230V AC Mains Over Voltage Protection Circuit

The 230V AC Mains Over Voltage Protection Circuit monitors incoming alternating current voltages to safeguard sensitive downstream electronics. Powering the control logic begins with stepping down and rectifying the mains power, utilizing components like the 1N4007 diodes and an LM7805 voltage regulator to supply a stable 5V DC rail for active circuitry. An arrangement of precise resistors (including the 56K, 1.5K, 560K, 62K, and a 10K 10-turn potentiometer) forms a voltage divider network that samples the scaled mains voltage. This sampled signal is fed into the LM358 operational amplifier, which is configured as a comparator with Schmitt trigger hysteresis to prevent erratic relay chatter near the threshold limit. When the input voltage exceeds the user-calibrated threshold set by the potentiometer, the LM358 output changes state. This output drives a BD139 power transistor, which acts as a low-side switch to energize the coil of the SRD-12VDC-SL-C PCB relay. The switching of the relay physically disconnects the load from the 230V AC mains line. Decoupling capacitors (0.1uF) filter out high-frequency noise to ensure stable comparison, while an indicator LED provides visual feedback of the active protection state. The entire circuit is compactly laid out on a generic clad board, and circuit schematics can be designed using Eagle PCB software.