What is "True RMS"? - Page 1
Forum section: electronics / testgear

author: Axtman
author_rank: Regular Contributor
timestamp: February 13, 2014, 04:11:03 pm
content: What is "True RMS" and why is it important (or not) for a multimeter?

author: TMM
author_rank: Frequent Contributor
timestamp: February 13, 2014, 04:27:04 pm
content: It's important if you want to measure AC signals that aren't perfect sine waves.Non-TRMS meters measure the peak voltage/current of the input signal and multiply by 0.707 which corresponds to the relationship between the peak voltage and root mean square voltage of a sine wave. The result is if you measure a 1Vrms square wave, a non-TRMS meter usually reports something less than 1Vrms because it is assuming the input is a sine wave.True-RMS meters perform a rms calculation (either digitally or analog), giving the true RMS reading -Interestingly, the non-TRMS meter in the video reads high when fed a square wave. Perhaps it is capturing some overshoot and that is where the high reading comes from.

author: Wytnucls
author_rank: Super Contributor
timestamp: February 13, 2014, 04:56:38 pm
content: True RMS meters aren't perfect either. Their reading accuracy will depend on the crest factor of the signal being measured. Very few can measure accurately an AC signal with a crest factor above 3.

author: zapta
author_rank: Super Contributor
timestamp: February 13, 2014, 05:18:36 pm
content: RMS gives you the DC power equivalent of an arbitrary signal (does not have to be AC). For example, if you connect a signal with X RMS volts to a resistor, you will get the same power dissipation as if you connect a DC voltage of X volts.All DMM in the AV range try to give the RMS value. Non true RMS meters give correct reading when you feed them with a pure sine waves. True RMS meters give correct reading also with some other signal shapes (depending on crest factor, DC offset, etc).As for being important, it depends what you are doing. For some applications it is, for others it's not.

author: Axtman
author_rank: Regular Contributor
timestamp: February 13, 2014, 05:26:46 pm
content: Thanks guys! Very informative, especially the video.So in the US is the house voltage 120 volts peak to peak or RMS?

author: mariush
author_rank: Super Contributor
timestamp: February 13, 2014, 05:49:54 pm
content: It's RMS ..

author: bronson
author_rank: Regular Contributor
timestamp: February 13, 2014, 06:13:21 pm
content: Yes, neutral-to-peak voltage in the US is 120*root(2)=170V, and peak-to-peak is 340V.

author: The Electrician
author_rank: Frequent Contributor
timestamp: February 13, 2014, 09:12:20 pm
content: Quote from: TMM on February 13, 2014, 04:27:04 pmThis is not correct. What non-TRMS meters do, is to measure the average of the absolute (full wave rectified) value of the waveform and them multiply by 1.1107. This is the relationship between the RMS and average (full wave rectified) of a sine wave; this is called the form factor (http://en.wikipedia.org/wiki/Form_factor_%28electronics%29).Quote from: TMM on February 13, 2014, 04:27:04 pmIt's not from overshoot. It's because the 1 volt peak square wave voltage is multiplied by 1.1107, not .707

author: Strada916
author_rank: Frequent Contributor
timestamp: February 13, 2014, 10:51:06 pm
content: Quote from: bronson on February 13, 2014, 06:13:21 pmThat means countries who have 240 mains will have 680VppV I don't think so.........120rmsV will have a ppV of 170VEarly morning read....

author: grumpydoc
author_rank: Super Contributor
timestamp: February 13, 2014, 11:00:47 pm
content: Quote from: Strada916Actually I think bronson is correct - for a sine wave, symmetrical about some 0V reference the half wave peak is sqrt(2)*rmsV and p-p is twice that. I don't see why mains would be different.

author: Macbeth
author_rank: Super Contributor
timestamp: February 14, 2014, 12:17:14 am
content: I've never quite understood the obsession with True RMS. I mean if a meter does a .707 on rectified AC volts, then who cares how accurate it is with square waves or such, as using a handheld meter on such volts is obviously measuring the mains 110-250 VAC. The electricity generators themselves can't guarantee much accuracy so our 240VAC in the UK is considered 230VAC just by EU harmonization.If I want to measure more interesting AC voltages (and for some reason need to know RMS) then I would just use an oscilloscope.Can anyone tell me of a situation were using a True RMS meter saved the day compared to using any ordinary VOM/DMM and maybe a 'scope?

