$4\mathrm{.}18.($a$)$ Snubber circuit for an SCR should primarily consist of capacitor only. But, in actual practice, a resistor is used in series with the capacitor. Discuss. R$,$ Land C in an SCR circuit meant for protecting against du$/$dt and di$/$di are $42,6$ MH and $6$ uF respectively, If the supply voltage to the circuit is $300$ V$,$ calculate permissible maximum values of du$/$dt and di$/$dt$.($Hint: $($b$)$ Rate of change of voltage across the thyristor at t $=0$ when the supply is switched on is given by