CMRR

On Tuesday, 28 April 2020 16:56:54 UTC+1, snipped-for-privacy@highlandsniptechnology.com

That might get interesting if the meters get used on 3-phase supplies.

John

That's a more expensive meter. Circuit common can be earth ground. But the CT outputs are isolated, so can be grounded, so there's still no reason to fight a big common-mode signal.

I also designed a multi-channel meter, the C180, for end-use load studies, that would meter any combination of phases and loads; Blondel's Theorem applies. Its electronics were grounded. We sold thousands of those, during the end-use-load-study fad. That's a nother story.

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Hmm, Re: old rotating disk things (and silly ideas*)

I know almost nothing about power measurement. I first wonder if you need a fast output. The disk meters have a built in averaging function. And so I'm sitting here thinking about ways to measure average power over some time. (total energy.) My first silly idea is to send a 'surrogate' of the voltage and current through a resistor and measure the temperature rise. (You'd have to have some means of getting rid of all the errors... Like some sort of differential gizmo that heats and cools (to environment) and known I*V as a calibration.. the details are a bit fuzzy. :^) But I was wondering if there are other 'silly' ways to measure power/ energy*time?

George H.

We had a voltage signal, from a resistive divider, and current signals from current transformers, or a current shunt for the single-phase India meter.

An ADC simultaneously samples voltage and current waveform pairs and the uP does the math. Software autozero, multiply, average for power, integrate for energy.

The sample rate need not be high; The Sampling Theorem does not apply. The survey meters sampled each channel pair around 27 Hz. We added some dither to the analog current signals to avoid ADC quantization and differential linearity errors. It was kinda fun at the time, but

We still sell a few of these:

mostly for aircraft 400 Hz testing, like APUs.

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Nice thanks. Some noise can be good I guess.

But sure resistor divider and current transformer was my input picture.. but then an opamp summing it into a resistor/ heater.. No! that can't work. I need the product... Can I program some fet as heater? (I was having this picture of a thermal oscillator, where the period was related to the power... or inversely related. long period = low power.)

Selling stuff is good.

George H.

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The old disk meters actually use two coils, one for current and one for vol tage. Their effect on an aluminum disk is proportional to the product incl uding the phase angle, in other words the complex product of voltage and cu rrent. The meter has fixed magnets that create drag making the rotational speed of the rotor proportional to the power being drawn.

So really the meter is measuring the voltage and the current including the relative phase, then finding the product though the EM effects on the disk.

Today's meter's do the same thing by measuring the instantaneous voltage an d current and finding the product. It is easy to find various circuits to multiply two quantities. It's not hard to find various ways to measure vol tage and current. Async voltage to PWM converters for both the voltage and the current taps, then use an XOR to find the instantaneous product. Smoo th the pulses and you have an analog signal proportional to the instantaneo us power.

Generate analog signals for the instantaneous voltage and current. Generat e the log of the signals and sum them giving power. This would require the two inputs to be biased since the input to the log can't go through zero. The bias would be subtracted out at the end potentially creating a problem of a small difference of two large signals.

How about using the voltage to control a pump with the output pressure prop ortional to the voltage. The opening of a valve can be made proportional t o the current and the volume of water would be the product measured by stan dard means. Or a gas could be used using the same measuring devices as a g as meter.

An excavator could be used to move boulders at a rate proportional to the v oltage and a size proportional to the current. The brake is held on propor tional to the phase lag. The fuel consumed is proportional to the energy.

An LED shines on a photocell with an LCD between them. The current to the LED is proportional to the instantaneous current in the wire. The number o f pixels activated in the LCD is proportional to the instantaneous voltage to the meter and the illumination to the photocell is the product. Actuall y, that might not be totally ridiculous. The LCD would need to be worked s ome way so it doesn't need an entire digital controller. Is there some sor t of shutter that could be proportional to a voltage? Or can the photocell response be modulated by a voltage?

Probably would have a limited range of accuracy. Like most measuring devic es.

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oltage. Their effect on an aluminum disk is proportional to the product in cluding the phase angle, in other words the complex product of voltage and current. The meter has fixed magnets that create drag making the rotationa l speed of the rotor proportional to the power being drawn.

e relative phase, then finding the product though the EM effects on the dis k.

and current and finding the product. It is easy to find various circuits t o multiply two quantities. It's not hard to find various ways to measure v oltage and current. Async voltage to PWM converters for both the voltage a nd the current taps, then use an XOR to find the instantaneous product. Sm ooth the pulses and you have an analog signal proportional to the instantan eous power.

ate the log of the signals and sum them giving power. This would require t he two inputs to be biased since the input to the log can't go through zero . The bias would be subtracted out at the end potentially creating a probl em of a small difference of two large signals.

Thanks, except for PWM (and averaging) multiplying signals is expensive or finicky. I hadn't thought about adding logs, thanks again.

The heat from a fet, where you servo the voltage and current has to equal the power... the trick would be measuring it.

George H.

oportional to the voltage. The opening of a valve can be made proportional to the current and the volume of water would be the product measured by st andard means. Or a gas could be used using the same measuring devices as a gas meter.

voltage and a size proportional to the current. The brake is held on prop ortional to the phase lag. The fuel consumed is proportional to the energy .

e LED is proportional to the instantaneous current in the wire. The number of pixels activated in the LCD is proportional to the instantaneous voltag e to the meter and the illumination to the photocell is the product. Actua lly, that might not be totally ridiculous. The LCD would need to be worked some way so it doesn't need an entire digital controller. Is there some s ort of shutter that could be proportional to a voltage? Or can the photoce ll response be modulated by a voltage?

