Gate Protection

• There is possibility of false triggering due to over voltage across gate cathode junction of the SCR. 
• The possibility of increase in the temperature of gate – cathode junction due to high current flows through it and this may result in damage of SCR. 
• The gate circuit is protected against over voltage by placing suitable value of resistance in series with it. 
• The voltage across SCR becomes very low and current through it very high when it is in turned on condition. 
• This will result in production of magnetic field and collapse of electrical field.

Radio interference

• There is possibility of induce voltage across other SCR gate – cathode junction due to change in electrical field and magnetic field resulting turn on the SCR. 
• There are other SCRs also turned on due to this malfunction. This effect of called as radio interference. 
• Therefore it is necessary to protect SCR gate – cathode circuit. 
• The gate circuit is protected by using shielded cable or twisted gate lead. 
• The effect of false triggering is reduced due to flux does not pass through twisted cable. 

RF filter

• The effect of radio interference is reduced by using RF filter. 
• The RF filter is made by using series inductor and parallel capacitor across the circuit. 
• The inductor decreases the effect of di/dt effect of anode current and capacitor reduces the effect of forward dv/dt voltage. 
• The calculation of inductor L and capacitor C is done as below.

        X_L = R_L ( Load resisance )

        L = R_L / 2πf

        X_C = R_L ( Load resisance )

        1 / 2πfC = R_L

        C = 1 / 2πf R_L

        Where f is corner frequency and R_L is load resistance

The protection of SCR against over voltage, over current and gate – cathode circuit is shown in the figure B. 

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