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For the circuit shown in the figure, the equivalent resistance between points A and B for the two cases (i) V_A>V_B and (ii) V_B>V_A is ______ and ______ respectively. (D₁ and D₂ are ideal diodes.)

Asked in GUJCET 2017 · Symmetric and complex networks

Figure: Symmetric and complex networks
Answer: (1) 25 Ω, ∞

Step-by-step solution

Idea: an ideal diode is a short circuit when forward biased and an open circuit when reverse biased.

In the figure both D₁ and D₂ point from A towards B, each in series with 50 Ω, and the two branches are in parallel.

(i) V_A>V_B: both diodes are forward biased and conduct.

The two 50 Ω branches are then in parallel: R=(50×50)/(50+50)=25 Ω.

(ii) V_B>V_A: both diodes are reverse biased and block, so no current flows between A and B: R=∞.

So the equivalent resistances are 25 Ω and ∞.

Why the other options are wrong

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