The main flaws of electric arc welds and factors determining the quality of the welded seams are considered in this paper. In practice, welding tools and their control systems are affected by some external factors causing deviation of the weld’s quality from the specified values. Currently used control systems of welding tools do not always satisfy the requirements for quality of welded joints. Major principles of developing the adaptive control system of the welding tool are considered here and the developed functional and block diagrams of microprocessor system are presented. General equations describing the welding process are also provided and the obtained transfer function of a closed system is described.