Selenium Rectifiers — Factors in Their 

 Application 



By J. GRAMELS 



(Manuscript received July 1, 1953) 



Selection of the proper selenium rectifier stacks for best results in the de- 

 sign of dc power supplies involves consideration of characteristics not ordi- 

 narily found in published data. This paper describes the data required for 

 the selection of selenium cell sizes and cell combinations, shows typical 

 voltage-current characteristics, and gives the results of extensive life test data 

 necessary for evaluating the life expectancy of the product. The life test data 

 indicate that there are substantial differences in the life expectancy of 

 selenium stacks as manufactured by various companies in this country. 

 Shorter life can be anticipated as the rms cell voltage ratings are increased. 

 In addition, the life is affected by the current density and the temperature 

 at which the selenium cells are operated. 



INTRODUCTION 



Since their introduction in this country about 1939, selenium rectifier 

 stacks have proved to be a useful means of converting ac power to dc 

 power for Bell System applications. These applications vary from 2 to 

 2,500 volts and in power sizes from a few watts to 10 kilowatts. Properly 

 designed, the selenium rectifier has a relatively long-life expectancy and 

 requires a minimum amount of maintenance. For these reasons, selenium 

 rectifier stacks are widely used in telephone plants for battery charging, 

 relay operation, plate and filament supply for vacuum-tube amplifiers, 

 bias supplies, telegraph and teletypewriter circuits. 



Up to 1952, about 245,000 rectifiers of all types have been manufac- 

 tured for the Bell System. These include tungar, copper-oxide, vacuum 

 tubes, thyratrons and selenium types. Of this total, about 25 per cent 

 are of the selenium type. 



Although Bell Laboratories studies of selenium rectifier stacks date 

 from 1939, rectifiers using such stacks did not enter the U^lephone plant 

 until 1945. From 1940 to 1945, however, selenium stacks were designed 



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