How to see effect of True RMS - Page 1
Forum section: electronics / beginners

author: mwb1100
author_rank: Frequent Contributor
timestamp: November 22, 2018, 05:14:56 pm
content: I've got a meter that doesn't do True RMS and a meter that does. I've read 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.Some of the information I've come across mentioned light dimmers - my house doesn't have any at the moment, but I suppose I could get one and put it in a box.

author: BrianHG
author_rank: Super Contributor
timestamp: November 22, 2018, 05:23:05 pm
content: If you have one of those cheap DC12v to 120v inverters with modified sine wave output, the difference between the True RMS readout and Non-True RMS readout should be big. If you have a pure-true-sine wave inverter, both meters should read the same.

author: spec
author_rank: Frequent Contributor
timestamp: November 22, 2018, 06:17:46 pm
content: Hi mwb1100This is just a bit of thinking out loud- but you did say you wanted to experiment.Get a resistor and somehow attach a temperature to voltage sensor chip to the resistor. Connect your multimeter, set to volts to the output of the temperature sensor.If you then apply a DC voltage across the resistor, you will get a reference temperature for true RMS. Then you can put any voltage signal across the resistor and get a temperature reading for comparison.For example if you put a 1V RMS sine wave across the resistor you would get the same temperature as for 1V DC.By the way, at one time, this is how true RMS was measured.

author: Karlo_Moharic
author_rank: Regular Contributor
timestamp: November 22, 2018, 06:42:54 pm
content: Get signal generator and try measuring voltage at different freq (within specified range of both multimeters).

author: mvs
author_rank: Frequent Contributor
timestamp: November 22, 2018, 07:22:36 pm
content: Quote from: mwb1100 on November 22, 2018, 05:14:56 pmAny device with bad power factor (some small switching power supplies, led an CFL lamps) will do that for AC current measurement.But out of safety reasons, i would suggest you to get old style 60Hz transformer with low voltage secondary coil and use diode in series with some dummy load (resistor, light bulb, etc.) to get non-sinusoidal current consumption.

author: Ian.M
author_rank: Super Contributor
timestamp: November 22, 2018, 07:55:54 pm
content: One really old-skool*method of comparing DC voltages/currents and AC ones that gave true RMS readout was to compare the brightness of two identical filament bulbs, one fed smooth DC via a rheostat, with either an ammeter in series or a voltmeter across the bulb, and the other fed the AC signal that one was interested in. Both bulbs would be considerably underrun so they glowed red to orange rather than being bright. The rheostat was adjusted till they were the same brightness, at which point the DC voltage or current was equal to the AC RMS voltage or current. Its certainly not a precision measurement, but with sufficient care and good enough apparatus one could get about 5% accuracy.Ideally you'd have a multi-pole switch to swap the bulbs, carefully selected bulbs for matched cold resistance and full voltage operating current, and a half silvered mirror so the images of the bulbs filaments could be inline and directly adjacent to each other for ease of brightness comparison.* Over a century ago, before good valves and copper oxide rectifiers.

author: Zero999
author_rank: Super Contributor
timestamp: November 22, 2018, 09:10:51 pm
content: If you don't have a signal generator, then make your own one. Build a 50Hz oscillator with a squarewave output from the 74HC14. Use one section to make a Schmitt trigger oscillator (the schematic and formula for calculating the component values can be found using a search engine) and another as a buffer. Couple it to the output of the meter with a suitable capacitor, say 100pF to 10nF. Once the reading has stabilised, a true RMS meter will read roughly half the circuit's supply voltage, since that's the RMS voltage of a squarewave oscillator, once the DC component has been removed. A non-true signwave multimeter will give a different reading.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: November 22, 2018, 09:13:22 pm
content: In practice, true RMS doesn't actually tell you a lot more than an averaging meter would. The true RMS value of, for example, a step waveform from a budget inverterdoesn'ttell you what rectified DC voltage it would give, which is mostly what matters for electronics. The only time it's important is with heating loads.Though, it's not the expensive option it once was so I guess it's as well to have it.The gotcha that catches the 'free energy' types out is to assume that TRMS Amps x TRMS Volts = True Power. No! Separate meters (or the same meter used twice) cannot allow for the phase difference in a reactive circuit. I suspect this is a common reason for them finding that power outseemslarger than power in.

