SELENIUM RECTIFIER APPLICATION CON8IDKH \Tff •Sk M73 



provided with a more suitable type of protective coating or iuMi. 'Hiese 

 finishes are available from most manufacturers. 



(6) When selenium stacks are mounted in cabineU or housing >nth 

 other heat-generating devices, they should he arranged in such a manner 

 that heat from the other components docs not roach the rectifier stacks. 

 The stacks should be mounted below the oilin ...mponents, and in such 

 position that the free flow of air through the net ificr stack is not impeded. 

 The stack should be mounted with the ass('inl)ly stud in the horizontal, 

 not vertical, position. When two or more stacks are moimicl m i h. -mic 

 housing, they should be in a horizontal plane with each other, or projx^rly 

 staggered. Cabinets should be provided with louvers to dissipate the heat 

 within the enclosure. 



ELECTRICAL RATINGS 



The electrical ratings of selenium rectifier stacks are based on their 

 voltage, current and thermal characteristics. All three must \ye consid- 

 ered carefully for initial design purposes, as any one can affect life ex- 

 pectancy. 



Voltage Ratings 



The voltage rating usually is expressed as the "reverse voltage rat- 

 ing." It is the maximum rms sinewave voltage above which an excessive 

 reverse current would flow and overheat the cell, causing breakdown. 

 However, the important consideration establishing the rating is the peak 

 voltage applied to the cell. If the applied voltage differs significantly 

 from a sine-wave, it is important that the applied peak voltage shall not 

 exceed 1.41 times the rated rms voltage. 



When selenium cells originally were manufactured in this country, 

 their rms reverse voltage ratings ranged from 14 to 18 volt«. Ratings 

 later increased to 26 volts. One manufacturer already has successfully 

 produced 33-volt cells for several years; and another supplier announced 

 recently a 40-volt cell. Cells rated at higher voltage have been produced 

 in the laboratory. For non-critical applications, such as low-cost radio 

 and television sets, some selenium manufacturers make cells rated at 45 

 volts rms. Generally, in such applications, a high reliability product is 

 not required and the units may have a relatively short life. 



The nominal dc output voltages obtained from various common basic 

 rectifier stacks assembled with cells rated at 18-, 26- and 33-volt rms are 

 listed in Table I. These ratings apply for stacks operating at mUsl dc 

 output current into a resistance load. 



