INDUCTANCES AND RESISTANCES 

 Case (3) (heavily loaded) 



Rs + Rl 



n^ 



{R,+ Ri)t 



Va-b = 



(K + r) + 



(Rs + Ri 



\e "'^ 



— e 



(.Rp + r)l 





These are plotted in Graph 20 for 



Rs + Rl 



Light load — 

 Matched load- 

 Heavy load — 



«2 



Rs + Ri 

 R. + R> 



n.y 



= 10(i?^ + r) 



R, + r 



= 0-\iR+r) 



as a function of time for various coupling factors. Clearly a real transformer 

 will not transmit a steady potential indefinitely, but it will do so for a short 

 time, i.e. if the input is in the form of a pulse. If the loading is heavy the 

 output pulse is small but is better sustained. The deficiencies of the output 

 pulse may be described by 'lag', 'sag', and 'undershoot' {Figure 4.35); sag 



Lag. 



shoot 



Input Output 



Figure 4.35 



and undershoot are numerically equal. Summing up, for a transformer to 

 transmit a pulse of a particular duration: 



(1) If sag and undershoot are to be small, L^, must be large and, if a low 

 output is acceptable, the load can be heavy. 



(2) If the lag is to be small, k must be as near unity as possible, particularly 

 if low sag has been achieved by heavy loading. 



Loaded transformer connected to constant-voltage type alternator — signal and 

 power transformer 



The situation is depicted in Figure 4.36, but, as in the case of the pulse 

 transformer, we work in terms of the equivalent circuit (Figure 4.37). Then 

 by inspection, 



out 



n. 



+ J'^l) 



'in 



Rl + Rs 



r^R^+j{\-k^)L^ + 



68 



n^ 



J^L^ 



Rl + Rs 



n' 



+ J^L^ 



