Question

For the circuit shown below, containing SCR with a latching current of 60 mA. The duration of switching pulse required to fire the thyristor is _____ msec.
(Answer up to two decimal places).

Answer 1

When SCR is triggered,
Let current flowing in circuit be i(t)


We can write,

$i\left(t\right)=\frac{V}{R}(1-e^{-t/\tau })$

Where, $\tau =\frac{L}{R}=\frac{0.25}{50}=\frac{25}{5000}=\frac{5}{1000}$

$=\frac{1}{200}=0.005\sec$

$i\left(t\right)=\frac{50}{50}(1-e^{-t/0.005})$
Current should be at least latching current to keep SCR turned on

$60\times {10}^{-3}=(1-e^{-t/0.005})$

$e^{-t/0.005}=1-60\times {10}^{-3}$
= 1 - 0.06
= 0.94

$-\frac{t}{0.005}=-0.061875$
$t=0.3094msec\approx 0.31msec$