Electronic power-on delay
Power for the delay circuit is derived directly from the mains by a bridge rectifier, D1-D4. The rectifier must be able to handle a current of up to 1 A. For safety reasons, resistor R1 ensures that capacitor C1 is discharged rapidly when the mains is switched off. The resistor should, therefore, be rated at 250 Va.c. or 400 Vd.c.
The direct voltage output of the rectifier is held steady at 24 V by zenzer diodes D6 and D7. The use of two zenzer diodes gives a stable voltage of 12 V for IC1. The rectifier output is smoothed by C2 and C3.
The timing of the circuit is based on the mains frequency: the clock signal is rectified is by D5 and then taken from junction R2-R3 at a level of about 11 V from where it is applied to pin 10 of IC1. After 2-10 clock pulses, outputs Q11 goes high. If contact 2 of DIP switches S1 is closed, the high signal (12 V) is applied to the gate of T1, an n-channel VMOS FET. This transistor then conducts, which causes the relay to be energized.
If, as shown in the diagram, all contacts of S1 are open, to a gate of T1 is permanently connected to the positive supply rail. This means that as soon as the supply is switched on, the clock input of IC1 is low via D9 so that the counter is disabled. The delays that can be selected with the DIP switch are shown in the table. The DIP switch may be replaced by a binary-coded decimal (thumb-wheel) switch.
Immediately after the supply is switched on, the IC is rest by a pulse is limited to a safe value by D8. When a new count cycle is started, the IC is, therefore, always in this zero state.
Diode D10 is a freewheeling device to suppress voltage peaks that are caused by the relay coil when the current through it is switched off.
The Current drawn by the circuit is determined mainly by the value of C1: in the diagram, it amounts to about 30 mA.
CAUTION must be exercised because the circuit connected directly to the mains. The circuit must, therefore, be tested with the aid of an isolating transformer. Furthermore, it impossible for main carrying parts to be touched.