1474 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



Current Ratings 



The current rating of selenium cells is based upon a normal current 

 density of 0.32 ampere for each square inch of active rectifying surface 

 for a single-phase full-wave bridge rectifier stack (4-1-lB) operating into 

 a resistance load. The rating applies to stacks in which the cells are sep- 

 arated by spacer washers so that the cells are cooled by convection air 

 currents. Small size cells mounted without spacer washers are rated at a 

 lower current density. 



The dc output current ratings for various selenium cell sizes are listed 

 in Table II. 



These ratings are based upon continuous operation in ambient tempera- 

 tures up to -f 35°C with unrestricted ventilation for convection cooling 

 of the stacks. For battery or condenser loads on single phase circuits, the 

 above current ratings usually are derated to 80 per cent of the above 

 values. 



Thermal Ratings 



Voltage and current ratings usually are based upon operation of the 

 rectifier stack in a normal ambient temperature of +35°C. It is im- 

 portant to note, however, that for selenium cell application, ambient 

 temperature is defined as the temperature immediately surrounding the 

 stack within the equipment enclosure, not the temperature area where 

 equipment is installed. Stacks operating at rated current and voltage 

 into a resistance load have temperature rises ranging from 15 to 30°C 

 when measured with a thermocouple in direct contact with the cell. 

 The temperature rise depends upon the manufacturing techniques used 

 by the various companies as well as the spacing of the selenium cells on 

 the assembled stack. For long life expectancy, the actual cell tempera- 

 ture should not exceed +75°C. 



Table I — Nominal DC Output Voltages of Rectifier Stacks 



Basic Stack 



Cell Rating, Volts rms. 



Single Phase 



Half -wave 



Full -wave 

 Center tap .... 



Bridge 



Three-Phase 

 Full -wave bridge 



1-1-lH 



2-1-lC 

 4-1-lB 



6-1-lB 



DC Output Volts 



18 



6 

 12 



19 



26 



9 

 18 



30 



33 



12 



12 

 24 



38 



