SELENIUM RECTI FIKK Al'lLK ATION CON'SI DERATIONS 



1489 



at 50 per cent normal current rating is about one liall tliat atrakd lur- 

 rent. Above normal rating, the aging is arreloratod to a greater degree. 

 These data were obtained on a particul.ti Mippli.i -^ m.i. k. with 26-volt 

 cells, but similar behavior has been observed on other supphers' produ(?t«, 

 including stacks assembled with 33- and 40-volt cells. 



The behavior of the reverse current during these aging studies indi- 

 cates that the reverse current generally decreases slightly during the first 

 few months of operation and then remains substantially constant there- 

 after. A few manufacturers' rectifiers, however, show the oppasite effect. 

 The current rises slightly before leveling off. For normal dc power apph- 

 cations, these changes are insignificant. 



As previously discussed, selenium stacks are rated for operation at 

 +35°C ambient temperatures and should be derated at higher ambients 

 for normal life expectancy. However, the problem frequently arises — 

 what life can be expected at high ambient temperatures and why cannot 

 aging qualities be predicted over a short time interval under accelerated 

 conditions, such as high ambient temperatures? Fig. 15 shows data ob- 

 tained on four different suppliers' product when operated at a -|-80®C 

 ambient temperature. The temperature of the selenium cells under 

 these conditions (ambient plus temperature rise) is about lOCC. For 

 comparison purposes, data are included for similar stacks aged at normal 



MANUFACTURERS 



Fig. 12 — Forward aging characteristics. Comparison of 26- and 33-volt cell 



ratings. 



