# What does "TRUE RMS" mean?

• posted

I have a TRUE RMS DVM. Hooked it in parallel with a digital scope to the function generator at 60 Hz. At zero offset, the DVM reading tracks the scope RMS calculation for sine/square/triangle waveforms.

The scope knows how to measure the RMS value of a sinewave with voltage offset.

4V DC is 4V RMS on the scope, but zero on the DMM.

A look at the DVM specs shows that the AC RMS measurement has a low frequency of 45 Hz. That explains the observation. BUT That does not sound like a TRUE RMS measurement????

• posted

Older AC meters would measure the peak-peak AC value or the average of the absolute value of the AC, then scale the result so that it was the same as the RMS value for a sine wave -- but it would be wrong for square, triangle, etc.

So, it was "truer" RMS.

```--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.```
• posted

Sorry, I left out:

"True" RMS meters block the DC, then run the signal through a circuit that actually takes the square of the absolute value of the AC, filters it, then takes the square root.

So, it is "truer" RMS than what came before.

If you know the DC component and you know the RMS value of the AC component, then you can calculate the overall RMS value of the wave, which is pretty cool.

Try measuring the RMS value of a signal that goes off of your oscilloscope screen, or that has rare big spikes and lots of little crud

-- see how "true" your RMS is then.

```--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.```
• posted

Well, it's better than the usual diddled (*1.11) average value that you see.

Best regards, Spehro Pefhany

```--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com```
• posted

"mike"

** True RMS means that within the specified frequency limits and max crest factor the meter responds to the rms value of the wave.

This is quite unlike most analogue meters and DMMs which respond to the

*average rectified value * of the wave and display a reading that is adjusted to give correct rms values only for sine waves.

Budget DMMs that display "true rms" have very limited frequency range ( ie

45Hz to 1000Hz ) and modest max crest factors like 3. They are useful the waves found in mains frequency PSUs.

Better DMMs may work accurately from DC to 100kHz or more and crest factors of up to 7.

The best use thermal methods and go out to GHz.

... Phil

• posted

Try using the DC range (on the DVM) when measuring DC voltages.

For a superimposed waveform or noise or PARD, the AC range will read zero, unless the AC oscillations are within the range set on the DVM. At a very fine range setting, you might read the ripple if it is slow enough. Most meters won't sample too fast. There are a lot of nice ones though that do read fast.

You have to use the millivolt scale and the meter has to be able to read at the frequency of the noise/ripple you are wanting a voltage of.

The scope is better for examining such signals.

What the meter maker means by "True RMS" is that the meter is better than the old, cheap "RMS" meters which simply averaged the value from the peaks. The new meters actually analyze the waveform from moment to moment.

• posted

Many true-RMS meters use AC coupling to remove the DC component, but = good ones also have an AC+DC option, which is simply DC coupled. The AC = coupling is very useful for things such as ripple on a DC signal. But the AC+DC measurement is necessary for things like variable duty-cycle rectangular waveforms and PWM signals. Generally you can determine the actual = true-RMS value of a signal with DC offset by using Vrms =3D sqrt( Vac^2 + Vdc^2).

In my work, I need to measure the true-RMS value of a short pulse of = power line frequency AC. If you do a computation of the value based on = samples, the value will vary widely for short pulses but then converge eventually = to the steady-state true-RMS value. I have found that it is better to use a sampling rate that is an integer multiple of the line frequency, so 1200 = Hz works well for both 50 Hz and 60 Hz, with 24 or 20 samples per cycle. = And when reading the RMS value, a sample of 100 or 200 mSec shows the least amount of "jitter" since it comprises exactly 5 or 6 complete cycles.

This is also SOP for many DC digital meters, so that AC line noise is essentially canceled out, resulting in relatively flicker-free readings.

Some discussion of this can be found in an article I wrote on Circuit Breaker Testing Technology:

Paul=20

• posted

"Dorothy with the Red Shoes on"

** DMMs ( hand held or bench types) with "true rms" ranges date back to the early 1980s when Analog Devices introduced their " true rms to DC" converter ICs.

A DMM is essentially an analogue meter with a numerical display based on a pulse counter and dual slope integrator. This arrangement can only measure DC voltages. In order to read AC ones, there is an AC to DC converter - either a precision rectifier or a true rms to DC converter inside the DMM. This is engaged when an AC range is used.

It is the performance of the latter circuits that determines the AC performance of the meter - plus the design and bandwidth of the input attenuator.

The effective sampling rate of most DMMs is only 3 or 4 times a second - ie the same as the display update rate.

... Phil

• posted

One of the earlier digital true-RMS meters is the HP 3403A, which came out in about 1972. It has AC, DC, and AC+DC modes.

Cheers

Phil Hobbs

