Electronics > Metrology

How do old (25+ year) digital multimeters measure RMS voltages?

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... and can the measurements be trusted when the signal has a bit of odd harmonic distortion?

I have an ancient "Maplin Precision Gold M-4510" multimeter which is similar to the 5010 shown here

It has been working great for all these years when doing DC measurements but now I'm trying to use it to verify the output from a microcontroller which is doing true RMS measurements from an isolation transformer stepped down mains voltage. The measurements are broadly in agreement, but the multimeter reading is suspiciously stable (1 part in a 1000) over a period of 30 seconds. Is that to be expected for mains voltage measurements in an urban environment? The micro is giving readings that fluctuate by about 2 parts in a 1000 over a 1 second period so I don't know which one is telling the truth.

The web page for the 5010 references an Intersil chip
which only mentions "RMS" once.

Do these things simply multiply the measured peak by 0.707 to get an approximate RMS reading, or are they doing something else?


I could be wrong, but unless the multimeter included a true RMS converter, then it looks like the Intersil chip will just use a conversion factor as you suspect.

It's not actually calculating an RMS from the peak x 0.707. It's rectifying and smoothing the DC, much like a power supplies bridge rectifier and RC filter. This average DC value is what the meter will display but with an adjustment to scale it to RMS. This will be calibrated with a trimmer so a typical mains derived 50/60Hz sinusoidal voltage will read correctly.


Suggest to be more specific, as you may know that most of bench system DMMs are 25+ year old as well. Fluke 8506A is using thermal convertors to measure ACV. 30+ year old 3458A using fast digitizing + AC conversion. Keithley 2001/2002's use AD637 + separate floating full-wave rectifier circuitry to enhance accuracy and frequency range.

Dr. Frank:
Look at this AD536, True RMS Converter, salvaged from my first DIY kit 4 1/2 digit DMM, built when I just left school in 1980, obviously.
These chips really square the voltage input, average, and draw take the root, so it's a real analog calculator for the integral of the input signal.

Geeze, that's been 36 years already! I'm vintage also..  :-//

True RMS DMMs could be recognized by a big "TRMS" sign on them, so yours obviously measures averaged AC volt only.

(The ICL 7106 is the 3 1/2 digits A/D converter and display driver only) 

When I worked in this Airforce calibration lab, also around 1980, we had many of these Fluke 6 1/2 digits state-of-the-art DMMs, I think, already the Fluke 8506A with their proprietary thermal RMS converter inside.

PS: To answer your question, even if your DMM only measures the average, and will not give the correct absolute RMS value, it will anyhow display any changes of the line voltage.
Maybe your ┬ÁP TRMS device is not stable; I would test that with a stable sinus signal, as higher harmonics may make a difference.


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