Design of automatic power-off switch for power outage and incoming calls

In some areas where power outages are common, electrical appliances are not turned off after a power outage. Once power is restored, they will automatically power on and work. This can result in wasted energy and shortened electrical appliance life, or even fire and other major accidents (such as various electric heating appliances). If the automatic power-off switch described in this article is installed, the electrical device will automatically be in a power-off state when power is restored after a power outage. Only manual intervention to press the switch can power on the electrical device. This device can be foolproof.
Working Principle

The circuit principle is shown in Figure 1 (click to download the schematic diagram). In the figure (a), the automatic power-off switch with capacitor step-down is used. The 220V AC voltage is stepped down by capacitor C1, rectified by VD2, and stabilized by VD1 to supply the DC working voltage to the relay J coil. Note that the normally open contact S is used in the relay. When the power comes in, since S is disconnected, the relay coil has no DC working voltage, and conversely, S cannot be closed, so the power output socket CZ has no power; when the appliance needs to be powered, the button switch AN needs to be pressed, and the relay coil gets the working voltage and S closes. After that, although AN is disconnected, the closing of S allows the relay to continue to work, which is called the "self-protection" function of the relay. When the power goes out, S is disconnected; when the power comes back, the circuit repeats the above process. The circuit in Figure (b) is more concise, eliminating a series of components such as step-down, rectification, and voltage stabilization in Figure (a), but a 220V AC relay is required. First, the price is expensive, and second, there is sometimes an annoying power frequency hum when working. You can choose according to the components at hand and your own hands-on ability.
Component selection and production
The relay in Figure 1 (a) is best to select a coil working voltage of 24V or 48V, so that the corresponding coil current is smaller to reduce the requirement for C1 capacity, and the contact current should be appropriately selected according to the load size. VD1 should be selected according to the working voltage of the relay. Generally, a voltage regulator diode with a voltage regulation value slightly higher than the working voltage of the relay coil 1W is selected. The withstand voltage of C1 should not be lower than 400V, 630V is better. The withstand voltage of C2 should also be appropriately selected according to the voltage regulation value of VD1.