-dmms-rms-measuring-capability/msg171571/#msg171571

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