Selenium rectifier: Difference between revisions

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==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 did not need speed. It was possible to punch 1/8-inch discs from a sheet of selenium diode. GE claimed that they could make reliable selenium diodes. A design was achieved for a DDTL circuit with two levels of [[diode logic]] feeding one [[Alloy-junction_transistor|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 [[Diode#Reverse-recovery_effectrecovery effect|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 (PLA) 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 were found to be not reliable and were replaced by silicon diodes. The logic family was packaged on [[IBM Standard Modular System|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>

==Further reading==