Device for influencing the reset value of a valve

A fluid operated drive assembly having snap characteristics for operating a closing control mechanism (126, 136, 118) comprising a fluid actuated motor (160 to 172) and biassing spring means (110) as well as magnet assembly (112, 114) including a stationary and a movable part made from magnetic material, driving members of said motor, said spring means and said magnet assembly acting on a lever (106) which in turn is connected to an output member (116) of the drive assembly, comprising abutment means (204) defining a minimum distance between the two parts of the magnet assembly, and comprising adjustment means (208) for varying the relative position between the two parts of the magnet assembly (112, 114) wherein the end faces of the two parts of the magnet assembly (112, 114) are adapted to be moved into a position of mutual surface contact thus forming said abutment means and wherein the adjustment means (208) act on one (112) of the parts of the magnet assembly (112, 114) for moving the latter in a direction that is perpendicular to the pivoting plane of the lever (106).