Zener diode as voltage regulator

Zener diode as voltage regulator

1. What a Zener Diode Is for Regulation

Zener diode = heavily doped p-n designed to break down (Zener) at precise $V_Z$ ($2$–$200$V) and operate stably in breakdown with current limiting, maintaining near-constant $V_Z$ despite current variation — used as shunt voltage regulator (and reference).

2. Circuit — Shunt Regulator

Unregulated DC Vin (e.g., rectified+filter, ~15V) → series resistor RS → node → Zener (reverse breakdown) cathode to node, anode to ground → parallel load RL across Zener

• RS limits current:

• Zener in reverse breakdown at $V_Z$ clamps node to $V_Z$ (if $I_Z$ within knee ($I_{ZK}$) to max ($I_{ZM}$)).
• Load $R_L$ sees $V_Z$ constant.

3. Working and Regulation

• Line regulation: if $V_{in}$ ↑, $I_{RS}$ ↑, extra current goes to Zener $I_Z$↑ (since $V_Z$ constant, $I_L = V_Z/R_L$ constant), so $V_L$ stays $V_Z$.
• Load regulation: if $R_L$ ↓ (load current $I_L$ ↑), $I_Z$ ↓ to keep $I_{RS}$ ≈ constant (since $V_{in},R_S$ fixed) ⇒ $V_L$ stays.

Condition for regulation: Zener must stay in breakdown:

and

chosen so

and

(power).

4. Design Example

Need Vout=5.1V from Vin=10V, load RL=500Ω (IL=10mA), Zener 5.1V, IZK=5mA, choose RS:
Total current needed I_RS = I_Z + I_L. Pick I_Z=10mA (mid) ⇒ I_RS=20mA ⇒ RS=(10-5.1)/0.02≈245Ω ⇒ standard 220Ω ✓
Check line variation Vin 12V: I_RS= (12-5.1)/220=31mA ⇒ I_Z=21mA still <IZM (e.g., 50mA) ⇒ okay.
For load variation RL=250Ω (IL=20mA): I_Z= I_RS -20mA =0mA? If I_RS=20mA, I_Z=0 ⇒ dropout ⇒ need larger RS or Vin.

5. Limitations and Modern Use

• Low efficiency (RS dissipates), only small currents (mA to 100mA).
• Modern: replaced by IC regulators (7805 etc., next topic) for higher current/efficiency, but Zener still used as reference.

6. Exam Points

• Draw shunt regulator circuit (Vin→RS→Zener||RL) — 5-7 marks, very frequent (almost certain to ask Zener regulator circuit).
• Explain $V_Z$ constant and $I_Z$ adjusts for line/load — 5 marks, very frequent.
• Give design $R_S=(V_{in}-V_Z)/(I_Z+I_L)$ and condition $I_{ZK}<I_Z<I_{ZM}$ — 5 marks, frequent.
• Mention $P_Z=I_Z V_Z$ limit and efficiency low — 3 marks.