1486 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



the transformer, ballast, load, etc. For most practical design considera- 

 tions, one or more aging taps on the input transformer to provide 5 to 

 10 per cent additional voltage, will compensate for the forward aging. 



It should be emphasized that the curves in Figs. 6,7,8 and 9 are based 

 upon empirical data obtained on new rectifier stacks and the charac- 

 teristics mil vary slightly depending upon normal manufacturing varia- 

 tions in the voltage current characteristics mentioned earlier in this 

 article. 



AGING 



Selenium rectifier stacks are subject to aging. Aging is defined as any 

 persistent change (except failure) which takes place for any reason in 

 either the forward or reverse resistance characteristics. The important 

 factor in selenium rectifier aging is the increase in the forward voltage 

 drop which results in a decreased dc output. For normal rectifier applica- 

 tions aging of the reverse current is not critical. 



For design purposes, a selenium stack is considered to have reached 

 the end of its useful life when the stack input voltage required to main- 

 tain rated output voltage exceeds the rms reverse voltage rating assigned 

 by the manufacturer. Operation beyond this limit will result in over- 

 heating of the selenium cells and rapid failure of the stack. 



The extent and rate of aging of selenium stacks cannot be predicted 

 mathematically. Aging characteristics must be determined by actual test 

 involving lengthy time consuming projects. To determine whether a 

 given rectifier stack will give satisfactory performance for five years, as 

 an example, tests would have to be conducted for this period. Aging can 

 be accelerated by operation at high temperatures or at load currents 

 above normal, but no accepted correlation exists between this type of 

 aging and that obtainable under normal operating conditions. 



Aging data were obtained on sample rectifier stacks obtained from 

 different manufacturers in this country. The samples were set up on life 

 test racks as single phase full wave rectifiers and were operated continu- 

 ously at normal room ambient temperatures. For the duration of the 

 tests, the 60-cycle ac input voltage to the stacks was kept constant at 

 approximately 10 per cent below the maximum rms voltage rating. The 

 stacks operated into a resistance load. The resistance was selected so 

 that the rectifiers operated at rated dc load currents. The resistance was 

 not changed thereafter. 



Long term forward aging characteristics of selenium rectifier stacks 



