Question

If the load resistance decreases in a zener regulator, the series current:

• A. decreases.
• B. stays the same.
• C. increases.
• D. equals the source voltage divided by the series resistance

Hint:

In a working Zener regulator, think of the series resistor and the constant Zener voltage as setting a constant total current supply (\(I_S\)). The Zener diode then "absorbs" whatever current is not drawn by the load to keep the voltage constant.

Answer 1

The Correct Option is B

Step 1: Analyze an ideal Zener regulator circuit. A simple Zener regulator consists of a series resistor (\(R_S\)) connected to an input voltage (\(V_{in}\)), and a Zener diode in parallel with the load resistor (\(R_L\)). The purpose of the Zener diode is to maintain a constant voltage (\(V_Z\)) across the load.
Step 2: Determine the series current (\(I_S\)). The series current flows through the series resistor \(R_S\). The voltage across \(R_S\) is the difference between the input voltage and the Zener voltage: \(V_{RS} = V_{in} - V_Z\). According to Ohm's law, the series current is: \[ I_S = \frac{V_{in} - V_Z}{R_S} \]
Step 3: Evaluate the effect of a change in load resistance. Assuming the regulator is operating correctly, the Zener diode maintains \(V_Z\) at a constant value. The input voltage \(V_{in}\) and the series resistance \(R_S\) are also constant. Since all the terms on the right side of the equation for \(I_S\) are constant, the series current \(I_S\) remains constant.
Step 4: Consider the current distribution. The series current splits into the Zener current (\(I_Z\)) and the load current (\(I_L\)): \(I_S = I_Z + I_L\). If the load resistance \(R_L\) decreases, the load current \(I_L = V_Z / R_L\) increases. Since \(I_S\) is constant, the Zener current \(I_Z\) must decrease to compensate. The regulator works as long as \(I_Z\) remains positive.