Among the numerous types of electrical resistance welding technologies, the most used is the spot welding.
To have a welding point is required a high intensity current.
For this purpose are used special transformers with a very little secondary voltage (only few volts).
Furthermore it is necessary to apply a considerable force to reduce the contact resistance between electrodes and sheets to be welded and to obtain a welded point with good mechanical characteristics.
This force is obtained by mechanical devices, driven pneumatically or by the operator force.
It is also necessary to adjust the welding time with great precision: for this purpose are used special electronic equipments.
An another version of the spot welding is constituted by the projection welding.
With this welding it is possible to realize more points at the same time. This is possible realizing projections on one of the two sheets to be weld.
These projections localise the passage of the current and determine the formation of the weld points
Next to the traditional welding monophase transformer, today are often used also groups treephase of transformers with rectifier on the secondary one.