The present invention is related to a method for measuring the instantaneous value of a monophase AC voltage signal V.sub.e, comprising the steps of: applying said voltage signal V.sub.e to a first input of a phase comparing circuit and a reference signal to a second input of said phase comparing circuit, in said phase comparing circuit deriving from the phase difference between said reference signal and said voltage signal V.sub.e a correction signal and outputting said correction signal, filtering said correction signal, using said filtered correction signal to adjust said reference signal, when the phase difference between said reference signal and said voltage signal V.sub.e is cancelled, determining the instantaneous value of said voltage signal V.sub.e.