Calculate Series Resistor and Power for Zener Shunt Regulator
Common Zener Presets
Input Voltage Vin (V)
Zener Voltage Vz (V)
Max Load IL (A)
Zener Power Pz (W)
Result
Rseries
R Power
Zener IZ(max)
Step-by-Step Calculation
Zener Regulator Formulas
I_R = I_Z + I_L (total through R_s)
R_s = (V_in - V_z) / I_R
I_Z(max) = P_z(max) / V_z
P_R = I_R² × R_s (power in resistor)
The Zener shunt regulator is the simplest voltage regulator circuit. A series resistor drops the voltage difference between input and Zener voltage. The Zener diode maintains constant voltage across the load by shunting excess current when load current decreases. Design requires worst-case analysis for both no-load (max Zener current) and full-load (min Zener current) conditions.
⚠Always derate Zener power by 2× (use 50% of rated power). During worst case (no load), all current goes through Zener. Verify I_Z does not exceed P_z/V_z. Use a 3-terminal regulator for loads >100mA.
Zener Regulator Design
Designing a Zener regulator involves finding the series resistor that limits current under worst-case conditions. The Zener must handle maximum current when the load is disconnected (I_Zmax = I_R). The resistor must handle the power dissipation when input is maximum. Zener impedance (Rz) causes small voltage changes with current, limiting load regulation.
Series Resistor
R_s = (Vin - Vz)/(I_Z + I_L). Choose I_Z ≈ 0.1×I_L(max). Higher I_Z = better regulation but more power loss.
Worst Case Analysis
No-load: I_Z = I_R (max Zener stress). Full-load: I_Z minimal (must stay above I_ZK). Use both conditions to verify design.
Line Regulation
How well output rejects input changes. ΔVz = Rz × ΔI_Z where Rz is Zener impedance (3-50Ω). Lower Rz = better regulation.
Load Regulation
How well output stays constant with load changes. ΔVz = Rz × ΔI_L. Example: Rz=10Ω, ΔI_L=10mA gives ΔVz=0.1V (2% for 5V).
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