Zener Diodes

Zener diodes break down at a precise reverse voltage and hold it nearly constant. Learn Zener and avalanche breakdown, the shunt regulator design (series resistor, minimum Zener current, power dissipation), line and load regulation, voltage references, and TVS protection.

A Zener diode is designed to operate in reverse breakdown. Once the voltage reaches V_Z, current can vary widely while the voltage stays almost fixed — giving a simple voltage reference, clamp or low-current regulator.

An analogy

A Zener is like an overflow spillway on a dam: water level (voltage) can't rise above the spillway height — extra water (current) simply flows over.

Breakdown

Below ≈ 5 V, Zener (tunnelling) breakdown dominates with a negative tempco; above ≈ 6 V, avalanche breakdown with positive tempco. Around 5.1–5.6 V they nearly cancel — the most temperature-stable Zeners.

Shunt regulator design

A resistor from the supply feeds the Zener and load in parallel. The resistor must supply the maximum load plus minimum Zener current at the lowest input; at highest input and no load, the Zener must safely dissipate all the current.

Limitations

Inefficient for large currents (resistor and Zener waste power), finite dynamic resistance r_Z causes the output to vary a little. For better regulation use a Zener + transistor or an IC regulator.

References and protection

Precision shunt references (TL431, LM4040) behave like programmable, stable Zeners. TVS diodes are Zener-like devices designed to absorb large surge energy (ESD, load dump) in nanoseconds.

Formulas

• Series resistor: R_S ≤ (V_in,min − V_Z) / (I_L,max + I_Z,min)
• Worst-case Zener power: P_Z = V_Z × (V_in,max − V_Z)/R_S (no load)
• Load regulation: ΔV_out ≈ r_Z × ΔI_Z

Worked example: Shunt regulator

Given: V_in = 12 V, V_Z = 5.1 V; I_L,max = 20 mA, I_Z,min = 5 mA

Find: R_S and worst-case Zener power.

• R_S = (12 − 5.1)/0.025 = 276 Ω → 270 Ω
• No load: I = 6.9/270 = 25.6 mA
• P_Z = 5.1 × 0.0256 = 130 mW

Answer: 270 Ω; P_Z = 130 mW (use a 500 mW Zener)

Worked example: Output ripple

Given: r_Z = 10 Ω; Zener current varies by 5 mA

Find: Output variation.

• ΔV = 10 × 0.005 = 50 mV

Answer: ≈ 50 mV

Common mistakes

• Wrong: Sizing R_S at nominal input only. — Right: Check minimum input (regulation) and maximum input (dissipation).
• Wrong: Using Zeners for high-current supplies. — Right: Use linear or switching regulators.

Applications

• Voltage references: Bias and reference points in analog circuits.
• Over-voltage clamps: Protect gates and inputs.
• Surge suppression: TVS diodes on data and power lines.

Key takeaways

• Zener holds V_Z in reverse breakdown.
• R_S from min input and max load.
• Check dissipation at max input, no load.
• Use references/TVS for precision/protection.

Going further

• Temperature coefficient: Best stability near 5.6 V.
• Zener + emitter follower: Higher-current simple regulator.
• Bandgap references: IC references with ppm/°C stability.