POWER ON DELAY

ON TIME DELAY CONTROL CIRCUIT

This simple time delay circuit is very useful for refrigerator, deep fridge, air conditioner or any power sensitive equipment.  It gives 3-4 minutes time delay when A.C. mains power supply resume.  If power fail during the running of compressor, it is necessary to give time gap to resume power to the compressor specially to the reciprocating compressors, since they require more power to compress the refrigerant in comparison to rotary compressors.

At power resumption both transistors remain in cut off position and base biasing of 3.6 V is available at the base of transistor Q5 through potential divider R10, R11.  Since capacitor C4 starts charging through resistor R12.  Transistor Q5 will not conduct until the voltage at its emitter remains below its base voltage.  When the voltage at emitter of transistor Q5 goes above the voltage available at its base (3.6V) it starts conducting and transistor Q6 gets base biasing since its base is connected directly to the collector of Q5.  The emitter of Transistor Q6 is connected to the gate of SCR through resistor R5 which causes SCR to fire and the relay is energised.  Capacitor C3 connected at the gate of SCR prevents it to misfire.  As SCR comes in conducting mode capacitor C4 is immediately discharged through diode D4 so that the circuit is ready for the next time delay cycle.

Power supply can be obtained from the existing device in which this circuit is to be placed or through a 12 Volt 300 milliampere transformer, a bridge rectifier and a smoothing capacitor of 1000 Mfd/25 Volt.