can someone recommend a really TRUE RMS meter?

Maybe what You need is a Scope ?

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Honest-to-Gawd true RMS meters are relatively uncommon, but they do exist. Look for something like this:

which actually measures the "heating power" of the signal in question, and therefore is getting you as close as you're likely to get to what "RMS" is supposed to be without capturing the waveform and doing the math (which generally would require a fairly sophisticated scope).

Bob M.

If it doesn't say "RMS" then it probably measures the peak & assumes a sinusoid.

If it does say "True RMS", then it's like the Fluke meter and measures the RMS of the AC portion of the signal -- but it's pretty easy to measure twice and calculate sqrt(a^2 + b^2), so what's the problem?

Lots of spikes, and lots of frequency content above 60Hz, may cause problems -- you're describing a signal with a high "crest factor", which will limit the meter's abilities, and I wouldn't just assume that a meter has good performance above a kHz or so, either.

Careful reading of the specs may tell the truth -- look for crest factor and bandwidth, and see what they say.

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A digital scope with a true-RMS calculation feature could give you the numbers, I suppose... or, just capture the data with a fast digital scope, upload it to your PC and do the math there.

Or, look around to see if you can find somebody who has an olde style HP 3400 true-RMS voltmeter (I got mine for a song at a local hamfest a couple of years ago). To measure RMS current you'd either need to have a low-impedance sense resistor installed in the current path, or use a current probe (HP 456A, or some sort of home-brew current transformer with resistive termination, and then some work to calibrate it).

The 3400 uses the two-thermocouples-and-a-feedback-loop method of RMS detection.

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A properly designed true-RMS meter can easily be more accurate than a scope. Scopes are no better than 1-2% on a good day. Analog Devices used to make a true-RMS sensor that used a heater and (iirc) a diffused silicon resistor bridge.

Cheers,

Phil Hobbs

There are a number of ICs and meters that measure true RMS by using the thermal method. Some have a 100Khz bandwidth, and others are better.

"Phil Hobbs"

** Which would cost thousands of dollars when wide bandwidth (ie MHz) is needed. ** Many (even budget model) digital scopes include " true rms " voltage measurements - with bandwidths the same as the range in use.

...... Phil

I dunno--I have an HP 400A with true-RMS, bandwidth ~ 10 MHz, cost on Ebay ~ $75. About 1% accuracy at this point--still better than your average scope. It was specified at (iirc) 0.25% of FS when new. It also has an analogue output.

Cheers,

Phil Hobbs

"panfilero"

** What are you really trying to measure ???

I suspect you have no clue about when knowing the "rms" current value is useful and when it is NOT useful.

...... Phil

"Phil Hobbs" Phil Allison

** I think you mean the HP 3400A, has +/- 5% accuracy from 3 to 10 MHz.

Will not read combined AC +DC.

What did it cost originally ?

A new, digital display one with high accuracy costs??

...... Phil

SHAMELESS PLUG: My Daqarta sound-card software has a true RMS Voltmeter. It actually does the root-mean-square computation. If you can trigger on the waveform (Daqarta has lots of trigger controls, so you probably can unless it is really noisy), then the computation will take place over an integer number of cycles. Otherwise, on an untriggered source the computation will be done over 1024 samples.

Daqarta also includes a signal generator, which (depending upon your particular situation) you may be able to use to initiate your pulses. In that case, Daqarta can use Gen Sync to trigger off its own generator and insure it knows the exact cycle length.

Or, if you have an external trigger source available (that is driving the signal), you can feed that into the other input channel and just use that as the trigger.

In any case where you can get a stable trigger, you can also use Daqarta's synchronous waveform averaging to reduce noise.

IMPORTANT: Note that although the RMS computation handles DC correctly, the sound card is AC coupled.

