Question

The equivalent resistance between $\text{A}$ and $\text{B}$ is

• A. zero if $V_A>V_B$ and $54\Omega$ if $V_A<V_B$
• B. $18\Omega$, if $V_A<V_B$
• C. zero if $V_A<V_B$ and $54\Omega$ if $V_A>V_B$
• D. $36\Omega$, if $V_A>V_B$

Answer 1

The correct option is A zero if $V_A>V_B$ and $54\Omega$ if $V_A<V_B$

If $V_A>V_B$, all the three diodes will be forward biased, so each diode will offer zero resistance. The current will flow as shown in the figure.


As the current is not passing through any resistance,

$\therefore R_{eq}=0$

If $V_A<V_B$, only $D_2$ is forward biased and the current will flow as shown in the figure.


As the current passes through both the resistance without any division, they are in series.

$R_{eq}=18+36$

$\therefore R_{eq}=54\Omega$

Hence, option $\left(\text{D}\right)$ is correct.