286 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



unit) using a neon tube for its reference. The grid-cathode bias of V5 con- 

 trols the secondary impedance of the transductor TD2 \'ia the ampHfier 

 V5, V6, TDl. The gain of the control amplifier and the sign and magni- 

 tude of the normal impedance of the secondary of TD2 are arranged to 

 giv^e a fall of approximately 0.25 per cent in the current from full system 

 load to short-circuit. 



Two advantages are obtained by emplojdng a magentically controlled 

 diode for V3 instead of an electrostatically controlled tube. The control 

 is directly proportional to the output current, and is not dependent upon 

 the stability of a series resistor, while the control circuits are isolated 

 from the output-circuit voltage, which simplifies maintenance of the 

 more complex parts of the equipment. 



Current Characteristics. 



Without current flowing in the auxiliary coil 1-2 (on "\'3 in Fig. 5) the 

 equipment has a normal constant-current characteristic, the value of 

 which, 322 ma, is preset by adjusting the mechanical position of the 

 coil assembly along the main axis of V3. 



Two neon tubes and two resistors form the bridge VI, V2, Rl and R2, 

 which is balanced when the voltage drop across Rl and R2 equals the 

 constant voltage across V2 and VI, the output voltage at which this 

 occurs being adjusted by RVl. At voltages below balance, current tries 

 to flow from y to x and at voltages above balance it flows from x to y. 

 The balance voltage corresponds to the voltage at which the sla\-e-unit 

 characteristic changes slope (C in Fig. 1). For voltages below balance 

 the rectifier J\IR2 prevents current from flowing in the winding 1-2 (on 

 V3), and in this range the constant current of 322 ma is maintained (see 

 DC, Fig. 1). For voltages above balance, current flows in the winding 1-2 

 and progressiveh' decreases the output current (CAB, Fig. 1). At 80 per 

 cent of the full link voltage, the contacts of relay M are operated and 

 disconnect the auxiliary coil 1-2 from the voltage-sensitive bridge and 

 connect it across the reference supph'. The current through 1-2 is then 

 set by the fine current control (RV2) to make the output current the re- 

 quired 316 ma. As previously stated, relay i\I does not automatically 

 switch back when the output voltage drops below 80 per cent of full link 

 voltage; the current characteristic of a master unit is EAF in Fig. 1. 



The master/master shut-down characteristic [Fig. 1, curve (c)] is 

 obtained by short-circuiting R3, which causes the constant current to 

 fall to 312 ma. Adjusting R^^2 would be equally effective, but short- 

 circuiting R3 does not permanently disturb the normal current setting. 