author: dacman
author_rank: Frequent Contributor
timestamp: November 22, 2018, 11:15:53 pm
content: I'll assume we are discussing voltage and amperes.If you have a function generator, try testing the different functions.Average responding meters are calibrated to indicate the RMS value of a sine wave (which means they are calibrated to indicate about 11% higher than the average).Average responding meters will indicate about 11% higher than the RMS value of a square wave.Average responding meters will indicate about 4% lower than the RMS value of a triangular wave (although the actual average is about 13% lower).For a rectangular pulse of 1% duty that rest at zero, an average responding meter will indicate about 9 times lower than an RMS meter (the average is actually 10 times lower).For a 4% duty pulse from zero, the average responding meter will read 4.5 times below the RMS value.RMS is related to heat which is what trips breakers and blows fuses and burns insulation and causes fires.

author: ArthurDent
author_rank: Super Contributor
timestamp: November 23, 2018, 02:38:52 am
content: The description by spec in post #2 is correct as to how it used to be done. Below is a diagram of how a low mass thermocouple is attached to a heater with a small glass bead inside a glass container with a vacuum inside and the heater is switched between D.C. and A.C.. The heating effect of the D.C. voltage is the same as the heating effect of the R.M.S. value of the A.C. voltage.I have an old thermal transfer voltmeter model 6A made by Holt and that had added circuitry, mainly voltage dividers, to allow calibration of D.C./A.C. voltages up to 1000 volts. The low voltage from the thermocouple was fed to an analog meter with adjustable sensitivity and the closer you got to a match, the higher the gain of the meter could be adjusted while still maintaining balance. The accuracy of the divider resistors and the meter accuracy wasn’t too important because as long as the circuit was stable enough to check comparison readings as you switched back and forth between the known accuracy D.C. and A.C. to be calibrated, that was all that was needed. Agreement of D.C. to A.C. better than .01% could be achieved.

author: David Hess
author_rank: Super Contributor
timestamp: November 23, 2018, 04:09:01 am
content: A square wave source is what I use to calibrate my average and RMS responding meters. The RMS meter will indicate the peak voltage of the square wave but a sine calibrated average responding meter will read 11% high.Going backwards, this is one way to calibrate a square wave source to be used to calibrate an oscilloscope.

author: spec
author_rank: Frequent Contributor
timestamp: November 23, 2018, 06:20:51 am
content: mwb1100Another way to measure true RMS of any voltage waveform is to use a chip, like the Linear Tech LTC1967. They are quite pricey though at around £8UK.https://www.analog.com/media/en/technical-documentation/data-sheets/1967f.pdfThe older Analog Devices AD567 RMS to DC converter is around £15, but I have some in metal cans, not surface mount. If you want one PM to get my address and send a SAE, and I will send you one. It Needs +-15V supply lines though.https://www.mouser.co.uk/datasheet/2/609/AD536A-1501510.pdfUPDATE- just noticed that you are in the US, so not sure about the SAE but PM me anyway if you want.

author: spec
author_rank: Frequent Contributor
timestamp: November 23, 2018, 06:31:38 am
content: Quote from: ArthurDent on November 23, 2018, 02:38:52 amThose were the days

author: Zero999
author_rank: Super Contributor
timestamp: November 23, 2018, 09:03:50 am
content: 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.

author: spec
author_rank: Frequent Contributor
timestamp: November 23, 2018, 09:41:15 am
content: Quote from: Hero999 on November 23, 2018, 09:03:50 amMy 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.

author: Zero999
author_rank: Super Contributor
timestamp: November 23, 2018, 09:56:11 am
content: Quote from: spec on November 23, 2018, 09:41:15 amThat'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.

author: The Electrician
author_rank: Frequent Contributor
timestamp: November 23, 2018, 10:46:26 am
content: Quote from: mwb1100 on November 22, 2018, 05:14:56 pmHave a look at this thread:https://www.eevblog.com/forum/testgear/testing-dmms-rms-measuring-capability/msg171571/#msg171571