An electric generator comprising a rotor with permanent magnets, configured for rotating about a rotation axis; at least one magnetic yoke with at least two arms extending axially inside or outside of the rotor so as to be adjacent to the radial inner or outer side, respectively, of the rotor; wherein the permanent magnets are arranged according to an Halbach array so as to maximize the magnetic field on the radial side of the rotor adjacent to the arms of the at least one yoke. Also, a valve for gas cylinder, equipped with a corresponding electric generator.