A new principle has been employed in an adaptive control for spot welding of unknown types of steel. The dynamics of the process is monitored, not materials properties or energy input etc. The rate of rise of the temperature, as reflected in the rate of rise of the resistance, provides all the necessary information. The control algorithm and a series of evaluative tests are described. Any conventional welding machine equipped with the new control will make sound welds the very first time, in HS or LC steel of two or multiple thicknesses, with electrode diameters as different as 2:1, and in the presence of shunting. A “teaching mode” is not required.