I am a bit confused on if the adapter can be used for power supply and also for voltage sensing. Although, in this article from the learn section it is stated to not use them but in the emonTx guide it shows “Plug in and connect the AC-AC adapter to provide voltage measurement and power”. Is it emontx specific ?
Yes. The input circuit of the emonTx V3 was changed to allow the use of the a.c. adapter for both measuring the a.c. voltage and supplying power. However, to avoid distorting the measurement, the power that may be drawn, hence the number of “add-ons” (temperature sensors etc) is strictly limited.
You cannot do the same thing with the emonTx V2, nor with the emonPi, which requires far too much power.
You might like to download the circuit diagrams for the emonTx V2 (essentially the circuit in ‘Learn’) and for the emonTx V3, and compare how the a.c. adapter secondary winding is connected and the position of the bias resistors and the capacitor. All those components still do the same job for getting the voltage input to the ADC, they are simply connected in a different order. This might not be clear at first, but it’s true.