This may not apply to all of these ZMPT101b modules. The modules I received from 2 different vendors are exactly alike except for different board markings. I would like to caution about potential safety issues. The transformer is a current transformer and there is a resistor on the line/mains input circuit primary that controls the current in the primary. What they use is a small "1206" SMD resistor. These resistors have a very small pad spacing and are typically rated for only 50V working. Since the transformer primary is 110ohms and the supplied resistor is 820k ohms, most of the voltage appears across the resistor; I do not believe this is safe! The voltage across this input resistor is many times the probably safe working voltage. If using this module, please consider this and consider possible remedies. I have modified the units I use by removing this small resistor and cutting the circuit trace to open a larger gap. I have then installed 2 x 1/8W leaded resistors that, in series, provide a safer alternative.
There are other issues with these modules that are not safety issues. The aforementioned input resistor is not a good choice for 120V line voltage and only slightly acceptable for 230V mains voltage. The resistor is supposed to set the primary current at between 1 and 2 mA. 820k ohms on even a 230V line only sets about 280microamps current; 280microamps in the secondary circuit (it is a 1:1 current ratio) is applied to a 100 ohm burden resistor to develop a voltage and this voltage is only about 28mV. It would be much better, require less amplification, if a proper current were set in the primary to begin. Thus, for a 230V mains, something like 150k ohms (using properly rated resistors, or series resistors to get the rating) is a much better choice and produces a higher voltage on the burden resistor on the secondary and requires less amplification (and noise).
Otherwise these are nice modules and seem to be good quality and it would be difficult to build something like this for the price of $2.50USD. I am attaching a schematic for the modules I describe. Since I had a difficult time finding a schematic, others may find this useful as well.
