Question

Potential drop across forward junction p-n diode is $0.7V$. If a battery of $4V$ is applied, calculate the resistance to be put in series, if the maximum current in the circuit is $5mA$.

• A. $660\Omega$
• B. $350\Omega$
• C. $475\Omega$
• D. $500\Omega$

Answer 1

Answer: (A)

Let the value of resistance be $R$ which is to be put in series.
${\left(I_f\right)}_{\text{peak}}=5mA=5\times 10^{3}A$
${\left(I_f\right)}_{\text{peak }}$ is maximum current in circuit and potential drop across $p$-n junction diode is $0.7V$.
It is knee voltage and if emf of applied battery is $E$ then.
$E=V_{\text{knee}}+{\left(I_f\right)}_{\text{peak }}R$... (i)
Putting above values in Eq. (i), we get
$4=0.7+5\times 10^{-3}R$
or $5\times 10^{-3}R=3.3$
or $R=660\Omega$