Question

The following diagram shows a Zener diode as a voltage regulator. The Zener diode is rated at \(V_z = 5\) V and the desired current in load is 5 mA. The unregulated voltage source can supply up to 25 V. Considering the Zener diode can withstand four times of the load current, the value of resistor \(R_s\) (shown in circuit) should be_______ \(\Omega\).

• A. 100
• B. 10
• C. 4000
• D. 1000

Hint:

The series resistor $R_s$ prevents the Zener diode from exceeding its power rating while ensuring sufficient current reaches the load.

Answer 1

The Correct Option is D

To determine the value of the resistor \( R_s \) in the Zener diode voltage regulator circuit, we need to consider the regulation requirements and Zener specifications.

Given:

• Zener Voltage, \( V_z = 5 \, \text{V} \)
• Load Current, \( I_L = 5 \, \text{mA} \)
• Unregulated Voltage, \( V_{\text{in}} = 25 \, \text{V} \)
• Zener Diode can withstand up to four times the load current.

Steps to Calculate \( R_s \):

1. Determine the maximum Zener current \( I_z \): \(I_z = 4 \times I_L = 4 \times 5 \, \text{mA} = 20 \, \text{mA}\).
2. Calculate the total current \( I_s \) through \( R_s \): \(I_s = I_L + I_z = 5 \, \text{mA} + 20 \, \text{mA} = 25 \, \text{mA}\).
3. Apply Kirchhoff’s Voltage Law (KVL) for the loop involving \( R_s \) and the Zener diode: \(V_{\text{in}} = I_s \cdot R_s + V_z\).
4. Rearrange the formula to solve for \( R_s \): 
   \(R_s = \frac{V_{\text{in}} - V_z}{I_s}\) 
   Substitute the known values: 
   \(R_s = \frac{25 \, \text{V} - 5 \, \text{V}}{25 \, \text{mA}}\) 
   \(R_s = \frac{20 \, \text{V}}{0.025 \, \text{A}} = 800 \, \Omega\).

Given the available options, the most suitable resistor value for \( R_s \) is 1000 Ω, ensuring current is within safe limits and achieving proper voltage regulation.