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This comes up pretty regularly, so I thought I'd post a simple comparison of measuring track voltage with pure sine waves and chopped waveforms using both standard meters, and true-RMS meters.  To double-check the results, I fired up my 'scope to verify the results and to view the actual waveform we're measuring and comparing.

If you're measuring a pure (or fairly pure) sine wave, most meters do a decent job.  This is from a pure sine wave transformer, but the TIU has a short spike of power that accounts for the small blip in the waveform.  The blip is due to the TIU drawing power briefly to charge it's P/S capacitors each half-cycle.  However, it doesn't really affect the measurements, so I left it.

First is the sine wave, note that all the measurements are within a couple tenths of a volt, not bad at all, and what we expected.

Cheap Digital Meter

Fluke true-RMS Meter

Rigol 'scope Measurement

Now, let's check what happens with a non-linear waveform, one generated by the TIU variable channel at half throttle, it's power input is a pure sine wave, but the output of the TIU variable channel is the classic electronic transformer chopped waveform.

Cheap Digital Meter

Fluke true-RMS Meter

Rigol 'scope Measurement

As you can see, the bench Fluke true-RMS meter and the 'scope agree within a few tenths, but the cheap digital meter is out in left field at 40% error!

FWIW, this not only applies to voltage measurements, but if you do current measurements, you'll see the same errors trying to measure AC current of a non-linear waveform with an inexpensive meter.

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FWIW, this not only applies to voltage measurements, but if you do current measurements, you'll see the same errors trying to measure AC current of a non-linear waveform with an inexpensive meter.

John,

I'm glad you added this to your comments because it occurred to me recently that, even with a pure sinewave voltage applied to them, some loads (particularly those with advanced electronics in them) may draw current in a non-sinewave pattern.

This means that you could be quite comfortable with the reading from your simple voltmeter if you know that your power source puts out a pure sine wave but you could still be mislead by getting a current reading that's off the mark due to non-sinewave current draw.

Mike

Last edited by Mellow Hudson Mike

John,

I'm glad you added this to your comments because it occurred to me recently that, even with a pure sinewave voltage applied to them, some loads (particularly those with advanced electronics in them) may draw current in a non-sinewave pattern.

This means that you could be quite comfortable with the reading from your simple voltmeter if you know that your power source puts out a pure sine wave but you could still be mislead by getting a current reading that's off the mark due to non-sinewave current draw.

Mike

Exactly, you can see that effect in a minor way with the spikes where the TIU was drawing current briefly.  If there was a load on the TIU P/S, that spike would be wider and probably have more of an effect on the measurements.

@ThatGuy posted:

Yes we all know true RMS is better, but for a basic look see analog is fine

As long as you don't mind the "look see" to be 40% in error!  Really, what's the point of measuring, why not just guess.  You'll likely be as close.

@ThatGuy posted:

I must have missed something are these model trains or are we building the lunar Lem? For the average joe using a conventional transformer these are fine, 40% yea maybe…..but to give an average draw…no issues.

@ThatGuy,

Your choice of extremes is way off.  Many, many, many, many things in common use today are more complex than the power equipment in the 1960's LEM.

Here are some examples where AC is involved that cannot be assumed to be a pure sinusoid, are much more complex than you realize, and are manufactured and in use in the millions or billions globally:

  • Furnaces
  • Refrigerators
  • Dishwashers
  • Microwave Ovens
  • Heat Pumps
  • Ground Fault Interrupters
  • Small Countertop Appliances
  • Power Inverters (RV, Solar, Wind, and Backup)
  • Clothes Washers and Dryers
  • Residential Lighting
  • Hair Dryers
  • Air Conditioners
  • Power Tools
  • Pumps
  • AC Drive Locomotives (1:1 Scale)
  • Rooftop Solar Power and/or Wind, particularly the inverter to connect them to your appliances/Heating/cooling.
  • Automobiles, alternators in all of them, and many other systems, like motor controllers as well, in hybrids and electric vehicles.

'Average', 'RMS' and 'True RMS' are alternative ways of measuring important characteristics of AC power.  Of the three, you imply that 'Average' is simpler and usually accurate enough.  This is absolutely true, as long as the waveform is a pure sinusoid.

When it is not the 'Average' you measure with your equipment will be wrong enough to encourage you to make a mistake that will, at some point, fry something.  It happens quite often and is very real.

Therefore the need to measure True RMS accurately is also very real, even for model trains.

In common terms the list above illustrates the size and exposure of the failure in the real world that an incorrect guess can impact.

It's not that expensive or complicated to get it the right the first time.

Mike

Last edited by Mellow Hudson Mike

GRJ, those cheap digital meters look like the ones you suggested to buy on Ebay in a previous thread.  I was thinking of ordering some of those voltmeters (and ammeters) for our club layout.  On our club layout, we have one TIU for all four lines.  The power to the tracks do not go through the TIU.  The TIU is just there to passively provide the DCS signal to all four tracks.  In this case, I think the cheap digital meters would probably do a decent job.  Maybe better than the meters on the Z-4000 since they often seem to be different than my Fluke meter.

The meter I show is in a box that I use for bench testing.  The input is an MRC Pure Power pure sine wave transformer, so it's reasonably accurate.  It has a low cost voltmeter and ammeter in the box, great for simple checks of something in the bench.  I really just want to see roughly what voltage is being applied and how much current the train is drawing.

Universal Meter Box for Layout or Workbench

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