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.
        XL = RL ( Load resisance )
        L = RL / 2πf
        XC = RL ( Load resisance )
        1 / 2πfC = RL
        C = 1 / 2πf RL
        Where f is corner frequency and RL 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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