Within an experimental research project financed by the European Union, a prototype semi-active oleodynamic damper has been conceived and manufactured in Italy starting from technology today currently adopted for passive energy dissipation devices. The new device was obtained by just adding two identical electrovalves and an external oleodynamic circuit to a commercially available silicon oil damper. Static and dynamic tests performed on the damper allowed to determine its stiffness and energy dissipation characteristics when passively operating (both electrovalves are always closed or open) as well as to measure the release and insertion times of the electrovalves during opening and closing tests. The experiments indicated a non-linear quadratic viscous constitutive law for the damper as well as operating times of electrovalves in good agreement with specifications given by the manufacturer. A semi-active assembly composed by the prototype damper and external flexible steel plates was then mounted and subjected to further tests in order to definitively characterize through a simple mathematical model the entire system and to completely identify its operating delays and their physical sources.