376 



THE BELL SYSTEM TECHNICAL JOURNAL, ^L\KCH l!)o7 



6. POWER SUPPLIES 



The design of a carrier system with low power drain made possible the 

 development of a low-cost, reliable dc power supply for the carrier equip- 

 ment. Because the central office carrier terminal was designed to utilize 

 standard central office voltages (24 or 48 volts), onl}' the power supply 

 for the remote equipment will be described here. 



Early exploratory studies showed that conventional power supply- 

 designs would miss the first and annual cost objective by an uncomforta- 

 ble margin. A number of unconventional approaches were studied: 



(a) Storage batteries charged over the carrier line. 



(b) Storage batteries placed in service with full charge and removed 

 to a central point for recharging. 



(c) Solar power plants. 



(d) Wind power plants. 



(e) Thermoelectric power plants. 



(f) Dry cells. 



In all of the above cases the power plant was either too costty, too 

 large, or technicalh- unfeasible, and none could prove in ovev the con- 

 ventional conversion of ac to dc where commercial power is available. 

 This was true despite need for a storage batterj- to operate the s^-stem 

 during ac power failure intervals and to provide peak ringing power. 



6.1 AC Rectifiier-Storage Battery Plant 



The basic elements of the power plant circuit, as shown in Fig. 21, are 

 the con\-ersion section represented by the step-down transformer Tl and 



VOLTAGE CONTROL 



f ft ■ -- 



AC STEPDOWN 

 TRANSFORMER 

 TRl 



■■^:'^^ 



16V 



-22V 



10 CELL 

 STORAGE 

 BATTERY 



Fig. 21 — Schematic of ac rectifier-battery power supply 



