This time delay switch circuit is useful to switch on an AC load such as lamps after the delay of three minutes. This protects the load from inrush current and transients at power on which may be detrimental to the device.
The time delay switch is directly connected to the AC lines to derive power to the components. Resistor R1 drops AC voltage and D1 rectifies the same to give low volt DC. The high value capacitor C1 act as a smoothing capacitor as well as the time delay device. That is the SCR conducts only after C1 is fully charged. It takes around 2 minutes. When C1 charges fully gate of SCR gets firing pulse and it conducts. This charges C2 which also give another one minute to fire the Triac trough R3. When the triac conducts, AC load gets electrical continuity and turns on. Capacitor C3 keeps the voltage level at the gate of triac and eliminates the back lash effect.
Time Delay Switch Circuit Schematic
Note:Take extreme care while handling the circuit, since it is at mains lethal potential. Do not touch any points when it is connected to mains. It should be constructed only by an experienced person.