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[[File:Selenium Rectifier.jpg|thumb|An 8 plate 160V160 450mAV 450 mA Federal brand Selenium Rectifier|224x224px]]A '''selenium rectifier''' is a type of [[metal rectifier]], invented in 1933. They were used in [[power supply|power supplies]] for electronic equipment, and in high current battery charger applications until they were superseded by [[silicon diode]] rectifiers in the late 1960s. The arrival of the [[alternator (automotive)|alternator]] in some automobiles was the result of compact, low cost, high current silicon rectifiers. These units were small enough to be inside the alternator case, unlike the [[selenium]] units that preceded silicon devices.
The rectifying properties of [[selenium]], amongst other semiconductors, were observed by Braun, Schuster and Siemens between 1874 to 1883.<ref>[https://books.google.co.uk/books?id=JXqKERUAdQcC&pg=PA342&dq=History+Selenium+Rectifier+Willoughby&hl=en&sa=X&ved=0ahUKEwiLnOfxp5vXAhWG2BoKHdHbAq8Q6AEINzAD#v=onepage&q=History%20Selenium%20Rectifier%20Willoughby&f=false books.google.co.uk]</ref> The photoelectric and rectifying properties of [[selenium]] were also observed by Adams and Day in 1876<ref>[https://books.google.co.uk/books?id=N_3uCAAAQBAJ&pg=PA2&lpg=PA2&dq=Adams+Day+1876+Selenium+Rectifier&source=bl&ots=ECJ9DHBjdr&sig=FKjLMGFdfot_YY4q2AS1v32p28A&hl=en&sa=X&ved=0ahUKEwjR9qPOq5vXAhXGiRoKHeehBVQQ6AEIODAG#v=onepage&q=Adams%20Day%201876%20Selenium%20Rectifier&f=false books.google.co.uk]</ref> and 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.<ref>Peter Robin Morris ''A history of the world semiconductor industry'', IET, 1990 {{ISBN|0-86341-227-0}} pages 13, 18</ref>
==Construction==
[[File:Structure selenium rectifier.svg|thumb|Typical structure of a selenium rectifier.{{clarify|contradicts associated text (no Bi/Ni layer)|date=August 2016}}]]
[[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 (metallurgy)|annealing]]. Each plate is able to withstand about 20 volts in the [[Peak inverse voltage|reverse direction]]. The metal squares, or disks, also serve as [[heat sink]]s 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 set]]s and [[photocopy machine]]s.
==Use==
[[File:MADDIDA Selen Rectifier.JPG|thumb|right|Selenium rectifiers used in 1950s [[MADDIDA]] computer.]][[File:Selenium rectifier.agr.jpg|thumb|Selenium rectifier from 1960s. Each plate is 1 inch square.]][[Receiver (radio)|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.
==Properties==
==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.<ref>[http://pubs.usgs.gov/of/2003/of03-018/of03-018.pdf Selenium, U.S. Department of the Interior]</ref> They are still manufactured for exact replacement purposes but are not designed into new equipment.
==Selenium diode computer logic==
In 1961 IBM started developing a low speed computer logic family<ref>[https://www.google.com/patents/US3218472 US Patent 3218472]: Transistor switch with noise rejection provided by variable capacitance feedback diode</ref> that used selenium diodes with similar characteristics to silicon but cost less than one cent. The [[computer terminal|terminal]] development departments were begging for low cost and didn’tdid not need speed. It was possible to punch eighth inch discs from a sheet of selenium diode. GE claimed they could make reliable selenium diodes. A design was achieved for a DDTL circuit with two levels of diode logic feeding one alloy transistor and no series input resistor or speed up capacitor. The family was called SMAL<ref name="ibm" /> or SMALL for Selenium Matrix ALloy Logic. The alloy transistor proved to be too fast for the selenium diode recovery. To solve this problem a selenium diode was connected around the base-emitter to slow it down. The two level logic was similar to the PLAs Programmable logic array that would come on the market many years later. Nearly any static logic function that yielded one output could be achieved with one transistor and a handful of cheap diodes. Several years later the selenium diodes indeed were not reliable and were replaced by silicon diodes. The logic family was packaged on SMS cards.<ref name="ibm">{{cite book |title=The 1060 Data Communications System |publisher=IBM |page=2 |url=http://ed-thelen.org/comp-hist/IBM-ProdAnn/1060.pdf}}</ref>
==References==
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