Working CT123 EmonTX sketch changes for USA

We have been working on the accuracy of the EmonTX calculations for use on US 120V power systems.

Using a clamp meter and the Ubiquiti mFi solution for comparison we now have a working sketch that is accurate to within 0.1amps

There was a fair bit of trial and error to get to this point and our next step is to find an AC-AC adapter in order to get a working CT123+Voltage sketch.

In the meantime we thought the rest of the community may appreciate seeing our changes.

The primary changes are obviously the voltage

if (CT1) {
    emontx.power1 = ct1.calcIrms(1480) * 120.0;                         //ct.calcIrms(number of wavelengths sample)*AC RMS voltage

Also, we found the following calibration number to deliver the most accurate value.

if (CT1) ct1.currentTX(1, 95.2);                                     
  if (CT2) ct2.currentTX(2, 95.2);                                 
  if (CT3) ct3.currentTX(3, 95.2);                                     


Robert Wall's picture

Re: Working CT123 EmonTX sketch changes for USA

A current calibration constant of 95.2 implies that either your current transformer is not of the claimed ratio, and/or your burden resistor is not the specified value. It seems likely that your supply voltage too is not the value that you think it is. You should also bear in mind that you are taking no account of the power factor of your load.

There is a page in Building Blocks that describes the derivation of the calibration constants.

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