AC to AC power adapter

I use 10V AC-AC adapter ( un-loaded ~13.0V) instead of a 9V AC-AC adapter, the Vrms in emontx seem too high (~254V, analogue multimeter ~237V ). What do I do to manage it?

You can do one, or possibly a combination, of three things:

  1. Change the voltage divider resistors inside the emonTx. You’ll find a page in Resources > Building Blocks that explains the maths, and the circuit diagram and PCB layout in the Wiki.
  2. Edit the sketch and change the calibration constant. How to do this is also in Building Blocks. You will need a programmer to connect a computer to the emonTx.
  3. If you are using emonHub and an emonPi, in the emonhub.conf file, in the section for your emonTx node, change the multiplier for the voltage in the line that starts "scales = ". The numbers correspond 1 to 1 with the name in the line "names = ".

We cannot guarantee proper operation of your emonTx if you are using that adapter as the only source of power to your emonTx.

If you provide more details of your transformer, i.e. the actual unloaded output voltage and the input voltage when that was measured, and the voltage limits of your electricity supply, I may be able to give you more guidance.

I’m having the exact same issue here. The default emonTX firmware gives me around 254V and multimeter ~237V.
The 3phase firmware for emontx gives me 264V.
Could you share your solution if you have found one ? I’m probably going to try fiddling with the calibration constant myself.

Aimless “fiddling” will most likely give you the wrong result. Instead of fiddling, you could try reading about calibration - you’ll find out why calibration is necessary, the theory for how to calculate the constant given your particular mix of hardware and you’ll find detailed procedures for how to calibrate your emonTx with the default sketch. As I wrote above, the general procedures are all in Resources. As you are apparently using the 3-phase sketch, you should read the general procedure, then read the comments at the top of the sketch and follow those. The calibration you end up with will be unique to your particular mix of components.

Thank you for your reply. I calibrated the emontx now and turns out VCal needed to be 247.0 .
Setup we have :
3phase monitoring with emonTx v3 powered by ac/ac 230V/9V idealpowerDE 06-09

ac measured with agilent U1232A

The theoretical value for the calibration coefficient is 260 for your adapter. Your value differs by 5%, which is exactly the tolerance on the a.c. adapter. So adding in resistor and supply tolerances, it is comfortably inside the possible range of values that one could expect.