One simple way to protect circuits from over voltage is the use of a Shunt Regulator IC TL431 to switch off the load if the voltage increases above the preset level. In unregulated power supply, if the voltage in the primary of the step down transformer increases, a corresponding increase in voltage will appears in the secondary also. This may seriously affect the circuit, if it uses voltage sensitive semiconductors.TL431 is an adjustable precision Zener shut regulator and its output voltage can be set between 2.5 volts to 36 volts using a voltage divider. It can functions like a Zener diode but with precision.
The voltage divider consists of the 22K variable resistor and two 1.5 K resistors. The voltage to the adjust pin of Shunt regulator can be adjusted through the variable resistor. Load is connected through the Common and NC contacts of the relay so that the load gets power from the power supply when the shunt regulator is off. If the input voltage increases above the preset level (as set by 22K), shut regulator conducts to trigger the PNP transistor (BC557). Relay energize and breaks the power to the load. Since the buzzer is connected to the NO contact of the relay, it sounds when the relay energize and indicates the over voltage.
Use a variable power supply to set the tripping point. Provide 14 volts ( 10V in 9 volt power supply) and adjust the 22K variable resistor till relay energizes and buzzer beeps. After setting, connect the circuit with the power supply and load as shown in the diagram.
Note: Actual symbol of Shunt regulator is not used in the diagram due to its non availability in the drawing programme. Actual symbol is shown above.