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.
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