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oltage. Their effect on an aluminum disk is proportional to the product in cluding the phase angle, in other words the complex product of voltage and current. The meter has fixed magnets that create drag making the rotationa l speed of the rotor proportional to the power being drawn.

e relative phase, then finding the product though the EM effects on the dis k.

Yes. It's a kind of psychological validation; somebody important wants it. The money can be useful too.

That meter uses one voltage coil line-line, and two current coils. That assumes that the 120-0-120 volt line is symmetric, which is imperfect but saves money in the meter.

onsdag den 29. april 2020 kl. 02.50.46 UTC+2 skrev jla...@highlandsniptechn ology.com:

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r voltage. Their effect on an aluminum disk is proportional to the product including the phase angle, in other words the complex product of voltage a nd current. The meter has fixed magnets that create drag making the rotati onal speed of the rotor proportional to the power being drawn.

the relative phase, then finding the product though the EM effects on the disk.

looks to me like it is just one line and one neutral

Right. The usual US meter has one voltage coil and two current coils. That internal element has those two current coils, but looks to be connected in the housing to only meter one current.

That would be very rare in the USA, to meter a single 120-N service. That's ancient.

onsdag den 29. april 2020 kl. 04.06.55 UTC+2 skrev jla...@highlandsniptechn ology.com:

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for voltage. Their effect on an aluminum disk is proportional to the prod uct including the phase angle, in other words the complex product of voltag e and current. The meter has fixed magnets that create drag making the rot ational speed of the rotor proportional to the power being drawn.

ing the relative phase, then finding the product though the EM effects on t he disk.

maybe for something like stair lights ?

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ne for voltage. Their effect on an aluminum disk is proportional to the pr oduct including the phase angle, in other words the complex product of volt age and current. The meter has fixed magnets that create drag making the r otational speed of the rotor proportional to the power being drawn.

uding the relative phase, then finding the product though the EM effects on the disk.

Not likely to need a separate meter for stair lights, but we do have a lot of outdoor lights on tall poles all supplied by the power company. Typical ly they are billed with a fixed fee, but I believe some locations meter the electricity and bill accordingly. That could easily be 120VAC rather than 240VAC.

I noticed the unit in the video had a crack in the glass, so likely removed from service while still working.

log(U) + log(I) = log(P)

If you need to sum (for averaging) individual power samples, you need to use antilog( log(P) ) before summing different samples. The exponential function required dynamic range is much larger than for either log(U) or log(I).

Just use abs() (full wave rectification) before the log(),

On Wednesday, April 29, 2020 at 12:46:36 AM UTC-4, snipped-for-privacy@downunder.com w rote:

voltage. Their effect on an aluminum disk is proportional to the product i ncluding the phase angle, in other words the complex product of voltage and current. The meter has fixed magnets that create drag making the rotation al speed of the rotor proportional to the power being drawn.

he relative phase, then finding the product though the EM effects on the di sk.

and current and finding the product. It is easy to find various circuits to multiply two quantities. It's not hard to find various ways to measure voltage and current. Async voltage to PWM converters for both the voltage and the current taps, then use an XOR to find the instantaneous product. S mooth the pulses and you have an analog signal proportional to the instanta neous power.

g can't go through zero. The bias would be subtracted out at the end poten tially creating a problem of a small difference of two large signals.

There will be times of zero current and/or voltage. It's not about the sig n. It's about the undefined nature of log(0) and what happens as you appro ach it. I can't think of a way to deal with this issue other than to add a bias. I suppose the two could be combined, abs() with a small bias added making the difference between large numbers only an issue at low values. B ut this is not a very practical solution anyway.

A bit of making a silk purse from a sow's ear... or gilding the lily. Pick one.

chnology.com:

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for voltage. Their effect on an aluminum disk is proportional to the prod uct including the phase angle, in other words the complex product of voltag e and current. The meter has fixed magnets that create drag making the rot ational speed of the rotor proportional to the power being drawn.

ing the relative phase, then finding the product though the EM effects on t he disk.

The last meter I saw for 120V was installed in 1945, and removed in 1966 from the garage of a house my parents bought when I was starting high scho ol. The original owner had built a small cottage to live in while the actua l house was built. After that, they cut away most of the wood floors to use it for a garage. My dad and I finished removing a center wall, and the res t of the floor because '60s cars were too wide for the missing wood. We had the separate 120V service turned off, then I ran 240 to it from the main h ouse. My first Electronics workshop was built on the back of that building while I was in high school.

for voltage. Their effect on an aluminum disk is proportional to the product including the phase angle, in other words the complex product of voltage and current. The meter has fixed magnets that create drag making the rotational speed of the rotor proportional to the power being drawn.

including the relative phase, then finding the product though the EM effects on the disk.

seems kind of dodgy, you need to keep those two clocks decorellated.

bias precision could be a problem, you have the same issue with the PWM scheme.

pass a current proportional to line voltage through a hall effect cell and derive the magnetic field from the line current

output voltage will be proportional to istantaneous power.

The instantaneous power product goes negative when the power factor is