Design and analysis of the principle of automatic power-off switch during power outage

In some areas where power outages are common, electrical appliances are not turned off after a power outage, and will automatically power on once power is restored. This can result in wasted energy and shortened electrical appliance life, or even fires 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 is foolproof.

How it works

The circuit principle is shown in Figure 1 (click to download the schematic diagram). Figure (a) is an automatic power-off switch with capacitor voltage reduction. The 220V AC voltage is reduced 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 is on, since S is disconnected, the relay coil has no DC working voltage, and in turn, 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 has been disconnected, the closure of S allows the relay to continue to work, which is called the "self-protection" function of the relay. When the power is off, S is disconnected; when the power is on again, the circuit repeats the above process. Figure (b) The circuit is more concise, eliminating a series of components such as voltage reduction, rectification, and voltage stabilization in Figure (a), but it needs to use a 220V AC relay, which is expensive and sometimes has 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

In Figure 1 (a), it is best to select a relay with a coil operating voltage of 24V or 48V, so that the corresponding coil current is smaller to reduce the requirement for the capacity of C1, and the contact current should be appropriately selected according to the load size. VD1 should be selected according to the relay operating voltage. Generally, a voltage regulator diode with a voltage stabilization value slightly higher than the relay coil operating voltage 1W is selected. The withstand voltage of C1 is not less than 400V, and 630V is better. The withstand voltage of C2 should also be appropriately selected according to the voltage stabilization value of VD1.