JEE Challenger
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PhyMedMain 2026 • 4th Apr - Shift 1

Time Constant of Circuit with Diodes and Capacitor

Consider a circuit consisting of a capacitor (20 μF20\text{ }\mu\text{F}), resistor (100 Ω100\text{ }\Omega) and two identical diodes as shown in figure. The resistance of diode under forward biasing condition is 10 Ω10\text{ }\Omega. The time constant of the circuit is α×10−3 s\alpha \times 10^{-3}\text{ s}. The value of α\alpha is _____.

Question Diagram 1

Options

A

2.22.2

Correct
B

2.02.0

C

2.12.1

D

2.42.4

Topics & Concepts

Current ElectricityCapacitorsSemiconductors

Step-by-Step Solution

To find the time constant of the circuit, we determine the equivalent resistance connected in series with the capacitor C=20 μFC = 20\text{ }\mu\text{F}.

In the parallel pair of anti-parallel diodes, one diode is forward-biased with resistance Rd=10 ΩR_d = 10\text{ }\Omega while the other is reverse-biased (ideal open circuit). Thus, the effective resistance of the diode network is Rd=10 ΩR_d = 10\text{ }\Omega.

The total equivalent resistance of the circuit is: Req=R+Rd=100 Ω+10 Ω=110 ΩR_{\text{eq}} = R + R_d = 100\text{ }\Omega + 10\text{ }\Omega = 110\text{ }\Omega

The time constant τ\tau of the RCRC circuit is: τ=ReqC=110 Ω×20×10−6 F=2.2×10−3 s\tau = R_{\text{eq}} C = 110\text{ }\Omega \times 20 \times 10^{-6}\text{ F} = 2.2 \times 10^{-3}\text{ s}

Comparing with τ=α×10−3 s\tau = \alpha \times 10^{-3}\text{ s}, we get α=2.2\alpha = 2.2.

Therefore, the correct option is A.

Time Constant of Circuit with Diodes and Capacitor | Physics PYQ Solution - JEE Challenger