WARNING: THE CIRCUIT MAY HANG WITH DANGEROUS AC MAINS POTENTIALS WHEN POWERED ON, SO OBSERVE UTMOST PRECAUTION AND CARE DURING THE BUILDING AND THE TESTING PROCEDURES.
The IC 4060 as also discussed in many of my previous articles yet again proves how versatile it is in its applications.
It’s definitely the heart of the circuit and solely completes the entire control functions and time delay operations.
As shown in the figure alongside, we see that the IC is configured in its usual and the most standard mode.
C5, R3 and P2 together as well as individually form the time delay determining components. P2 may be adjusted to set the delay-ON time period anywhere from 1 to 30 minutes, for the present application it may be set for obtaining a delay of around 1 minute.
The circuit is powered through a standard transformer/bridge power supply circuit which is further regulated using a 7805 voltage regulator IC.
Pin # 12 of the IC is its reset pin and the IC remains functional (oscillates) as long as this pin is kept at a logic low potential or ground.
Initially when power is switched ON, C1 resets the IC so that it starts counting from zero onwards and not from any intermediate value and also at that very instant T1 on receiving a base bias conducts to ground pin 12 of the IC, initiating the counting process.
The IC starts counting, and the moment the set time delay period elapses, its pin #3 goes high, triggering the Triac. At the same time, the high or the positive status from pin #3 reaches pin #11 also via D6, immediately seizing the counting process so that the IC “freezes" at that position.
Since the load is connected via the triac, during this time it remains switched ON and operates as long as the input mains power is stable.
Now suppose due to some power disorder the mains voltage gets interrupted (even for a second), the base voltage to T1 correspondingly gets inhibited so that it stops conducting for that fraction of a second. Such occurrences instantly reset the IC, switching off the triac and the load.
Even if the mains voltage is restored at the very next second the load remains switched OFF until the circuit has finished counting and the set delay period has elapsed.
The above feature very effectively diffuses any possible chance of developing a spike/transient and also makes sure that the circulating fluid inside the refrigerator receives enough time to settle before restarting. The system thus very faithfully serves the function of a perfect refrigerator appliance compressor failure protector.