Zener Diode

A Zener diode conducts in reverse breakdown at a fixed, tightly controlled voltage. This makes it the simplest voltage reference and regulator in electronics. The LM78xx series IC uses a Zener-based reference internally, but discrete Zeners like the BZX55C5V1 appear in every low-power regulator and clamping circuit.

Core Concept

An ordinary diode breaks down in reverse bias and is damaged. A Zener is designed to break down safely and repeatedly. At the Zener breakdown voltage V_Z, the reverse current increases sharply while the voltage across the diode stays nearly constant. This clamping action is what makes it useful.

Two physical mechanisms cause breakdown. At voltages below about 5 V, Zener breakdown dominates: a strong electric field tears electrons directly from covalent bonds. Above 5 V, avalanche breakdown dominates: carriers accelerate and collide with atoms to create new electron-hole pairs. Most devices called Zener diodes above 6 V actually use avalanche breakdown.

In a shunt regulator, a series resistor Rs connects the supply to the Zener. The Zener sits in parallel with the load RL. If the input voltage rises, the extra current flows through the Zener, and the output voltage stays fixed at V_Z. The BZX55C5V1 holds 5.1 V across a wide range of currents from 5 mA to 70 mA.

Key Equations

Series resistor selection: Rs = (Vin_min - V_Z) / I_Z_max where I_Z_max = I_Z + I_L_max

Power dissipation in Zener: P_Z = V_Z * I_Z — must be less than rated power (e.g., 500 mW for BZX55 series).

Zener dynamic resistance: r_z = delta_V_Z / delta_I_Z — typically 5 to 40 Ω. Smaller r_z means better regulation.

Line regulation: delta_V_out = r_z * delta_I_Z — lower r_z gives smaller output variation with input change.

Given:
Vin = 12 V (DC)
V_Z = 5.1 V (BZX55C5V1)
I_L = 20 mA (load current)
I_Z_min = 5 mA (minimum for regulation)
P_Z_max = 500 mW

Why this formula:
Rs must drop (Vin - Vz) while supplying both load and Zener current.

Formula:
Rs = (Vin - V_Z) / (I_Z_min + I_L)

Substitution:
Rs = (12 - 5.1) / (0.005 + 0.020)
Rs = 6.9 / 0.025

Calculation:
Rs = 276 Ω → use standard 270 Ω

Check Zener power at no load (worst case):
I_Z_max = (Vin - V_Z) / Rs = 6.9 / 270 = 25.6 mA
P_Z = 5.1 * 0.0256 = 130 mW < 500 mW (safe)

Final Answer:
Rs = 270 Ω (standard value)
Max Zener dissipation = 130 mW (within 500 mW rating)

Exam Tip: The most tested trap is forgetting that worst-case Zener power occurs at no load (RL disconnected), not at full load. At no load, all the current from Rs flows through the Zener. Also, Zener voltage has a temperature coefficient: low-voltage Zeners (< 5V) have negative tempco (voltage drops with temperature), while high-voltage ones (> 6V) have positive tempco. A 5.6 V Zener is near zero tempco, which is why it is preferred in precision references.

Key Properties

• Zener diodes are specified by V_Z at a test current I_ZT. For BZX55C5V1, V_Z = 5.1 V at I_ZT = 5 mA.
• Dynamic resistance r_z is low for high-power Zeners. A 1N4733A (5.1 V, 1 W) has r_z ≈ 7 Ω, giving tighter regulation than the 500 mW BZX55.
• Temperature coefficient: below 5 V the Tempco is negative (about -2 mV/°C). Above 6 V it is positive (about +2 mV/°C). Near 5.6 V, Tempco is near zero.
• Minimum Zener current I_Z_min must be maintained for the diode to stay in breakdown. Below this, regulation fails and output drops.
• Power rating determines Rs. A 500 mW Zener with 5 V output can handle up to 100 mA of Zener current before it overheats.
• Zener diodes are used in overvoltage protection (crowbar circuits), voltage references, level shifting, and waveform clipping.

Quick Revision

• Zener operates in reverse breakdown at constant V_Z.
• Below 5 V: Zener mechanism (field ionization). Above 6 V: avalanche mechanism.
• Rs = (Vin - Vz) / (I_Z + I_L). Choose I_Z ≥ I_Z_min from datasheet.
• Worst case power dissipation is at no load (all Rs current through Zener).
• Dynamic resistance r_z: smaller is better. Typically 5 to 40 Ω.
• 5.6 V Zener has near-zero temperature coefficient, preferred for references.
• Exam trap: Using full-load current to check Zener power rating. Always check at no load, where Zener current is maximum.