Thread tension control device for a sewing machine

A thread tension control device for a sewing machine comprising a tension stud of which one end is fixed in part of a sewing machine main body, a pair of tension discs disposed on the tension stud slidably in its axial line, said tension discs being supported rotatably about the tension stud, a pneumatic cylinder engaged with at least one of the pair of tension discs, said pneumatic cylinder having a drive rod for moving one tension disc to other tension disc side by force, and an air regulator interposed between the pneumatic cylinder and an air supply source, thereby adjusting the pressure to the thread engaged with the tension stud and held between the pair of tension discs,

• wherein said air regulator is composed in an electropneumatic system for converting between voltage and air pressure, and the applied voltage to the electropneumatic air regulator is controlled on the basis of an instruction from a microcomputer, so that the set pressure of the air supplied to the pneumatic cylinder is adjusted.