Mains overvoltage protection circuit with excellent performance

In areas where power is scarce, the power supply voltage is very unstable. It is only around 170V at peak times and exceeds 250V late at night. In order to protect the increasing number of household appliances, it is necessary to configure an overvoltage protector. The overvoltage protector I made uses the precision voltage stabilizing integrated circuit TL431C, which has accurate voltage setting and reliable operation. It uses a capacitor to store electric energy to drive the relay, so the static power consumption is low, only 0.1W. It adopts short-circuit power supply protection, making it easy to use at home. Electrical appliances can be effectively protected and have the advantages of ease of use. The specific circuit is shown in the figure. It uses capacitor step-down and simple voltage-stabilized power supply circuit. When the power supply voltage is normal, the TL431C control electrode voltage is less than 2.5V, the TL431C circuit does not conduct, and the relay contacts do not close. When the mains voltage reaches 250V, the current after rectification and filtering reaches 3.3mA, and the voltage drops on R3, diode and voltage regulator tube (including light-emitting tube) are 11V and 26V respectively. The 37V voltage is divided by R1 and R2 and the voltage at point A is 2.55V. The TL431C circuit is turned on, relay K is closed, and the mains is short-circuited, causing the power supply overcurrent to trip or the fuse to blow, thus protecting household appliances. Here, a small value resistor is connected in series in the contact loop to prevent excessive short-circuit current from burning the contacts. Since capacitor C3 has a large capacity and stores a large amount of charge, when added to the relay coil, the relay contacts can maintain a closing time of 0.2 seconds, which is enough to cause the overcurrent device to trip or the fuse to blow. The current limiting capacitor C1 is a 0.47μF/630V polyester capacitor. In order to prevent incorrect connection to the 380 line voltage, C1 should have the necessary withstand voltage. Depending on the power of the relay, small or ultra-small large and medium power contact relays should be selected, such as JQX13-F/024-1Z or JZC-22F/024-1Z. The 5Ω current limiting resistor is a 5W cement resistor, and other components are selected according to the figure. The entire circuit can be installed in a small plastic box, with the power pin fixed at the bottom and the light-emitting diode exposed on the surface of the plastic box.