This is one simplest form of Over voltage Protector for the Bread board power supply. Usually we use 12 volt DC power supply to power the bread board. If the CMOS devices are used in circuit assembly, over voltage can damge them permanatly. This circuit limits the voltage to the bread board to 12 volts. If the voltage increases above 12 volts due to a high voltage surge in the AC lines ,the circuit cut off the power to the bread board. It can also be used in any device like Chargers that works on 12 volt DC .
Circuit working is simple. A Zener diode and an SCR works as a Voltage controlled switch. When the input voltage is 12 volts or less, Zener remains off keeping the SCR also off. Relay will be in de- energized state and power to the bread board will be available through the Common and NC (Normally Connected ) contacts of the relay. When the input voltage increases abouve 12 volts, Zener ( ZD ) conducts , SCR fires and latches. When SCR conducts, Relay energize and breaks the contacts to cut off the power to the device.
SCR has the property of latching once triggered ,so even if the Zener turns off SCR remains latched and relay energized. Relay can be turned off only by removing power to the Anode of SCR by pressing the Push switch.
Green LED indicates input power and Red LED indicates power off to the device. Diode DI protects SCR from back e.m.f when the relay turns off.