Zener Shunt Regulator

Circuit: V_s → R_s → node → Z_D to GND, R_L to GND
The zener maintains V_out = V_z by shunting excess current. R_s limits total current.

Supply Voltage V_s

Zener Voltage V_z

Common: 2.7, 3.3, 3.9, 4.7, 5.1, 6.2, 8.2, 9.1, 12V

Load Current I_L (max)

Current drawn by the load (0 for no load)

Min. Zener Current I_z_min

Minimum to keep zener in regulation (~5mA typical)

Zener Max Power P_z_max

From datasheet (e.g. 500mW, 1W)

📐 Design Formulas

Series Resistor

Rs = (Vs − Vz) / (Iz + IL)

Use max IL and min Iz
for worst-case Rs

Total Current

I_total = (Vs − Vz) / Rs
I_z = I_total − I_L

Check I_z > I_z_min

Power Dissipation

P_zener = Vz × Iz_max
P_Rs = (Vs − Vz)² / Rs

Iz is max when IL=0

Load Resistor

R_L = Vz / I_L

Regulation fails if
R_L < Vz×Rs/(Vs−Vz)

Vs ──┬── Rs ──┬── Vout = Vz │ │ │ Z_D (zener, cathode up) │ │ GND GND RL from Vout to GND

❓ FAQ

How does a zener shunt regulator work?

The zener diode is reverse-biased and connected in parallel with the load. When supply voltage varies, the zener adjusts its current to maintain a constant voltage across the load. The series resistor Rs absorbs the voltage difference and limits the total current.

What is the minimum load resistance for regulation?

Regulation fails (zener turns off) when all current goes to the load. Minimum load resistance: R_L_min = V_z × R_s / (V_s − V_z). Below this, the zener cannot conduct and regulation is lost.

How do I choose the zener power rating?

Maximum zener power occurs with no load (IL = 0). At that point, all current flows through the zener: Iz_max = (Vs − Vz) / Rs, giving Pz_max = Vz × Iz_max. Use a zener rated at least 2× this value for reliability.

When should I use an LM317 instead of a zener?

Use a zener regulator for simple, low-current applications where efficiency isn't critical. Use the LM317 for higher load currents (up to 1.5A), better regulation accuracy, and adjustable output voltage.