18 May 2017

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 bolow.
              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.

You may also like :
Static AC Circuit Breaker
Snubber Circuit
Class D Commutation
Series Inverter
Working Principle of DC Generator

No comments:

Post a Comment