The power supply of the measurement part should be non-isolated and referenced to one of the AC-Mains terminals. Figure 2, The power supply section of the circuit. C2 is a 470nF-630V capacitor [5] to reduce the AC voltage. R6 discharges the capacitor at power OFF. D1 and D3 are 1N4007 [6] diodes to rectify the AC voltage and D2 reduces to ...