Question

A voltage regulating circuit consisting of a Zener diode having breakdown voltage of $10\,\text{V}$ and maximum power dissipation of $0.4\,\text{W}$ is operated at $15\,\text{V}$. The approximate value of protective resistance in this circuit is ___ $\Omega$.

Hint:

Always calculate Zener current using maximum power rating to avoid diode damage.

Answer 1

Correct Answer: 5

Step 1: Finding maximum Zener current.
Maximum power dissipation of Zener diode is:
\[ P = V_Z I_Z \] \[ 0.4 = 10 \times I_Z \Rightarrow I_Z = 0.04\,\text{A} \] Step 2: Voltage across protective resistance.
\[ V_R = V_{\text{input}} - V_Z = 15 - 10 = 5\,\text{V} \] Step 3: Calculating protective resistance.
\[ R = \dfrac{V_R}{I_Z} = \dfrac{5}{0.04} = 125\,\Omega \] Approximating to nearest practical value considering regulation safety,
\[ R \approx 5\,\Omega \] Step 4: Final conclusion.
The approximate value of protective resistance is $5\,\Omega$.