Calibration Values for my CT

Hi everyone,

I just want to make sure I am understanding this correctly as I don't seem to be getting very accurate readings.

ct1.current(1, 300.3)

My CT has 3000 turns, however it has a built in burden resistor so the formula i need is that it outputs 0.333mV @ 100A so that calibration value should be 300.3

Since it has a built in burden resistor I don't need to incorporate one in my circuit. What if I was using the emontx?

ct1.voltage(2, 117.9, 1.7)

From what I understand the middle value should be my mains voltage. All my receptacles are close to 118 volts as I live in the US. The middle value matches this, is that correct? I still don't understand how to calibrate the phase shift which I currently have set to 1.7

Thank you.

Robert Wall's picture

Re: Calibration Values for my CT

You have several problems. First, your choice of a voltage-output CT with a maximum voltage of 0.333 V rms was a poor one. Your emonTx requires at least 1 V rms (I always suggest 1.1 V rms) and your Arduino 1.6 V rms at max current for best accuracy, so having only 0.33 V will limit resolution and damage your accuracy especially at low power.

If you cannot replace your CT or remove the burden resistor (Magnelab will supply CTs with an internal burden, calibrated for 1 V output, on request) then you must at least remove the burden resistor on the emonTx PCB. Your 300.3 is only correct when you have no second burden.

Second, the second parameter in voltage( ) is not the voltage, it is the voltage calibration constant. Read through all the articles in Building Blocks about calibration. Everything is explained there, including calibration for phase & timing errors.

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