Question

The triggering circuit of a thyristor is shown in figure. The thyristor requires a gate current of 10 mA, for guaranteed turn-on. The value of R required for the thyristor to turn on reliably under all conditions of $V_b$ variation is

• A.
  1200 $\Omega$
• B.
  1600 $\Omega$
• C.
  10000 $\Omega$
• D.
  800 $\Omega$

Answer 1

The correct option is D

800 $\Omega$

$V_b=12\pm 4V$

$V_{b(min)}=8Vand{V}_{b(max)=16V}$

The thyristor must turn on even for minimum value of $V_b$.

Gate current for guaranteed turn-on = $I_g=10mA,$ neglecting voltage between gate and cathode.

Therefore, required value of
$R=\frac{V_{b(min)}}{I_g}=\frac{8}{10mA}=800\Omega$