Selenium rectifier
A Selenium rectifier is a type of metal rectifier, invented in 1933. They were used to replace vacuum tube rectifiers and in high current battery charger applications where previously only DC generators could be used. Vacuum tubes could not put out the tens of Amperes needed for charging automobile batteries.
Construction
Selenium rectifiers are made from stacks of square, or round, 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 of 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, or disks, 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 television sets and photocopy machines.
Use
Radio and television receivers used them from about 1947 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 around 20 seconds or more in order to warm up, precision valved equipment taking up to an hour or more in order to become stable. Selenium rectifiers were gradually replaced with silicon diodes who have efficiencies near to 100% at high voltages. However Selenium rectifiers had capability to act as current limiters which can temporarily protect rectifier from short circuit or provide stable current for charging batteries.
Replacement
Selenium rectifiers had a shorter lifespan than desired. During catastrophic failure they produced significant quantities of malodorous and highly toxic fumes that let the repair technician know what the problem was. By far the most common failure mode was a progressive increase in forward resistance, increasing forward voltage drop and reducing the rectifier's efficiency. During the 1960s they began to be superseded by silicon rectifiers which exhibited lower forward voltage drop, lower cost, and higher reliability.[1] They are still manufactured for exact replacement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 except for some automotive battery chargers.[2]
References
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