Design and production of no-load automatic power-off device for household power supply

The function of this power no-load automatic circuit breaker is that when all the electrical appliances in the home are turned off, it can automatically cut off the power supply of the line, and when the electrical appliances are turned on, it will automatically supply power to the line.

Working principle: This device is connected to the household incoming line. When there is no load at the output end, the normally open contacts J1-1 and J1-2 of relay J are disconnected, the output end is isolated from the mains, and the normally closed contacts J1-3 and J1-4 of J are closed, and the battery E and C2, D3, etc. are connected to the circuit. The battery circuit is used for self-starting when the load is connected after the mains is cut off.

When the load at the output end is connected (such as turning on the light), E forms a loop with D3, J1-3, J1-4, load, RP2 and the light-emitting diode in the photocoupler IC. The light-emitting tube in IC emits light, the photosensitive thyristor is turned on, and J is attracted. J1-3 and J1-4 disconnect the battery circuit, and J1-1 and J1-2 are closed to connect the mains power to the load. Once the load is powered on, current is generated in the line, and the secondary of the current transformer B generates an induced current, which is rectified by D1 and filtered by Cl, and then replaces E to keep IC on. J1 remains in the attracted state, and the load works stably. When the load l is turned off, the primary and secondary currents of B disappear, IC is cut off, and J is released.

J1-1 and J1-2 cut off the line power supply, and J1-3 and J1-4 are closed to connect E and other devices to the circuit, and the circuit enters the standby state again.

When the line is powered off, if the human body accidentally touches the output phase and neutral wires at the same time, since only a very low DC voltage is applied to them, the current generated in the circuit is extremely small, and it is impossible for the IC to work and J to be attracted, thus effectively preventing electric shock from the two wires (this is something that the leakage protection switch cannot protect). Due to the magnetic saturation characteristics of B, the secondary voltage of B will not increase indefinitely.

In the circuit, D2 and D3 are isolation diodes, which are designed to prevent the DC power of B secondary from charging E after rectification after J is attracted. After the load is connected, C2 is charged first, and then IC works and J acts. During the period when J1-3 and J1-4 are disconnected but J1-1 and J1-2 are not closed, the charging voltage of C2 continues to make IC work, preventing J from being released again, so that the circuit cannot work reliably. In this circuit, the minimum starting load design value of the starting circuit composed of battery E is greater than the minimum load design value of the induction circuit composed of B to attract J, so once the circuit is started, J can be reliably attracted. In addition, since the relay has the characteristic that the release current is much smaller than the initial attraction current, J needs to be released under a smaller load than the circuit during initial operation. Therefore, once J is released, it will not cause the starting circuit composed of E to start again with a small load. In this way, it is ensured that the relay will not produce the oscillation phenomenon of attracting and releasing, attracting and releasing... under the critical load state, and the circuit works stably.

Component selection and production: Current transformer B can use an E-shaped transformer core of about -10.15VA, with 20 turns of φ2.5 enameled wire for the primary and 400 turns of φ0.21 enameled wire for the secondary. lC uses MOC3041 thyristor photocoupler, which has a large load current and can directly drive the relay to work. J uses a relay with a coil voltage of 220V, the contact current should not be less than 10A, and there are two pairs of normally open and normally closed contacts. E uses a 6V or 9V stacked battery, and theoretically E has a fairly long service life (in years). During production, several components can be welded together, or welded with a small circuit board and installed in a bakelite or plastic box. The cross-sectional area of ​​the power connection line should not be less than 2.8mm2.

Debugging: (1) Turn on two 15W bulbs without load, adjust RP2 so that J can be attracted (it is also possible to release after being attracted and then attract again); (2) Then adjust RP1 so that J will not be released after being attracted, and continue to reduce the resistance value of RP1 so that J can still be attracted after one bulb is turned off. (3) Appropriately adjust the capacity of C2 so that J can be attracted once the load is connected. No other debugging is required.