236 



HANDBOOK OF MECHANICAL DESIGN 



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Relay «l_. 

 >— 1 ■ ■ . 



Relay 



+ - + 



Fig. 483. — Vacuum tube. Condenser charged or discharged through a resistor closes a relay after definite time, 

 using direct current. When switch is open, the condenser discharges slowly through shunt resistor. This lowers the 

 negative potential on the grid, and at the critical value the plate current will rise enough to operate the relay. Full 

 line voltage may be applied to the condenser to obtain longer time delay. 



Fig. 484. — In this circuit, operation is maintained for a predetermined time after the starting impulse has stopped. 

 When the button has been pressed, the filament gets current in series with relay winding 1, and the relay pulls up, 

 locking in the circuit. The second contact charges the condenser negative, and no plate current flows. When button 

 is released, the relay stays closed until condenser discharges. Then the plate current flows through the second relay 

 winding in opposition to the first, releasing the armature. Applicable to direct current or rectified alternating current 

 only. 



Reciifier 



Relay 



Re sis lor, 



•y^ 



Condenser 



PolenHo- 

 mef'er 



^ Transformer 



Swiich 

 Fig. 485. — In the Wes'tinghouse electronic relay, there is no temperature error, reset is instantaneous, adjustment 

 is easy, and first cost is low. When the switch is closed, the tube passes current. As the current increases, the increas- 

 ing IR drop from the potentiometer causes a charging current through condenser. The IR drop across the resistor 

 because of this current applies the negative bias to the grid. Plate current cannot build up very rapidly, because the 

 faster it increases, the more negative the grid becomes. After a time period, adjustable through potentiometer, the 

 plate current will operate relay. The time delay is proportional to the product of resistance and capacitance. Long 

 delays require large resistors, and short delays correspondingly small resistors. Maximum time delay with this device 

 is about 3 min. About 0.05 see. is the minimum. 



