1470 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



mdely into communication systems for World War II military projects. 

 Since 1945, selenium rectifier power supplies have increased rapidly. For 

 example, in 1945, out of a total of 9,000 rectifiers of all types, about 

 1,000 or 11 per cent used selenium. In 1951, about 30,000 out of 45,000 

 rectifiers, or 67 per cent, were of the selenium type. 



There are many companies in this country who manufacture selenium 

 stacks but the quality and behavior of the various manufacturers' 

 products show considerable variation, particularly in regard to life ex- 

 pectancy. For many years the Laboratories has carried on an extensive 

 testing program to evaluate properly the various suppliers' rectifiers. 

 As a result of these continuing investigations, the Laboratories is in a 

 position to select cell sizes and combinations of selenium rectifier stacks 

 for use in new power applications. Specifications then are written on one 

 or more suppliers. These specifications have a three fold purpose: (1) 

 They are used as a purchasing and inspection document, (2) they cover 

 the electrical and mechanical requirements necessary for proper elec- 

 trical design and equipment layout, and (3) they are useful in maintain 

 ing records of the electrical and mechanical characteristics. 



CELL MANUFACTURE AND STACK ASSEMBLY 



A selenium rectifier cell is an elementary rectifying device having one 

 positive electrode, one negative electrode and one rectifying junction. 

 Cells are made by coating a chemically treated base plate, usually alum- 

 inum, with a thin layer of purified selenium to which a halogen element 

 (chlorine, iodine or bromine) has been added. This mixture is applied to 

 the base plate by one of the following methods: 



1. Sprinkled or dusted on and subjected to heat and pressure. 



2. Deposited by an evaporation process in an evacuated chamber. 



3. Dipped in molten selenium and spun. 



The selenium then is converted to the desired crystaUine structure by 

 heat treatments. During the heat treatment, a blocking or barrier layer 

 is formed on the exposed surface of the selenium. This layer is further 

 developed by various chemical means, which are "trade secrets" with 

 each manufacturer. A thin layer of low melting point alloy, the front 

 electrode, is then sprayed on the selenium. 



The manufacturing process is completed by electrically ''forming" the 

 cells by applying a pulsating dc voltage in the non-rectifying direction for 

 a specified time interval. 



Selenium cells are assembled on an insulated bolt or stud, mth in- 

 dividual cells separated by metal spacer washers to allow free passage of 

 air for cooling the assembly. Contact terminals are brought out in various 