author: Wytnucls
author_rank: Super Contributor
timestamp: February 14, 2014, 12:21:28 am
content: http://www.testequipmentdepot.com/application-notes/pdf/hvac/hvac-applications-where-true-rms-rest-tools-matter_an.pdf

author: mjkuwp
author_rank: Unknown
timestamp: February 14, 2014, 12:49:47 am
content: Macbeth, it matters if you are reading current. Around the house the only loads I know of that are sinusoidal current draw are old fashioned incandescent light bulbs and.... toasters? Other types of light bulbs and most anything with a switching power supply will have a complex waveform.There probably are situations where an AC wave intended to be sinusoidal gets distorted from these kinds of loads. Maybe not largely distorted but some amount.

author: The Electrician
author_rank: Frequent Contributor
timestamp: February 14, 2014, 05:37:14 am
content: Quote from: Macbeth on February 14, 2014, 12:17:14 amHave a look at this thread:https://www.eevblog.com/forum/projects/led-light-bulb-meaurements/msg207417/#msg207417

author: BravoV
author_rank: Super Contributor
timestamp: February 14, 2014, 05:56:11 am
content: Quote from: Macbeth on February 14, 2014, 12:17:14 amImo, hobbyist that plays with low voltage DC circuits or battery based, its very unlikely they will need a true rms measurement.The Fluke document pointed by Wytnucls is good example.Scope is not practical for this kind of measurement, also they're far more expensive than ordinary scope.But if you're working with mains voltage and high power stuffs like hvac, a 10% off measurement result sometimes will ended up with bad thing, either you got sack and loose your job or you blew up the budget.Say you're assigned to do some measuring of a 100 Kilo Watt electrical installation, the purpose is to allocate the company's electrical budget, if your reading off by 10% = 10 Kilo Watt and you're responsible for the measurement "AND" the procurement of the measuring device, will you pay that miscalculated electrical billing with your salary ?

author: TMM
author_rank: Frequent Contributor
timestamp: February 14, 2014, 06:16:39 am
content: Quote from: The Electrician on February 13, 2014, 09:12:20 pmInteresting, that explains a lot

author: The Electrician
author_rank: Frequent Contributor
timestamp: February 14, 2014, 06:48:24 am
content: The most common situation where a hobbyist would need true RMS is in the area of capacitor input rectifier power supplies.Have a look at this thread:https://www.eevblog.com/forum/testgear/testing-dmms-rms-measuring-capability/msg171571/#msg171571The current drawn by the typical capacitor input rectifier power supply, the kind that hobbyists like to build, requires derating the transformer by a factor of about .6 because the current drawn from the transformer secondary is like the waveform shown in post #1 of that thread. The true RMS value of that current must be no more that the rated current of the transformer to avoid overheating it. True RMS must be used because the RMS value of the current is the value that must be used in the familiar I^2*R formula for the power dissipation in the resistance of the copper winding of the transformer.

author: boo9
author_rank: Contributor
timestamp: August 17, 2014, 10:21:42 pm
content: Quote from: The Electrician on February 13, 2014, 09:12:20 pmSo it looks like that computing AVG value of the signal (rectified) is no problem since any dinky DMM does that.AVG = (v1 + v2 + ... + vn) / nThen Why is such a big deal to compute RMS on the chip ? it's just a formula like the AVG, just different.RMS = sqrt( (v1^2 + v2^2 + ... vn^2)/n )I am guessing it is trivial to compute AVG in analog circuit/computer, but the complexity of sqrt() or power() operators in analog cirucits are not that trivial.Please correct me.

author: G0HZU
author_rank: Super Contributor
timestamp: August 17, 2014, 11:57:49 pm
content: Quote from: Macbeth on February 14, 2014, 12:17:14 amMost scopes are very noisy and can't measure small signals. A decent old school true rms voltmeter can read down to about 30uV in a 20MHz bandwidth.eg a decent meter will be able to measure band limited noise out to, say, 10MHz even if the rms voltage of the (LF through 10MHz) noise is only 30uV.