1480 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



at 33 volts, rms. Therefore, '*%4 or 2 cells in series in each of the four 

 rectifying elements of the full wave bridge circuit will be required. From 

 Table II, a 2-inch square cell is needed to carry the specified load of 1 

 ampere dc. As previously described under "Symbolic Notation," the 

 rectifier stack would be designated as 4-2- IB combination using 8 cells. 



Selenium cells rated at 18 or 26 volts rms also could be used, but the 

 total number of cells in the stack would be increased as shown in Ta- 

 ble III. 



Obviously, the higher the cell voltage rating, the less total cells and 

 consequently a smaller and lighter stack is obtained. However, other fac- 

 tors, particularly life expectancy, must be evaluated as described later 

 in the text. 



After the stack cell combination has been established, the required ac 

 input voltage is obtained from Fig. 6. At 100 per cent normal load cur- 

 rent (1 ampere dc from Table II) and 24 volts dc, the ac voltage for a 

 basic stack is about 30 volts, rms. Since there are two cells in series in 

 each rectifying element, 2 X 30 or 60 voJts rms input voltage would be 

 required for a new stack. 



If cells rated at 26 volts rms are selected, three cells in series would be 

 required. For the specified 48 volts dc output, ^% or 16 volts dc per 

 series cell, is obtained. Again referring to Fig. 6, an input voltage of 20.5 

 volts per cell or 61.5 volts total is required. This compares to 60 volts 

 for the previous illustration. The higher input voltage results from the 

 added voltage drop of the additional cells in the rectifier stack. 



This stack design is satisfactory for operation in ambient tempera- 

 tures not exceeding H-35°C. For operation at higher ambients, the stack 

 should be de-rated as shown in Fig. 2. For example, if the stack is re- 

 quired to operate in an ambient of +60°C, the dc output current rating 

 would be 50 per cent of the normal rating. The 2'' square cell previously 



I . 1.75 



< 



? 1.50 



»- 



m 1.25 



ir 



3 1.00 



o 



° 0.75 



Q 



OC 



^ 0.50 



3 

 FORWARD 



< 30 



z 



- 25 



(- 



\ 20 

 a. 



3 



^ 15 



if) 



i ,0 



10 12 



14 16 18 20 22 24 26 



REVERSE 



RMS VOLTS 



Fig. 5 — Temperature characteristics of 4-1 -IB stacks with 2" x 2" cells nited 

 at 26 volts rms and 1.0 ampere dc. 



