Power sources can be divided into two categories: Direct current (DC) power sources,
direct current power sources are the commonest power sources and are characterised by their high stability; this is because they are based on the fact that an electric arc will tend to stabilise naturally if it is powered at constant voltage and generated on a wire fed at a constant rate. Given the flexibility of the process, there is some flexibility in the choice of voltage and wire feed rate parameters. In this way it is possible to obtain drop transfer, from the welding material to the material to be welded, using either the "short arc" immersion procedure or the "spray arc" procedure

Pulsed current power sources in this case it is not the voltage size that is regulated in the power source, but the current, which is not held constant but modulated with a train of impulses (hence the name "pulsed"). The purpose of the impulses is to force the drop to detach itself from the welding material; in this case the arc is not stabilised naturally, so that the impulses and wire feed rate must be perfectly synchronised to achieve an acceptable weld.
Both the in first and second case, at least two knobs are used for regulation; recent research in the industry has led to the development and marketing of "synergetic" type welding machines in which the operator uses only one control knob.
The manufacturer memorises the optimal welding parameters in the power source and these can be recalled and/or corrected by the operator, depending on the particular job requirements.

The different polarities when connecting the poles of the power source to the material to be welded identify two operating modes:

i) Direct current with straight polarity connection
With electrode negative connection, the torch is connected to the negative pole and the material to be welded to the positive pole of the power source; this type of connection is only used in welding with tubular wire (FLUX).

ii) Direct current with reverse polarity connection
When welding with this operating mode, the torch is connected to the positive pole and the piece to be welded to the negative pole of the power source; this is the most frequently used type of connection.