Relay driver

  
The relay operates a certain time, td, after power is applied to it, and then it operates for a length of time, tc. The SCR fires when the voltage on Ci reaches VA. This operates the relay, which stays activated until the current charging C2 drops below the dropout current. To keep the relay in its activated position indefinitely (tc =00), eliminate C2 and choose R2 just large enough to keep the relay coil current within its related limits.
Relay driver  - schematic

Typical component values for td = 30 seconds and tc = 2 seconds are: Rl = 1.5 megohms, R2 = 10 k ohms, R3 = 3 k ohms, Cl = 47 µ¥, and C2 = 100 µ¥. The SCR is a 2N1877 and the relay is a Potter Brumfield PW-5374. A value of 12 Vdc is assumed for V~.




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