AC Current Measurement

Let's say we have +/-20 Amps of AC current flowing at 250V into (Source) o r out of (sinking) a node. (The node is the grid and we can either supply o r sink current to that node). Measuring current is easy but how do I determ ine which direction it is going. Am I sourcing or sinking? If I sense the differential voltage across a 1mR sense resistor, limiting my power loss to 0.40 watts, and trying to resolve a 100 mA current change in direction, my CMRR would be -128 DB, not easy to do. There has got to be a better way. Cheers, Harry

Reply to
Harry D
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Using a current transformer, you look at the current in comparison with the AC line voltage.

As you start to draw power, let's say a resistive load, the current will go one direction compared to the voltage.

If you put power back into the grid, the current humps would go the other direction... Assuming the power factor is 1...

No power, the current will be flat zerol of course.

For PF lower than 1, it can be a bit of both directions but if you were drasing power from the grid, then the net current would be negative. More one way or the other.

Reply to
boB

AC current doesn't really have a direction at one point on a wire.

Power does. To measure power magnitude and sign, you need to measure both voltage (relative to a return wire) and current and multiply+average, or equivalent.

The usual way to measure AC current is with a current transformer.

Reply to
John Larkin

OK

I don't know exactly what you're after here but I'll wing it. I assume you know what alternating current is. There is no direction of cur rent flow, so I am assuming maybe you men you want to determine which end i s the source.

Current readings will do you no good for that, the current is the same in a series circuit. If you want to know which end is "up" the best way is prob ably with an extremely extremely extremely accurate voltmeter with more res olution than anyone would ever though they would need. But for this...

If you are at mains frequency forget any and all time domain shit. I am pre tty sure none of them even today could tell you accurately at like line fre quencies.

That is it. People do not give enough detail. If you did I could figure out quite a bit more. Are you working in a sewer or on a utility pole ? Or is this in a building. At how many points can you get to this particular wire ?

Or is this some project in your LAB-OR-ATORY ? You need bandwidth, wind you r own. If you don't need high frequency just wrap ten turns of #12 THHN wir e around it and feed a 1KHz square wave through it. Get an audio equalizer and make that coil's waveform a square wave. You'll be fairly accurate thro ugh the audio spectrum.

Then for your actual measurement, after the scope is off having done its th ing, use a TRMS reading meter.

Maybe you have no interest in what I said here, but I have done well figuri ng out how to measure, calibrate and align things without using those multi thousand dollar machines that are probably good for no other purpose in the world ever.

I can measure your AC current from about 35Hz to about 150KHz. And I don't have to spend a dime to do it.

Reply to
jurb6006

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or out of (sinking) a node. (The node is the grid and we can either supply or sink current to that node). Measuring current is easy but how do I deter mine which direction it is going. Am I sourcing or sinking?

g my power loss to 0.40 watts, and trying to resolve a 100 mA current chang e in direction, my CMRR would be -128 DB, not easy to do. There has got to be a better way.

** A current transducer (or transformer) will give you an easily read and S AFE voltage analogue of the actual current.

The easiest way to identify which way it is flowing is to use a scope in X- Y mode - with voltage and current waves on each input.

For in-phase waves, the slope of the line on the screen will be opposite in each case.

.... Phil .... Phil

Reply to
Phil Allison

) or out of (sinking) a node. (The node is the grid and we can either suppl y or sink current to that node). Measuring current is easy but how do I det ermine which direction it is going. Am I sourcing or sinking?

ng my power loss to 0.40 watts, and trying to resolve a 100 mA current chan ge in direction, my CMRR would be -128 DB, not easy to do. There has got to be a better way.

Nor does it sink or source from a node. Whatever current comes into a node has to go out, it can't pile up.

And to determine which is the source, which is the load, you just need to look at the direction the current is flowing and the voltage. Imagine a battery hooked up to a resistor. If I don't know which is which, they are hidden, just look at the polarity,which wire is the positiv e and then which way current is flowing in that wire. The current will be flowing from the source to the load. With AC it gets a little more complicated because the battery keeps changing voltage and reversing polarity so you have to check the voltage and current at the same point to figure out which way it's flowing and integrate the power over time if that's what you're after.

