Ideas for inexpensive True-RMS Metering?

Don't waste your time trying second- and third-best solutions. Go to Analog Devices' web site and acquaint yourself with their wide selection. We're featuring the ADE7753 in H&H AoE III, you could start there. 20-pin ssop, $4.61 at DigiKey. It's single phase, but they have more complex 3-phase versions with the same architecture, so what you learn on one is extendable.

Still very expensive. The same amount of $ buys you a complete micro with ADCs.

All of these are easy to do except the last one. The waveforms can be pretty ratty and it may be difficult to measure this. Why phase difference? Although in getting the rest I suppose you could calculate this.

If you are trying to make a globally acceptable product, you may also have to use three voltage transformers (in addition to the current transformers).

I am not sure about the current situations, but many countries used

The cheapest way to do this is to buy a small uP that has an on-board ADC and mux. Scale down the line voltages with resistive dividers. A single-phase meter or Y-connected meter can let the uP ride on neutral, and sense neutral current(s) with cheap shunts. Other configurations will generally need current transformers.

A 10-bit ADC works pretty well, 12 is even better. Add some dithering noise to the current signals.

Sample voltage and current inputs and do the math. You can even acquire waveforms. The trickiest math is phase angles.

One of the voltage dividers can also be used to run a timer channel to get frequency.

The algorithms to get metering-quality power measurements are a bit tricky. RMS volts and amps are pretty obvious.

You should be able to do a pretty good 3-phase power meter with a $4 uP, or maybe even a $1 one.

John

The cheapest way to get an inexpensive true RMS multimeter is to simply go buy an inexpensive true RMS multimeter.

You could not possibly jump into a game such as this with such nievete.

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Presumably you mean AC average voltage, AC average current, and you want those to be RMS averages? For power factor, you need power, i.e. average of (voltage * current) as well. Either you'll require three multipier circuits of some sort, or an equivalent in software.

"Phase difference" only applies to sinewaves, what approximation would be appropriate if your current waveform is... awkward?

To get the power value correct, you have to simultaneously sample the V and I (you can't alternate measurements with a single ADC unless you know some strong limits on slew rates).

I've always wanted to use measurements of (V+k*I) and (V-k*I) so the microprocessor would just have to look up the square of the digitized values (the difference of those squares is 4*k*V*I), that gets you a power measurement without integer multiply instructions.

What does that have to do with designing polyphase wattmeters?

John

Remember to put sufficient number of resistors in series so that the combined resistor voltage rating is 1.5 to 2.5 kV depending on the national standards, so that voltage peaks can be handled without flashover.

That is a horrible idea when used with TN-C (or measuring on the TN-C side of a TN-C-S system) wiring system, in which Protective Earth (PE) and Neutral are interconnected at the load. A test equipment wiring fault or a blown shunt resistor could cause the full phase voltage on the cases of multiple load equipment.

Putting current transformers on the phases is the safe way of doing it and works both with wye and delta loads.

That would make for some interesting ground loops. Using safety ground as a current carrier is illegal here.

A test equipment wiring

As would any open in the single ground wire. Scary, especially at 240 volts.

But they're big, expensive, and nonlinear.

John

Big? If you are sensing dozens of amps and the alternatives are shunt resistors and current transformers, there aren't any 'small' candidates.

Expensive? Not a problem in onesies, I'm not sure what a few thousand DOES cost. OK, probably that's true.

Nonlinear? That only happens with large core field, and current transformers with active burdens solve that problem neatly. It doesn't HAVE to be a small-value burden resistor, that was a simple example.

Actually, the 'big' and 'nonlinear' are connected; you need the size to keep the nonlinearities down, and the use of an active burden reduces both the size and the nonlinearity.

'Nievete' seems to be a cheap Chinese clone of better quality instrumentation, hence the warning.

michael the naive

Hall effect sensor?

It used to be legal (or at least common practice) to connect the neutral to both the neutral and ground terminals when "upgrading" residential outlets from 2-prong to 3-prong.

Is that not now (or not ever) legal?

I did the multiple-outle demo with two 1206 low-ohm resistors, in parallel, per channel. Gain is essentially free.

We punch or photoetch our own current shunts out of manganin, for maybe 20 cents each. They'd be pennies in serious volume. The 1206 low-ohm resistors are cheap, too.

The linearity problem isn't at large fields, it's at low ones. Silicon steel permeability drops seriously at low fields, enough that an affordable CT won't meet Ansi C12 accuracy requirements without messy compensations. An active burden doesn't help the copper loss problem; it's easy to use a passive shunt that's ting compared to winding resistance. Active burdens burn power, too.

In real life, no.

John

I don't believe that was ever "legal". The practice now is to use a GFCI on ungrounded outlets and label them as such.

Take a look at

While a pole pig in the USA feeding a single house might actually be a true TN-S system, larger systems are most likely TN-C-S systems, so tampering with the TN-C side grounds is a bad idea.

A sane person would not even think about tinkering with the neutral/ground with some test equipment :-).

Typically individual grounding electrodes are required at each house in Europe, but typically the 230/400 V feeder cables and overhead lines are 3L+PEN only.

That's what AC/DC clamp meters use ;)

Grant.