Bi-directional Watt Meter

[snipersquad100]

Can anyone tell me how I can monitor the direction of the current going to and from my electric meter?
I got a watt meter on the positive cable but I don't know if it's coming from the grid or going to the grid via my solar panels.
my solar setup is DIY so no fancy grid tie inverter telling you the whole picture.
Thank you.

[David Hess]

The phase between the current and voltage reverses 180 degrees when power is reversed so you need a better watt meter.

[Brumby]

The old fashioned electromechanical disc meters were good at this.  They would run backwards if you were feeding power back into the supply network.

[snipersquad100]

Yes, those turning disc meters were great, I wish I still had one.

David Hess, No shit. a better meter is what I'm asking advice for. 

Why are you answering a question I never asked? "The phase between the current and voltage reverses 180 degrees when power is reversed" I know what a sinusoidal wave
form looks like, you fucking idiot.

[JPortici]

dude, chill.

[dmills]

Current transformer to get the current waveform, small centre tapped mains transformer to provide the voltage reference, a diode ring mixer to do the multiplication, lowpass and display on a centre zero moving coil meter, job done?

Use the mains voltage reference to switch the diodes in the mixer (Effectively use it as the LO), and feed the current transformer output in where the RF would normally go, moving coil meter goes across the IF (DC coupled) output.

73 Dan.

[fourtytwo42]

Current transformer to get the current waveform, small centre tapped mains transformer to provide the voltage reference, a diode ring mixer to do the multiplication, lowpass and display on a centre zero moving coil meter, job done?

Use the mains voltage reference to switch the diodes in the mixer (Effectively use it as the LO), and feed the current transformer output in where the RF would normally go, moving coil meter goes across the IF (DC coupled) output.

73 Dan.

I saw you post this idea somewhere else and I am intrigued, wouldn't this require a center tapped ct ?
All the 50hz split core ct's I have seen used in this application are single winding, no center tap.

[Simon]

dude, chill.

He will be chilling for 2 days!

[dmills]

I saw you post this idea somewhere else and I am intrigued, wouldn't this require a center tapped ct ?
All the 50hz split core ct's I have seen used in this application are single winding, no center tap.

Divide the burden resistor for the CT into a series pair and use the junction (it will be small compared to the scaling resistor for a 50uA or so movement anyway), I see no reason to think it would not work, yea, a few tens of uA DC in the cores, but meh.

The little mains transformer does of course need a centre tap, but that is trivial.

What is more interesting to consider is the possibility of eliminating the mains transformer and doing the thing with a cap divider and equalising resistors, it would all be live chassis, but I think it would probably still work.

Regards, Dan.

[schmitt trigger]

Some of the power meter ICs from Analog Devices have a "reverse power" output precisely for this scenario.

[fourtytwo42]

I saw you post this idea somewhere else and I am intrigued, wouldn't this require a center tapped ct ?
All the 50hz split core ct's I have seen used in this application are single winding, no center tap.

Divide the burden resistor for the CT into a series pair and use the junction (it will be small compared to the scaling resistor for a 50uA or so movement anyway), I see no reason to think it would not work, yea, a few tens of uA DC in the cores, but meh.

The little mains transformer does of course need a centre tap, but that is trivial.

What is more interesting to consider is the possibility of eliminating the mains transformer and doing the thing with a cap divider and equalising resistors, it would all be live chassis, but I think it would probably still work.

Regards, Dan.

Thank you Dan I should have thought of that as I often use divided burdens to avoid a full bridge rectifier! I am certainly going to give this method a go because my present system (sampling mpu) has trouble discriminating direction with light distorted loads such as ccfl's. I have a gut feel this simple analogue solution is much better

Regards
Fourtytwo

[richard.cs]

I have done analogue ac power measurement as part of a solar dump load board I built, in this case the voltage and current signals were scaled and preconditioned with op-amps and fed into an AD633. The real power resulting has a strong 100 Hz component which is then filtered to DC. In my device this was used to provide a monitoring output, and also integrated (again in analogue) to track energy use within a 1 Watt-Hour window. Not the cheapest approach but it works very well. I've been meaning to post a write-up on here for over a year and not got around to it.

For a simple visual indication of power I do rather like dmills idea of mixer with mains-derived LO. It seems a very elegant approach and I see no reason that splitting the burden to get a virtual centre tap should not work.

[dmills]

Anything has trouble at low loads, just because the magnitudes become rather puny.

CCFLs (And LED lights with built in switchers) can be notoriously harmonic rich and sometimes of surprisingly piss poor power factor, I would expect to be having to sample at well over 1kHz do get these right with a digital solution.

A log reading meter might be an idea (Read out in dBW!), much easier to see the small stuff.

You will want the LO drive to be large compared to the burden voltage from the CT, and probably want the CT voltage to be small compared to two diode drops, but that should not be a problem. 

Regards, Dan.

[fourtytwo42]

I tried the circuit (enclosed) and got a result shown here (pdf) but the output doesn't discriminate between leading and lagging phase Have I made an error maybe ?

[In Vacuo Veritas]

Yes, those turning disc meters were great, I wish I still had one.

