### AC measurement

Hi,

I just built up a PCB with the same CT schematic like EmonTx (only ADC circuit for CTs and AC measurement, no AC powering or temperature sensors)  ran the hardware successfully with your EmonLiB!

Thanks a lot! Very easy and straight  forward!

But now I want to include the AC measurement for higher accuracy.

Due to the mounting position I am thinking about using the already existing bell transformer.

Would it be sufficient? If yes, do I need to adapt something in the circuit? The transformator supplies 8 volt ac.

I hope you can give me some hints.

Regards,

Ulli

### Re: AC measurement

But now I want to include the AC measurement for higher accuracy.

Do you mean that you want to measure AC voltage as well as AC current?

The AC voltage from your bell transformer should be OK for this purpose.  You would need to include the Building Blocks circuit which creates a smaller version of this signal for the processor to measure.

When the door bell is ringing, the voltage is likely to drop so your measurements will be affected.

### Re: AC measurement

Yes, I meant measuring AC voltage with the following functions of the emonLib library:

emon1.voltage(2, 234.26, 1.7);  // Voltage: input pin, calibration, phase_shift
emon1.current(1, 13.333);       // Current: input pin, calibration.

I attached my current schematic and want to ask whether you see any mistakes?

What do I have to adept if the AC voltage is 8V AC of the bell transformer, instead of 9V of an AC-AC power adapter?

I recalculated the CT current calibration value to 13.33 based on 150 Ohm burden resistor.

How do I need to adept the voltage calibration factor? Do I need to change some resistor values regarding the AC voltage measurement or is a calibration value change sufficient?

### Re: AC measurement

You probably will not need to change anything. The 9 V transformer typically gives 11 - 12 V on no load, and I would expect your bell transformer to give a similar voltage. In any case, the voltage is not critical as long as you know the relationship between the mains voltage and the transformer's output voltage. Given that, everything you need to calculate the calibration coefficient is in Resources > Building Blocks > CT and AC power adapter installation and calibration theory.

### Re: AC measurement

Great thanks so far!

### Re: AC measurement

One question regarding the voltage calibration. I got the following formulnar for the emonTx V2:
voltage constant = 230 × 11 ÷ (9 × 1.20) = 234.26

230 is the main voltage, 11 is the scaling factor based on the voltage divider R13 & R14 (v2=11, v3 =13), 9 is the AC  adapter output voltage and 1.2 is the transformer without load correction.

So far so good...but in v3  the resistors are 10k  and 120k...shouldn't the scaling factor be 12 instead of 13?

Additionally, could you give me a hint how to find the right power factor/phase angle?

### Re: AC measurement

"shouldn't the scaling factor be 12 instead of 13?"
No. You need to refresh your knowledge of voltage dividers.
And don't rely on the 20% regulation (factor of 1.2) - it depends on the details of the transformer design and varies between models and manufacturers.

"could you give me a hint how to find the right power factor/phase angle?"
The only realistic way is by trial and error. Again, the procedure is written out in detail in Building Blocks.