ElectroHub

Zener Diode Regulator Calculator

A zener held in reverse breakdown clamps its voltage. Put a resistor in front of it and you have the simplest regulator there is — provided the resistor passes enough current to keep the diode in breakdown at maximum load, and not so much that it cooks the diode at no load.

Zener Shunt Regulator

Series resistor R_s feeds a zener diode (V_z) in parallel with the load. See how the current splits between the diode and the load, and the power each part dissipates.

Current through R_s
Load current I_L
Zener current I_Z
Zener dissipation P_Z
Resistor dissipation P_Rs
  1. 1.Given: V_in = 12 V, V_z = 5.1 V, R_s = 330 Ω, R_L = 1 kΩ
  2. 2.FormulaI_Rs = (V_in − V_z) ÷ R_sSubstituteI_Rs = (12 − 5.1) ÷ 330ResultI_Rs = 20.9091 mA
  3. 3.FormulaI_L = V_z ÷ R_LSubstituteI_L = 5.1 ÷ 1000ResultI_L = 5.1 mA
  4. 4.FormulaI_Z = I_Rs − I_LSubstituteI_Z = 0.0209 − 0.0051ResultI_Z = 15.8091 mA
  5. 5.FormulaZener power P_Z = V_z × I_ZSubstituteP_Z = 5.1 × 0.0158ResultP_Z = 80.6264 mW
Common trap: The zener must survive the no-load case — with the load disconnected, all of I_Rs flows through the diode, so size its wattage for the worst case (V_z × I_Rs), not the normal operating point.

The formula

R_s = (V_in − V_z) / (I_load + I_z(min)) ; P_z(max) = V_z × (I_R − I_load(min))

V_in
unregulated input voltage (volts)
V_z
zener voltage (volts)
I_load
maximum load current (amperes)
I_z(min)
minimum current to hold breakdown, typically 5 mA

Worked example

12 V in, 5.1 V zener, load up to 20 mA, holding 5 mA in the diode.

  1. R_s = (12 − 5.1) / (0.020 + 0.005) = 6.9 / 0.025
  2. Worst case is no load: all 25 mA flows in the zener
  3. P_z = 5.1 × 0.025 = 0.128 W

R_s = 276 Ω (use 270 Ω), with a zener rated at least 0.25 W.

Where you'll use it

Cheap references and clamps. It is deliberately wasteful — the resistor passes full current whether the load needs it or not — so beyond a few tens of milliamps a proper regulator wins on both heat and cost.

The laws behind it

Parts this applies to

Resistor (fixed)Potentiometer / Rheostat / TrimmerResistor Array / NetworkThermistor (NTC / PTC)Varistor / MOVSupercapacitorLED (light-emitting diode)MOSFET

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