Selenium rectifier

Selenium rectifiers were used to replace vaccuum tube rectifiers for higher current applications. They consisted of stacks of square plates coated with selenium. Each plate was able to withstand about 20 volts in the reverse direction. They were widely used in battery chargers. Vaccum tubes could not put out the tens of Amperes needed for charging automobile batteries.

Radio and television receivers used them from about 1955 to 1975 to provide up to 250 volts of plate voltage. They were much more efficient that vaccuum tube rectifiers. Unlike vaccuum tubes, they did not need 20 seconds to warm up before operating.

Selenium rectifiers had a short life. When they failed they generated a nasty stink that let the repair technician know what the problem was. They were replaced by silicon rectifiers which exhibited lower forward voltage drop, lower cost, and higher reliability.