I need to read an analog voltage

goodnight I am doing a project of surge with an autotransformer

As you vary the voltage supplied by the mains of my country varies its output

this is by sensing voltage across a control winding of the autotransformer whose readings are 2.5v - 4v

which I am by sensing in the analog pin A0

I need to know that voltage sensing that enable power stage and thus achieve steady output voltage to 120 VAC 60Hz

when making my design to detect apply the following arrangement

a bridge rectifier and a capacitor and a variable resistor

one pin connected to Vcc
pin center to pin analog A0
and the last to GND

pero no me percate del tiempo que le toma al capacitor de carga y descarga

which at the time of the readings by analogRead the capacitor discharge time is outside the parameters set

my question is

as I can read this voltage

since my Arduino UNO reads 0 - 5V

which it is the same from 0-255

thank you very much