In one line
In a Zener shunt regulator, a series resistor drops the extra input voltage while the Zener diode, connected in reverse across the load, adjusts its own current so that the load voltage stays fixed at \( V_Z \).
Explained simply
Picture a water tank with an overflow pipe set at a fixed height. If more water comes in, more flows out of the overflow; if more is drawn by the tap, less goes to the overflow; the water level stays the same. The Zener diode is the overflow pipe: it takes whatever extra current is needed to keep the voltage level constant.
Technically, the circuit is: unregulated \( V_{in} \) → series resistor \( R_S \) → Zener diode (reverse biased) in parallel with load \( R_L \). As long as the Zener operates between \( I_{Z(min)} \) and \( I_{Z(max)} \), \( V_{out} \approx V_Z \).
How it works
- Current through \( R_S \): \( I_S = (V_{in} - V_Z)/R_S \). It splits as \( I_S = I_Z + I_L \).
- Line regulation (input changes, load fixed): if \( V_{in} \) rises, \( I_S \) rises; since \( I_L = V_Z/R_L \) stays the same, the extra current flows through the Zener. The extra voltage appears across \( R_S \), so \( V_{out} \) stays at \( V_Z \).
- Load regulation (load changes, input fixed): if the load draws more current, \( I_Z \) falls by the same amount, keeping \( I_S \) and \( V_{out} \) the same.
- Regulation fails if \( I_Z \) drops below \( I_{Z(min)} \) (too little input or too much load) or exceeds \( I_{Z(max)} \) (overheating).
- Because the Zener is in parallel with the load, this is called a shunt regulator.
Key formula
\( I_S = \dfrac{V_{in} - V_Z}{R_S}, \quad I_L = \dfrac{V_Z}{R_L}, \quad I_Z = I_S - I_L \)
\( V_{in} \) = unregulated input (V), \( V_Z \) = Zener voltage = output (V), \( R_S \) = series resistor (\( \Omega \)), \( R_L \) = load (\( \Omega \)), \( I_S, I_Z, I_L \) = series, Zener and load currents (A).
Quick example
\( V_{in} = 12 \) V, \( V_Z = 6 \) V, \( R_S = 100\,\Omega \): \( I_S = 60 \) mA. With a 20 mA load, \( I_Z = 40 \) mA. If the input rises to 14 V, \( I_S = 80 \) mA and \( I_Z = 60 \) mA, but the output is still 6 V. (Illustration only; numericals are not asked in this module.)
Common mistakes
- Connecting the Zener in forward bias in the regulator diagram.
- Omitting the series resistor; without it the Zener current is not limited and the diode burns.
- Connecting the Zener in series with the load instead of across it.
How MU asks it
Numericals are not expected in this module (MU syllabus), so questions are theory with diagrams. Typical: "Explain the Zener diode as a voltage regulator with a neat circuit diagram" (5–8 marks). For full marks draw the circuit with \( R_S \), Zener (reverse), \( R_L \) and current directions, write \( I_S = I_Z + I_L \), explain both line and load regulation separately, and state the limits \( I_{Z(min)} \) and \( I_{Z(max)} \).