# AC voltage sensing circuit

In a project I'm working on, I am designing a Totem pole PFC, and for the controller (using a digital controller) I need to sens the input voltage, which is an ACvoltage and feed it to the ADC of the digital controller and do the necessary calculations.

My question is regarding a circuit that senses and scales down the input voltage so I can feed it to the controller.

I tried this circuit:

Idealy the results will be as follows:

And the output voltages are:

V1 = VN * R2/(R1+R2)

V2 = VL * R2/(R1+R2)

The problem is that I don't know the voltage of the phase VL and of the neutral VN in regard to the ground GND, so I don't know how to choose the right resistors R1 and R2 and secondly the input of the op amp IN+ can't have less than (-V)-0.1V = -0.1V, so I think it will burn out if V1 or V2 goes under -0.1V.

Should I be worried about these problems and how can I find solutions for them, and is there any circuit with similar behaviour that I can use.

Thank you.

• If I were you, I would put a small transformer. Power rating is not important, and one can be found with any desired turns ratio. And one lethal advantage is, of course, isolation. And the isolation lets you to choose any pin as a reference (i.e. GND). May 18, 2017 at 18:16