ZMPT101B AC Single Phase Voltage Transformer Sensor Relay is the best for the DIY project, where we need to measure the accurate AC voltage with the voltage transformer. This is an ideal choice to measure the AC voltage using Arduino/ESP8266/Raspberry Pi like an open-source platform. In many electrical projects, an engineer directly deals with measurements with a few basic requirements.ZMPT101B is a high-precision voltage Transformer. This module makes it easy to monitor AC mains voltage up to 1000 volts. A tiny little thing the size of a bouillon cube. Holds up to 4kV per breakdown voltage, the ratio of turns is 1: 1, but this is a current transformer of 2mA: 2mA. We feed it a current and remove the current. The input current is simply set by the resistor in series R1, and we use a sampling resistor R2 in parallel to get the output voltage.