Electronics Engineering (ELEX) Board Practice Exam

The charging time constant for an inductor is defined as the time taken for the voltage to reach what percentage of the initial voltage?

1. 50%

2. 37%

3. 75%

4. 90%

The correct answer is: 37%

The time constant for an inductor in the context of an RL circuit (a circuit consisting of a resistor and an inductor) is characterized by the behavior of the current as it changes when a voltage is applied. Specifically, the time constant (often denoted as τ) for an RL circuit is defined by the formula τ = L/R, where L is the inductance in henries and R is the resistance in ohms. When a voltage is suddenly applied to an inductor, the current does not instantly reach its maximum value; instead, it gradually increases according to an exponential function. The voltage across the inductor also behaves in a similar way. It typically takes about one time constant for the current (and the voltage across the inductor) to reach approximately 63.2% of its final steady-state value. However, the question specifically asks about the percentage of the initial or applied voltage, and in this exponential charging process, the voltage across the inductor rises to about 37% of its final steady-state value after one time constant. This percentage represents a critical point in the charging process, helping to visualize the behavior of energy storage components like inductors. Thus, the correct answer emphasizes this fundamental concept of inductor behavior in