Metal rectifier
A metal rectifier is an early type of
In some countries the term "metal rectifier" is applied to all such devices; in others the term "metal rectifier" normally refers to copper-oxide types, and "selenium rectifier" to selenium-iron types.
Description
Metal rectifiers consist of washer-like discs of different metals, either copper (with an oxide layer to provide the rectification) or steel or aluminium, plated with selenium. The discs are often separated by spacer sleeves to provide cooling.
Mode of operation
The principle of operation of a metal rectifier is related to modern semiconductor rectifiers (Schottky diodes and p–n diodes), but somewhat more complex. Both selenium and copper oxide are semiconductors, in practice doped by impurities during manufacture. When they are deposited on metals, it would be expected that the result is a simple metal–semiconductor junction and that the rectification would be a result of a Schottky barrier. However, this is not always the case: the scientist S. Poganski discovered in the 1940s that the best selenium rectifiers were in fact
Performance
Compared to later silicon or germanium devices, copper-oxide rectifiers tended to have poor efficiency, and the reverse voltage rating was rarely more than a few volts. A number of rectifier discs would need to be used in series to provide an adequate reverse breakdown voltage figure – a
Applications
Radio receivers
Metal rectifiers were also used as envelope detector (AM demodulator) diodes in radio receivers. The WX6 Westector was a typical example. It was about the size and shape of an AAA battery, with threaded posts at each end to which connections were made.
Selenium rectifiers were once widely used as high-tension rectifiers in transformerless radio and TV sets, before cheaper silicon diodes became available. Although they were reasonably efficient in this application, (at least compared to vacuum-tube rectifiers), their internal resistance tended to increase as they aged. Apart from reducing the available high voltage, this tends to make them run hotter, producing an unpleasant smell as the selenium starts to evaporate.
TV sets and photocopiers
Specially designed selenium rectifiers were once widely used as
Electrolysis
Metal rectifiers have been replaced by silicon diodes in most devices, however there are certain applications where the replacement of metal rectifiers with silicon units has proven impractical. These are mostly in electroplating, aluminium smelting and similar high-current low-voltage industrial applications, where the lower forward voltage drop of metal rectifiers is more important than their reverse breakdown voltage.
References
External links
- Rectifier used as an AM-demodulator
- Cougar Electronics - example of "legacy" selenium rectifier manufacturer
- The Copper Oxide Rectifier This document discusses the history of the copper oxide rectifier, and, in particular, the use of Chilean copper in its manufacture; the thallium catastrophe that befell the manufacture of selenium rectifiers is also discussed.