Voltage Measurements, true-RMS vs conventional meters

@gunrunnerjohn posted:

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

GRJ-

Great information as always!  Quick question as I don't have a scope to check.  Is the ZW-L a chopped waveform?  I suspect it is chopped since its a "modern" transformer.  I'm sure you know this off the top of your head.

Nice but half your pics are not coming through.  I have those cheap meters incorporated in my layout.  I'm a pure sign wave guy as I don't use my TPC or TIU to run conventional.  My meters are more of a guide than a true reading of the voltage/current anyway.

I see all 6 pics from GRJ.

@LionelAG posted:

I see all 6 pics from GRJ.

I'm not seeing the Fluke pics or the last O-scope pic.  Weird.  Something to do with Firefox.  can't see them on my computer or my Iphone but if I use Safari, I see them.

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

@Mellow Hudson Mike posted:

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.

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.

Whatever works for you.

I see a Fluke 117 come up on sale at work, but don't know if it's True RMS. Any thoughts?

I have a Klien true RMS meter.  Seem to be accurate. That was the cheapest meter  I bought. As my buddy used to say, it only costs a little more to go first class.

Gunner,

How would you rate the analog Simpson 260/270 series?

My go-to meter most of the time.

@cbojanower posted:

I see a Fluke 117 come up on sale at work, but don't know if it's True RMS. Any thoughts?

The Fluke 117 is a true-RMS meter.  It says so right on the front of the meter!

@PRRMP54 posted:

Gunner,

How would you rate the analog Simpson 260/270 series?

My go-to meter most of the time.

That's obviously an old-school analog meter, so it won't deal with the non-linear waveforms like a true-RMS meter.  OTOH, I think I still have the 260 in the attic storage somewhere.

@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

@gunrunnerjohn posted:

That's obviously an old-school analog meter, so it won't deal with the non-linear waveforms like a true-RMS meter.  OTOH, I think I still have the 260 in the attic storage somewhere.

Old school it may be, but it is still in production as the 260-8.

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

FWIW Lowes has the Fluke 117 online for $150.00 today (9/10) It's a true RMS meter. Not sure how long the sale is for. Its due in by 10/15, but you may be able to order online and if your store has them in stock, get the same price