1484 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



selected would now be rated at 0.50 X 1 or 0.5 ampere. Therefore, for 

 the specified 1.0 ampere load, two cells in parallel are required or the 

 next larger cell size, (3" square) could be selected. The 3'' square cell 

 would be rated at 0.50 X 2.2 or 1.1 ampere. 



At 60°C ambient, the input voltage for new or aged stacks should not 

 exceed 92 per cent of the normal rms voltage rating. 



An alternative method of calculating the input voltage required for 

 any stack cell combination may be obtained by the use of the formulas 

 shown in Table IV. 



Since the selenium rectifier stack is a non-linear device whose charac- 

 teristics vary depending upon the circuit and operating conditions, the 

 values given in Table IV are not precise but they are reasonably accurate 

 for most design purposes. 



To illustrate the formulas method, let us determine the input voltage 

 for the previous example, that is, a single-phase full-wave bridge stack 

 to supply 1 .0 ampere dc at 48 volts into a resistance load . As previously 

 determined, the stack cell combination would be a 4-2- IB circuit. The 



Table IV — Calculating the Input Voltage Required for Any 

 Stack Cell Combination 



Eac = Stack input volts, rms. 



Edc = Average dc output volts. 



K = Circuit form factor. 



n = Number of cells in series in each rectifying element. 



DV = RMS forward voltage drop at specified dc output current (see Fig. 9). 



* For center tap circuits, Eac is the voltage to the mid-tap on the transformer. 



