Selenium rectifiers were used in d-c welding power supplies until the development of silicon, or solid state, rectifiers. Selenium rectifiers provided a convenient means of changing alternating current to direct current. This ability was based on the characteristic of permitting the current to pass freely in one direction, while blocking, or greatly limiting, its passage in the opposite direction.
If the rectifier cell readily passes current in a forward direction, it indicates that the resistance to current flow in this direction is low. The resulting forward voltage drop across the cell thus must also be low. Conversely, if the current flow in the opposite direction is blocked, or held to a minimum value, the reverse resistance must be high. Consequently, the voltage drop across the cell in the reverse direction must also be high. The minimum value of the reverse voltage drop is limited by the breakdown potential of the barrier layer of the rectifier cell.
Selenium rectifiers had the advantage of being able to accept high-voltage surges without breaking down. Cooling was easily achieved, because selenium rectifiers were usually made up of a number of plates. This also made it easy to add more rectifying surface if required for a specific application. The disadvantage of the selenium rectifier was that it required more physical space in the power source. It was an important stepping stone in the development of solid state rectifiers.