David Hess, No shit. a better meter is what I'm asking advice for. 

Why are you answering a question I never asked? "The phase between the current and voltage reverses 180 degrees when power is reversed" I know what a sinusoidal wave
form looks like, you fucking idiot.

Male menopause, huh?

[richard.cs]

I tried the circuit (enclosed) and got a result shown here (pdf) but the output doesn't discriminate between leading and lagging phase Have I made an error maybe ?

No, it doesn't do that because it's projecting a two dimensional quantity onto a one dimensional output. It gives you the magnitude and sign of the real component of the power which is sufficient to do what you the OP originally asked for, know whether power is flowing into or out of  his house through the electric meter. Leading and lagging phase by the same amount affect the real power by the same amount so you don't see it in the output, and for the application given is unimportant. A normal domestic electric meter only records the real component.

If you want to know the reactive component you could build a second one with the voltage reference at 90 degrees (you would have to shift it with more components, although in simulation you can just add 90 degrees) then that one would give you the magnitude and sign of the reactive power and you would have the full set of information.

[fourtytwo42]

It gives you the magnitude and sign of the real component of the power which is sufficient to do what you originally asked for

Ooops please don't confuse me with the banned OP! I only hijacked the thread to carry on the discussion but thank you for your explanation I will go away and cogitate

[richard.cs]

Ooops please don't confuse me with the banned OP! I only hijacked the thread to carry on the discussion but thank you for your explanation I will go away and cogitate

Sorry about that.

Thanks for modelling it by the way, though I think some of the currents need re-scaling to get something that would work with real components, the BAT54s won't like that amount of current and it's not really needed.

[dmills]

Yea, 1k or so in series with each of the mains transformer outputs would seem to be indicated...

I make it about 950mV of output with power flowing one way, -950mV with power flowing the other with 100R limiting the diode current in each leg of the transformer, and whatever the CT is that you are simulating.

I would add a trimmer somewhere to trim the zero because the phase shift thru the transformer will cause some offset.

Looks reasonably usable, but you are grossly over driving the diode ring with 5V out of the CT, try more like a hundred mV, linearity will be much improved.

[dmills]

Been playing, and here is another variant, a '3904 and '3906 as switches this time, but no need for the centre tapped voltage reference transformer.

The nice thing is that by replacing the base resistor with a cap dropper you can probably get near enough 90 degrees phase shift if you want to look at reactive power instead.

I think you can probably (at the cost of two more transistors and a couple of caps) come up with something having both real and reactive indication outputs.

I am thinking an EDN circuit ideas submission?

Regards, Dan.

[station240]

Ooops please don't confuse me with the banned OP! I only hijacked the thread to carry on the discussion but thank you for your explanation I will go away and cogitate

OP isn't banned, just in 'the cooler' for 2 days.
Not that I blame him, having in the past gotten academic replies to simple questions like "so what does your project do ?", and no I never did get a coherent reply to that question, even after 3 attempts.

Maybe when the OP returns I might have something of use to him. 

[schmitt trigger]

I understand the OP's frustration, because similar situations have happened to me.
But he did not have to use foul language.

Anyways, across the many forums for many topics which I have lurked over the past 10 years, my impression is that the discourse has become angrier and angrier.

[fourtytwo42]

Been playing, and here is another variant, a '3904 and '3906 as switches this time, but no need for the centre tapped voltage reference transformer.

The nice thing is that by replacing the base resistor with a cap dropper you can probably get near enough 90 degrees phase shift if you want to look at reactive power instead.

I think you can probably (at the cost of two more transistors and a couple of caps) come up with something having both real and reactive indication outputs.

I am thinking an EDN circuit ideas submission?

Regards, Dan.

That is really cool Dan, I didn't get a chance to further play with the other after the suggestions here but this one is just Wowww!! I was thinking about how to use my existing non-centre tapped voltage trafo without consuming to much power and you just fixed it for me Thank you kind sir and I would definitely rate that an EDN circuit idea
Regards, John

[dmills]

Note that this is more of a phase sensitive ammeter then a true power meter, as the transistors operate in saturation as switches rather then as a true multiplier.

As long as you know the supply voltage you can scale for real power, but it will not for example read half as much on a 120V circuit as it does on a 240V one at a given current, but that is probably acceptable for most uses.

Regards, Dan.

[fourtytwo42]

Note that this is more of a phase sensitive ammeter then a true power meter, as the transistors operate in saturation as switches rather then as a true multiplier.

As long as you know the supply voltage you can scale for real power, but it will not for example read half as much on a 120V circuit as it does on a 240V one at a given current, but that is probably acceptable for most uses.

Regards, Dan.

Yes I see that but I think for both me, the OP and many other GTI people we are interested in the direction rather than super accurate power readings. IMOP this simple circuit replaces acres of complex software! In my case I have had real trouble on a simple micro dealing with all the weird current waveforms generated by various bits of domestic kit and here we have a simple analogue low pass filter that will integrate the lot and spit out a reliable result without a line of code, ohh utopia indeed Once again Dan thankyou for your simple solution