A Zener diode is an ordinary PN junction diode that is deliberately operated in reverse breakdown. While a normal diode would be destroyed by reverse breakdown, a Zener diode is designed for it: once the reverse voltage reaches the Zener voltage (Vz), the voltage across the diode stays almost constant even as the current through it changes widely. That single property makes it the simplest voltage regulator in electronics.

Figure 1: Zener shunt regulator circuit (a) and the Zener I–V characteristic (b). In breakdown the voltage stays at Vz while the current varies. (Schematic, not to scale.)

How the regulator works

The circuit is a series resistor (Rs) feeding a Zener diode connected in parallel with the load. The Zener is reverse-biased: its cathode faces the positive supply. As long as the Zener current (Iz) stays between its minimum (knee) current and its maximum rated current, the output voltage is locked to Vz.

Regulation happens automatically. If the input voltage rises, the extra voltage drops across Rs because the Zener takes more current. If the load draws more current, the Zener takes less — the total current through Rs stays roughly the same, so the drop across Rs stays the same, and the output stays at Vz. The Zener "shunts" away whatever current the load does not need, which is why this is called a shunt regulator.

Two breakdown mechanisms exist: the Zener effect dominates below about 5.6 V, and avalanche breakdown dominates above about 5.6 V. In practice both are called Zener breakdown, and the 5.6 V Zener is the most temperature-stable of all.

Design rules

• Choose Iz in the middle of the range: Iz must stay above the knee current (a few mA, so the diode is properly in breakdown) and below Iz(max) = Pz / Vz, where Pz is the Zener's power rating.
• Rs sets the total current: Rs = (Vin − Vz) / (Iz + IL), where IL is the load current.
• Check the worst cases: at full load, Iz must still be above the knee; at no load, Iz is at its maximum, so Pz = Vz × Iz(max) must stay below the Zener's rating, and the power in Rs must stay below its rating.
• Design at the minimum input voltage: if Vin falls, Iz falls first — if Iz reaches zero, regulation is lost.

Worked example — 5.6 V regulator from a 12 V supply

Design a regulator with Vz = 5.6 V, unregulated input Vin = 12 V (minimum), load current IL = 20 mA, using a 5.6 V, 0.5 W Zener diode.

Step 1 — load resistance: RL = Vz / IL = 5.6 / 0.020 = 280 Ω.

Step 2 — Zener current limits: Iz(max) = Pz / Vz = 0.5 / 5.6 = 89 mA. Choose a working Zener current of Iz = 10 mA — well above the knee and well below the maximum.

Step 3 — series resistor: Rs = (Vin − Vz) / (Iz + IL) = (12 − 5.6) / (0.010 + 0.020) = 6.4 / 0.030 = 213 Ω. The nearest standard value is 220 Ω.

Step 4 — check with Rs = 220 Ω: total current Is = (12 − 5.6) / 220 = 6.4 / 220 = 29.1 mA. At full load, Iz = 29.1 − 20 = 9.1 mA — still in breakdown. At no load, Iz = 29.1 mA and Pz = 5.6 × 0.0291 = 0.163 W, below the 0.5 W rating. Power in Rs = Is² × Rs = (0.0291)² × 220 = 0.186 W — use a ½ W resistor for a safe margin.

Step 5 — line regulation check: if Vin rises to 14 V, Is = (14 − 5.6) / 220 = 38.2 mA, so Iz = 38.2 − 20 = 18.2 mA — still below 89 mA, and the output stays at 5.6 V.

Key points to remember

• A Zener diode in reverse breakdown holds a nearly constant voltage Vz while its current varies — that is the basis of the shunt regulator.
• Rs = (Vin − Vz) / (Iz + IL); design it at the minimum input voltage.
• Worked example: 12 V in, 5.6 V Zener, 20 mA load → Rs = 220 Ω, Is = 29.1 mA, Iz = 9.1 mA at full load.
• Check both extremes: Iz above the knee at full load, and Zener and resistor power below their ratings at no load.
• The Zener effect dominates below ~5.6 V and avalanche breakdown above ~5.6 V; a 5.6 V Zener is the most temperature-stable.