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SCR Positive Overvoltage Protection Circuit

2016-09-19 09:30  
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In most power supplies there is a series regulating element, such as a transistor or aFET . On one side there is normally a relatively high voltage, and on the other is the final output voltage.In order to ensure that the required voltage on the output of the series regulator, control circuitry monitors the output voltage and applies a suitable signal to the series regulator device so that the required voltage is obtained.The silicon controlled rectifier (SCR) is rated to handle at least the current of the power supply. It is connected in parallel across the 12 V dc output lines, but remains inert until a voltage appears at the gate terminal. This triggering voltage is supplied by the zener diode. At potentials less than 14 V the zener will not conduct current. But, at potentials greater than 14 Vdc the zener conducts and creates a voltage drop across the 330 ohm resistor that will fire the SCR. When the SCR  turns on, the output lines of the power supply are shorted to ground.This will blow the primary fuse or burn out the transformer if there is no primary fuse. A circuit is required that very quickly detects that the voltage is rising too high and then shuts down the power supply while not allowing the voltage to rise. While this may seem to be a tall order a simple overvoltage protection circuit can be made relatively easily based around a thyristor circuit
 

Overvoltage protection circuit