The power supply for this analogue circuit, which affords delays of up to 330 seconds, is taken directly from the mains. The direct voltage at the output of the bridge rectifier is held at 22 V by zenzer diode D5. Resistor R6, which enables C1 to discharge rapidly as soon as the mains is switched off, must be rated at 250Va.c. or 400Vd.c.
The delay is provided primarily by C4, which is charged via C3, whose impendence at 50 Hz is about 10 M(ohm), and half-wave rectifier D6-D7. After a given period, the potential across C4 will be 12 V higher than the source voltage of T1, which is set with P1. The gate of T1 has the same potential as C4. Network R2-C5 serves to suppress any superious voltage peaks.
When the potential across C4 becomes higher than the source voltage of T1, the FET begins to conduct and this will result in T2 being switched on. Moreover, the voltage across the relay is fed back to the gate of T1 via D8. This feedback ensures that T1 and T2 are quickly driven into saturation.
Once the relay been energized transistor T3 will be switched on via R5-C6. When this happens, C4 will be discharged through the transistor, so that the circuit is quickly back in its initial state, not short delay on power-up is, therefore, not shortened by the residual charge on C4. In spite of C4 being discharged, the relay remains actuated because the gate voltage of T1 is held steady via D8. Only when the supply voltage is switched off will the relay be deenergized.
Note that the circuit is connected electrically to the mains so that great caution should be observed during any testing and operation.