Selenium rectifier

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Selenium rectifiers were invented in 1933. They were used to replace vacuum tube rectifiers and in high current battery charger applications where only dc generators could be previously be used. Vacuum tubes could not put out the tens of Amperes needed for charging automobile batteries.

They consist of stacks of square aluminum or steel plated with about 1 μm of bismuth or nickel. A much thicker layer of selenium (50 to 60 μm) which has been doped with a halogen is deposited on top the thin metal plating. The selenium is then converted into polycrystalline gray (hexagonal) form by annealing. Each plate is able to withstand about 20 volts in the reverse direction. The metal squares also serve as heat sinks in addition to providing a mounting place for the selenium disks. Plates can be stacked indefinitely to withstand higher voltages. Stacks of thousands of miniature selenium disks have been used as high voltage rectifiers in televisions and photocopy machines.

Radio and television receivers used them from about 1955 to 1975 to provide up to a few hundred volts of plate voltage. Vacuum tube rectifiers had efficiencies of only 60% compared to the 85% of selenium rectifiers. They are ready to operate instantly, unlike vacuum tubes, which require 20 seconds to warm up.

Selenium rectifiers had a shorter lifetimes than desired. 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.^[1] They are still manufactured for exact replacement purposes but are not designed into new equipment.^[2] When they are replaced with silicon rectifiers output voltage is too high because of their greater efficiency. Series resistors must be added to prevent overvoltage.