Ohm’s law states that when a variable electrical voltage is applied to an object, the strength of the electric current flowing through it changes in proportion to the voltage. This means that the electrical resistance, defined as the quotient of voltage and current, is constant, i.e. independent of voltage and current. The basic formula is:
U = R ⋅ I with R = electrical resistance, U = electrical voltage and I = electrical current. Strictly speaking, Ohm’s law only applies to direct current, but can also be applied to alternating current in an adapted form. It describes the relationship between current and voltage in a simple way. For resistance welding, Ohm’s law can be used to describe and explain the relationships. In the case of resistance welding, the resistance R is decisively described by the electrodes, the intervening materials (welded parts) and the electrodes. If there were no resistance there, then nothing would happen because no voltage would drop.
The voltage drop results in a power conversion P, which can be expressed in heat (Joul’s law). This heat finally ensures the melting and the creation of the bond, of course only if the right parameters and above all the right control are chosen.
This also makes it easy to see that a different welding procedure is required for materials with low resistance, for example, aluminium or copper, than for steel. With low-impedance materials, a much higher current must be used than with high-impedance materials. This fact is directly reflected in the design of the welding parameters and the electrical components.