Accurate Measurement of AC Votlage and Current

[sabu31]

Dear All,

I want to measrure AC voltage and current for an energy monitoring project. I am using differential amplifier configuration for sensing the voltages. However, i am getting error of more than 1 %. The voltage range is between 110V -250V RMS. The opamp used is MCP6022. Similar issue I am facing with Current. The current range is maximum of 200A and a CT (200/5) is used. The accuracy at lower currents is poor. And I need to get at least 1% accuracy. The sensing resistors used is having 1% tolerance.

WHat is the best approach and any references for the same.

 

[FvM]

How do you measure AC? What is your specification, true RMS or something else? MCP6022 seems well suited as such. 1% accuracy is no rocket science and should work with 5 V supply. For CT, it's essential to understand the shunt requirements.

 

[KlausST]

Hi,

We need to see your full schematic.

And we need to see your code.

******
Following there are a lot of questions. Most of them are essential go get the result you expect. I mean: you are worried about 1% error, so you need to describe the "expected result" in first place.


to your text step by step:

I want to measrure AC voltage and current for an energy monitoring project.

I guess grid tied. What´s the frequency range of your grid. Range! - This is min to max.
WHAT exactly do you want to measure? true V_RMS? true I_RMS? P_apparent? real_power?
HOW OFTEN do you need a measurement data output?

If you want V_RMS and/or I_RMS ... do you want them including DC or excluding DC? If "excluding", then what cutoff frequency do you want?
(Many handheld true RMS meters work show true_RMS_excluding_DC.

(I could give an explanation to every single point ... why the information is nedded and how "ignoring" the point is likely to get (expectable) errors. )

I am using differential amplifier configuration for sensing the voltages. However, i am getting error of more than 1 %.

This tells nothing. Not how it is connected, what other parts are involved ... nothing we can use to calculate expectable errors.
Also what does 1% mean? is it 1% of full scale, or 1% of the shown value?
And how do you know about the "true value"? What is your reference?

My standard resistors have 1% tolerance. So I´m not worried when the value is 1% off. And capacitor tolerance usually is even higher. 5%, 10% or higher...

The voltage range is between 110V -250V RMS.

If it fluctuates .... in that hige range ... then for comparison you need to ensure to measure the exact same time window on both your measurement devices. How can you ensure this?

The opamp used is MCP6022.

The OPAMP is usually far more accurate than the parts around the OPAMP. ...

Similar issue I am facing with Current. The current range is maximum of 200A and a CT (200/5) is used. The accuracy at lower currents is poor.

This - again - tells nothing. --> post the link to the CT datasheet. It´s rather likely that the CT´s error + burden_error is more than 1%.

The sensing resistors used is having 1% tolerance.

What do you mean with "sensing resistors"? Are there special sensing resistors .. or do you mean the burden resistor ... or do you mean all used resistors?

But generally ... if the resistors are specified with 1% tolerance ... this means you have to expect a worst case accuracy error of 1%.

WHat is the best approach and any references for the same.

"The best approach" depends on your requirements, specifications.

*****
Indeed I´m not sure .. when you talk about 1% accuracy error .. do you really mean "accuracy" or "precision". In doubt: read about their defintions.

Klaus