1472 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 

 DESIGN CONSIDERATIONS 



The proper selection of selenium stacks for use in dc power supplies 

 involves a number of important factors that must be carefully consid- 

 ered. 



(1) Circuit requirements must be carefully analyzed so that alllowance 

 may be made for the normal variations in the voltage-current charac- 

 teristics of each manufacturer's product as well as variations that exist 

 for the same stacks processed by different manufacturers. For a fixed ac 

 input voltage, differences in the forward voltage drop may vary the dc 

 output voltage at least zb 3 per cent from the mean value. If this cannot 

 be tolerated, selenium cells have to be carefully graded and selected to 

 obtain uniformity or other circuit adjustments have to be provided. 

 Special selection of cells, obviously, will increase the cost of the product. 



(2) The engineer must take into account the magnitude of the changes 

 in the voltage-current characteristics of the rectifier stacks over the 

 specified temperature range of his project. At very low temperatures, 

 output voltages may be 5 to 10 per cent lower than at normal room tem- 

 peratures. For high temperature operation, the stacks must be properly 

 derated for both current and voltage to prevent overheating and rapid 

 failure. 



(3) Selenium rectifiers age with time. Compensation for this aging 

 should be provided if load requirements warrant. The project engineer 

 should determine what life he expects or requires of the application. For 

 military applications life requirements may vary from minutes to thou- 

 sands of hours. On other applications, such as telephone and elevator in- 

 stallations, it is desirable to design selenium rectifiers for life expectancies 

 of ten to twenty years or more. 



(4) The equipment engineer must anticipate the differences in me- 

 chanical details of the same stack assembled by different suppliers. There 

 is no standardization in the selenium industry regarding cell sizes or 

 mechanical details such as the overall length and height of the stack 

 and particularly the type of mounting. However, a committee for the 

 National Electrical Manufacturers' Association is attempting to stand- 

 ardize these mechanical details so that stacks assembled by different 

 suppliers will be mechanically interchangeable. 



(5) Unless otherwise specified, rectifying stacks are coated with vari- 

 ous types of paints and varnishes for protection against moisture in 

 normal conditions of humidity. For military projects and other applica- 

 tions where selenium rectifiers may be exposed to high humidities, fun- 

 gus, salt or other corrosive atmospheres, the rectifier stacks must be 