No much more to go on from what's given, as to whether this is a one time test or he's designing a new widget. Looking at a power metering IC might be an option, depending on what he's doing.

Reply to
Whoey Louie

) or out of (sinking) a node. (The node is the grid and we can either suppl y or sink current to that node). Measuring current is easy but how do I det ermine which direction it is going. Am I sourcing or sinking?

ng my power loss to 0.40 watts, and trying to resolve a 100 mA current chan ge in direction, my CMRR would be -128 DB, not easy to do. There has got to be a better way.

Grid voltage can be constant so I still have the problem of measuring the voltage difference across a sense resistor in a 250 V CM. That is difficul t to do with a 60 Hertz power line. Averaging will help but there has got t o be a better way. I have the same problem with the CT. Cheers, Harry

Reply to
Harry D

ce) or out of (sinking) a node. (The node is the grid and we can either sup ply or sink current to that node). Measuring current is easy but how do I d etermine which direction it is going. Am I sourcing or sinking?

ting my power loss to 0.40 watts, and trying to resolve a 100 mA current ch ange in direction, my CMRR would be -128 DB, not easy to do. There has got to be a better way.

ive

Let's say we are net metering in a 240Vac, 20 Amps, 60 Hz power line in a digital power meter. Cheers, Harry

Reply to
Harry D

) or out of (sinking) a node. (The node is the grid and we can either suppl y or sink current to that node). Measuring current is easy but how do I det ermine which direction it is going. Am I sourcing or sinking?

ing my power loss to 0.40 watts, and trying to resolve a 100 mA current cha nge in direction, my CMRR would be -128 DB, not easy to do. There has got t o be a better way.

SAFE voltage analogue of the actual current.

X-Y mode - with voltage and current waves on each input.

in each case.

I need this circuit for a Power Meter that will do net-metering in a 20A,

240 Vac 60 Herts power line. It will have a digital readout. Cheers, Harry
Reply to
Harry D

If you need to ask such questions, you shouldn't be doing this on your own.

Jeroen Belleman

Reply to
Jeroen Belleman

urce) or out of (sinking) a node. (The node is the grid and we can either s upply or sink current to that node). Measuring current is easy but how do I determine which direction it is going. Am I sourcing or sinking?

miting my power loss to 0.40 watts, and trying to resolve a 100 mA current change in direction, my CMRR would be -128 DB, not easy to do. There has go t to be a better way.

itive

e

a digital power meter.

Then what I posted applies, including the suggestion to use a power meterin g chip. I'm sure they have plenty of app notes too.

You have two hot wires coming in, yes? Look at that at any instant in time when there is voltage, ie not when it's crossing zero and you have what looks like a DC voltage, yes? One wire is positive, one negative, yes ? Measure the direction of the current in the positive wire. The current will be flowing on the positive wire from the source to the load. The power metering chips do that and compute the voltage, current, power, etc on the fly.

Reply to
Whoey Louie

ce) or out of (sinking) a node. (The node is the grid and we can either sup ply or sink current to that node). Measuring current is easy but how do I d etermine which direction it is going. Am I sourcing or sinking?

iting my power loss to 0.40 watts, and trying to resolve a 100 mA current c hange in direction, my CMRR would be -128 DB, not easy to do. There has got to be a better way.

nd SAFE voltage analogue of the actual current.

n X-Y mode - with voltage and current waves on each input.

e in each case.

, 240 Vac 60 Herts power line. It will have a digital readout.

Why reinvent the wheel? Buy a digitial power meter. Or if you need to build, then a power metering chip, and a lot of other components, it's not trivial. That chip will multiply voltage by current in real-time to get the power. Without doing that, if say you measured voltage with one circuit or instrument, current with another, you won't know the power u nless you also know the phase relationship between the current and voltage and that it doesn't vary. If it's a resistive load, that isn't an issue, but if it's typical loads for a net metering application, eg non-incandesce nt lighting, motors, appliances with power supplies, then the voltage and current will not be in perfect phase and it changes.

