SELENIUM RECTlFIKli AITLK ATK . \ ( . r\>l Dl.i. \ l lu.\.^ 1177 



of characteristics in a given grade. More than one grade of cell may Ix; 

 assembled in a rectifier stack. The art of manufacturing selenium cells 

 is such that, considering production over a yearly period, diflFerences in 

 the forward voltage drop at rated current may vary as much as ±30 to 

 50 per cent from the mean value. In the reverse direction, a larger per 

 cent spread exists in the reverse current, particularly in cells or stacks 

 made by different suppliers. 



Fig. 4 shows dynamic characteristics plotted on a linear scale to illus- 

 trate variations of the forward voltage and reverse current character- 

 istics of selenium rectifier stacks that are processed by two different 

 suppliers. Variation of this magnitude exist, not only from supplier to 

 supplier, but also may occur in a particular suppliers' product. 



Selenium rectifier stacks in common with other semi-conductor recti- 

 fiers, have a negative temperature coefficient of resistance in the fonvard 

 direction. The forward voltage drop at a specified current decreases as 

 the ambient temperature increases (see Fig. 5). In the reverse direction, 

 the reverse current decreases as the temperature is lowered to approxi- 

 mately — 20°C. Below this temperature, there is no apparent change in 

 the current except at the higher voltages. At the higher voltages, the 

 current again tends to increase. 



STACK DESIGN 



The dc output voltage-current characteristics for various ac input 

 voltages of basic rectifier stacks (one cell per rectifying element) are 

 represented in Figs. 6, 7 and 8. The data were obtained by maintaining 

 a constant 60-cycle rms voltage at the stack input terminals and do not 

 take into account transformer regulation or other regulating devices that 

 may be used. 



Fig. 6 shows the single phase full-wave bridge characteristics for re- 

 sistance loads. 



Fig. 7 shows the single-phase full-wave bridge characteri-stics ior battery 

 loads. It vnW be observed that, with single-phase circuits for a given dc 

 output voltage, lower ac input voltage is required for a battery load than 

 for a resistance load. Capacitor loading is somewhat similar in output 

 characteristics to battery loading, but the value of output voltage is de- 

 pendent upon the magnitude of the capacity, the quality of the capacitor, 

 and the current drawn by the load. For batt^^ry loading, the output volt- 

 age is dependent upon the type of battery, the condition of the battery, 

 the battery voltage and the charging current rate re(|uire<l. 



For these reasons, it is difficult to accurately predict the exact input- 



