A selenium rectifier is a type of metal rectifier, invented in 1933. They were used to replace vacuum tube rectifiers in power supplies for electronic equipment, and in high current battery charger applications.
The photoelectric and rectifying properties of selenium were observed by C. E. Fitts around 1886 but practical rectifier devices were not manufactured routinely until the 1930s. Compared with the earlier copper oxide rectifier, the selenium cell could withstand higher voltage but at a lower current capacity per unit area.
Selenium rectifiers are made from stacks of aluminum or steel plates coated 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.
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, partially because vacuum tube rectifiers required heating. Selenium rectifiers have no warm-up time unlike high vacuum rectifiers. Selenium rectifiers were also cheaper and simpler to specify and install than vacuum tubes. However they were later replaced by silicon diodes with high efficiencies (close to 100% at high voltages). Selenium rectifiers had the capability to act as current limiters which can temporarily protect the rectifier during a short circuit and provide stable current for charging batteries.
A selenium rectifier is about the same size as copper oxide rectifiers, but much larger than a silicon or germanium diode. Selenium rectifiers have a long but not indefinite service life of 60,000 to 100,000 hours depending on rating and cooling. The rectifier can show some unforming of the rectifier characteristic after long storage. Each cell can withstand a reverse voltage around 25 volts, and has a forward voltage drop around 1 volt, which limits the efficiency at low voltages. Selenium rectifiers have an operating temperature limit of 130 C, and are not suitable for high-frequency circuits.
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. They are still manufactured for exact replacement purposes but are not designed into new equipment.
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