Reply to
Whoey Louie

urce) or out of (sinking) a node. (The node is the grid and we can either s upply or sink current to that node). Measuring current is easy but how do I determine which direction it is going. Am I sourcing or sinking?

imiting my power loss to 0.40 watts, and trying to resolve a 100 mA current change in direction, my CMRR would be -128 DB, not easy to do. There has g ot to be a better way.

and SAFE voltage analogue of the actual current.

in X-Y mode - with voltage and current waves on each input.

ite in each case.

0A, 240 Vac 60 Herts power line. It will have a digital readout.

unless

cent

Also take a look on Ebay, there are power measuring modules for sale there starting at $5. IDK what they are rated for, nor what you're doing, so proceed at your own risk.

Reply to
Whoey Louie

ce) or out of (sinking) a node. (The node is the grid and we can either sup ply or sink current to that node). Measuring current is easy but how do I d etermine which direction it is going. Am I sourcing or sinking?

ting my power loss to 0.40 watts, and trying to resolve a 100 mA current ch ange in direction, my CMRR would be -128 DB, not easy to do. There has got to be a better way.

he voltage difference across a sense resistor in a 250 V CM. That is diffic ult to do with a 60 Hertz power line. Averaging will help but there has got to be a better way. I have the same problem with the CT.

What's so hard about measuring the voltage across a resistor? That isn't the hard part. The hard part is measuring the voltage and current at each point in time, multiplying it and integrating it to get power and energy. Which is why I said use a power metering chip.

Reply to
Whoey Louie

A standard CT will output 5 amps. You could get a 20:5 or 40:5 CT, something like that. It would dump into a burden resistor, and give you a big, isolated, legally insulated signal.

There are also DVM accessory clamp-on current transformers, which output a scaled voltage.

You still need to include the line-line voltage to determine the direction of power flow. There are power meters that do that.

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Reply to
jlarkin

I have dissected two electronic utility meters (different makes) and neither had any wound components. Current measurement was by hall effect chip clamped to the current carrying busbar with magnetic circuit closed by some magnetic material. Voltage by a string of high value resistors.

piglet

Reply to
piglet

I have seen the innards of a meter at the UK factory where it was being made. It sensed current with a 4-terminal resistor which was a short strip of manganin electron-beam welded to copper busbars in a 4-terminal configuration. The voltage drop across the current sense resistor was amplified with an op-amp. The mains voltage was divided down with a series string of resistors. The rest of the work was done with a microcontroller.

John

Reply to
jrwalliker

The mount-on-the-exterior power meters of yore were human-readable. The modern digital ones I see are... much less friendly to a user, and favor networked operation by power utilities.

What, exactly, would be a good model of such a meter (to fit a standard meter base) for an end user? Landis and Gyr have a bunch of offerings, but trying to figure out accuracy and whether the current can be indicated, is gonna take some work (they MUST have a document somewhere, but I'm not seeing details on the website).

Reply to
whit3rd

IDK, good points. I didn't mean to necessarily buy the kind of bulky power meter that plugs into a meter base. Those, I agree, not clear that most or any would display amps or even anything other than Kwh. But some of them probably have some kind of interface to transmit real-time data, what with all the solar folks wanting to monitor their solar array and such. I had pointed him to Ebay and something like this is more what I had in mind:

271952136069

That's the item number, just put that into the Ebay search bar. The link was a mile long. It's $13 and they've sold 3100 of them. Not much info on it there though, but it sure looks nice and it shows volts, amps, power and energy, rated at 100A, 260V. It shows it wired directly up to a load. How that widget handles 100A, IDK. :)

But if not that, then I bet there are other off the shelf modules that do what he wants.

Reply to
Whoey Louie

Never mind how it handles 100A, it shows a schematic with a CT and it includes one.

Another thought, I wonder how that cheap module handles the power direction? Does it show positive or negative for the current/power? Details like that are always a problem with that Chinese cheap stuff. You'd think it probably does because they are using some kind of power meter IC that should be capable of that, would do it automatically. Doesn't cost much to try it though, compared to building one.....

Reply to
Whoey Louie

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