Editing Rectifier (section)

==== Selenium and copper oxide rectifiers ====
{{Main|Metal rectifier}}
[[File:Selenium Rectifier.jpg|thumb|upright=0.7|[[Selenium rectifier]]]]
Once common until replaced by more compact and less costly silicon solid-state rectifiers in the 1970s, these units used stacks of oxide-coated metal plates and took advantage of the [[semiconductor]] properties of [[selenium]] or copper oxide.<ref>H. P. Westman et al., (ed), ''[http://lccn.loc.gov/43014665 Reference Data for Radio Engineers, Fifth Edition]'', 1968, Howard W. Sams and Co., no ISBN, Library of Congress Card No. 43-14665 chapter 13</ref> While [[selenium rectifier]]s were lighter in weight and used less power than comparable vacuum tube rectifiers, they had the disadvantage of finite life expectancy, increasing resistance with age, and were only suitable to use at low frequencies. Both selenium and copper oxide rectifiers have somewhat better tolerance of momentary voltage transients than silicon rectifiers.

Typically these rectifiers were made up of stacks of metal plates or washers, held together by a central bolt, with the number of stacks determined by voltage; each cell was rated for about 20 V. An automotive battery charger rectifier might have only one cell: the high-voltage power supply for a [[vacuum tube]] might have dozens of stacked plates. Current density in an air-cooled selenium stack was about 600 mA per square inch of active area (about 90 mA per square centimeter).
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