```--
Dr Philip C D Hobbs
Principal Consultant```
• posted

** A thermal voltmeter - not a DMM.

I mentioned the type in my previous post in this thread.

But the HP3403a is only good to 10MHz.

** Way too big.

... Phil

• posted

The early TrueRMS and wide bandwidth meters even today are essentially (RF)dummy loads with a thermometer.

It is interesting to note that English people talk about RMS (Root Mean Square) values, while in many European languages something equivalent to effective value (e.g. Effektivwert) is used, i.e. the DC level that would produce the same heating effect in a resistive load as the AC/DC signal being measured.

• posted

It's digital, anyway. Analogue ones go back to at least 1966.

No, it's 100 MHz. The 3400A is the analogue one from 1966 that goes to

10 MHz.

Takes the same amount of shelf space as a normal bench DMM--it's just a bit taller.

Its main drawback is that its most sensitive range is 10 mV, whereas the

3400A goes down to 1 mV.

Cheers

Phil "Add 1 dB for noise" Hobbs

```--
Dr Philip C D Hobbs
Principal Consultant```
• posted

I've seen the phrase 'EE' used on 1930s equipment here, meaning electrical equivalent. Its a lot more intuitive than rms.

NT

• posted

I don't know about that. True RMS meters theoretically don't make a lot of assumptions about their input signals, whereas there's a very long history of measurements based on peak- or average-reading AC meters with their scale dorked to read the correct RMS value, assuming that the input waveform is sinusoidal.

For noise waveforms, average-reading meters read 1 dB low--a little matter of 10% error.

"Electrical equivalent" would leave me wondering what the underlying assumptions are. "True RMS" is a lot clearer.

Cheers

Phil Hobbs

```--
Dr Philip C D Hobbs
Principal Consultant```
• posted

"Nutcase Turd

Arent the old fashioned 2 coil meters rms?

** That sounds like a power meter.

Got SFA to do with my post about DMMs.

Can you read at all ??

.... Phil

• posted

I think effective value is more intuitive.

Look at the HP link at page 17, what this circuit actually does, could be described with the following analogy.

Two identical 50 ohm dummy loads with identical thermometers are used. The (modulated) RF signal to be measured, is fed into the other dummy load and the temperature is observed.

A DC supply is connected to the other dummy load through a potentiometer and the potentiometer is adjusted, until the other thermometer reads the same as the first thermometer. The potentiometer output voltage is measured using any ordinary DC volt meter. The DC voltage reading is the effective value of the modulated RF waveform.

• posted

"Nutcase Turd"

I've seen the phrase 'EE' used on 1930s equipment here, meaning electrical equivalent. Its a lot more intuitive than rms.

** Its totally ambiguous while " rms" is very precise.

The term comes from statistical analysis - the "root of the mean square " of a series of numbers.

The physics connection is pure serendipity ....

... Phil

• posted

e
t

It means equivalent heating effect. RMS is a mathematical description of how one calculates it, EE is a description of what it means.

NT

• posted

```--
Then, instead of the ambiguous and counter-intuitive "EE" it should be
"EH" or "EHE".```
• posted

Theres really no reason for it to be any of them, it could just as well be IHP, identical heating power. Its entirely trivial.

NT

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.