616 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



circuit which carry direct current. One is through the combination of 

 the varistor Vi and the 200-ohm resistance, which is bridged across the 

 set. Another is through the winding A of the coil, the transmitter and 

 22-ohm resistance. The third is through winding B of the coil and the 

 varistor V2. 



Fig. 4 shows the dc and ac resistance of the Vi and V2 varistors 

 at various loop currents when connected in the telephone circuit in the 

 normal way. The magnitude of the direct currents which flow through 

 the varistors under these conditions differs from the currents shown 

 in Fig. 1 and, therefore, the resistances are different. However, the 

 inherent property of decreasing in resistance with increasing current 

 is evident, as is also the fact that the ac resistance is much lower than 

 the dc resistance. From this it is apparent that the varistors will introduce 

 shunt losses and that these losses will be greater for ac than for direct 

 currents because the resistance of the varistor is lower in the former 

 case. 



The variation in transmitting plus receiving loss with loop current 

 is shown in Fig. 5. These curves are plotted in terms of the level of the 

 302-type set. If no equalization were provided in the circuit, the full 

 transmitting and receiving gain offered by the new transmitter and 

 receiver would be obtained at all loop currents. This condition is in- 

 dicated by the broken Une at the top of the diagram. With the equaUza- 

 tion introduced by the varistors, the loss is graded from an insignificant 



10,000 



6000 

 4000 



§ 2000 



^^ 1000 



u 



< 600 



i 400 



ID 



200 



set. 



0.03 0.04 0.05 0.06 0.07 08 0.09 0.10 0.11 0.12 0.13 0.14 0.15 016 

 LOOP CURRENT IN AMPERES 



Fig. 4 — Characteristics of Vi and V2 varistors in circuit of 5(X)D telephone 



J 