Also note that you'll need to calibrate your system first. Daqarta has an auto-calibration option that determines the relative step sizes of the sound card mixer using a loopback connection, but you'll also need a full-scale range measurement. If you already have a meter that reads AC Volts, you can use that as your reference, with a sine wave input (that can be provided by Daqarta, if you wish). You just take a

reading and do a simple calculation to get the value to enter into Daqarta's Full Scale Range dialog. All described in great detail in the Help system.

Since your signal can run larger than sound cards can handle, you should also provide an input attenuator to keep things in the +/-3V range or so. (The card probably won't be damaged by +/-12, but the input ranges will clip.)

Daqarta is US$29 to purchase (Personal/Hobby license), but it sounds like this may be only a short-term need. If so, you may be able to do your whole job within the 30-day/30-session trial period. Enjoy!

Best regards,

Bob Masta DAQARTA v3.50 Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, FREE Signal Generator Science with your sound card!

No, I have one of those at home, vintage probably 1972 by the look of it. The one at work is quite definitely a 400A and is much newer. I used to use 400ELs back in the day, but they were peak-reading.

The accuracy spec includes the 1-pole rolloff of the input amplifier--when you correct for that, it's much better than a scope. And the 400A is much better than the 3400A.

Well, since the cross term contributes zero average power, that isn't too serious--if it's an issue, he can measure both and add root-sum-square fashion.

You're moving the goal posts on me. I doubt the OP has a time machine, so it doesn't have much practical significance what it cost originally. (I forget where I saw the line, "Time travel is no longer a thing of the past." ;)

Dunno. I'm happy with my 400A until they pry it from my dead fingers, and the OP can easily get one too, I think.

Cheers,

Phil Hobbs

"Phil Hobbs" Phil Allison

This is a 3400a.

** The HP 400A is an antique, non true rms millivolt meter.

** No - because HP 400As are simply not true rms meters.

Post a link to your example.

.... Phil

Whoa, ok sounds like the opinions on this issue are all over the place, from people who think a SCOPE would be better, others a METER...etc are these old thermal-based rms meters really better than today's digital ones for measuring RMS? Why would a METER be better than a SCOPE... don't SCOPES have much higher bandwidth and more capable of measuring this?

What I'm trying to do: measure the current and voltage going into each winding of a brushless fan motor, each winding is recieving some ugly looking pulses in order to run the motor, I'm intersted in find the power per each winding.

Thanks J

then you also need to know the phase angle betwen the voltage and current..

I suspect in this case, getting a wide bandwidth TRUE RMS reading is the least of your problems.....

Analog devices or someone makes ICs that can measure the REAL power of an AC circuit.,.. I think that is the way you need to go,,,if you make independent RMS measurments of the voltage and current, you still have the phase angle problem... You need to measure them both in a combined device that can multiply them together on a point by point basis to account for the phase angle..., not just multiply the overall RMS numbers...

Mark

As far as I know, most DVMs still put an analog RMS-to-DC converter in front of their regular DVM stuff. But fast SAR ADCs are cheap these days, so one could just random or dither sample the input and do the RMS as math. That would be cheap and deadly accurate, well into the MHz range where the analog things quit.

John

I need the phase angle? This might be going off on a tangent a bit, but the way I'm finding the power used by each of the 3 windings is by putting a shunt resistor (R) in series with the winding and measuring the votlage across it (Vrms,shunt) and then dividing Vrms/R to find the current (Irms) going to each winding, then I am going to measure the voltage at the winding (Vrms) with respect to ground. And then just take the product Pavg = Vrms*Irms .... I don't see why I would need the phase angles to find the power.

Thanks J

This page explains why you need "...the phase angles to find the (real) power.":

In your case, the current waveform is not sinusoidal, so the notion of phase angle becomes more complicated.

A 3-phase wattmeter would give the results you want, but as a practical matter, you should probably use an oscilloscope with trace math to carry out the measurement of your 3-phase power. See:

You could sample the (instantaneous) voltage and current at some fast rate, and calculate it in a micro.

Good Luck! Rich