A selenium voltage doubler rectifier is a type of power conversion circuit that uses selenium rectifier elements to double the input AC voltage. It operates based on a voltage multiplier principle, typically using a combination of diodes and capacitors to achieve the doubling effect. Selenium rectifiers offer advantages such as high surge current resistance to overload conditions compared to early silicon diodes. These rectifiers were widely used in power supplies, industrial equipment, and high-voltage applications before being largely replaced by modern semiconductor-based rectifiers.