Zener Diode Resistor Calculator
Size the series resistor for a zener diode shunt regulator from supply voltage, zener voltage, load current, and desired zener bias current.
The calculator reports the theoretical resistance, next higher E24 value, current distribution, and component power so hardware designers can assess regulation and thermal margin.
Use worst-case supply, load, zener tolerance, knee current, and power derating from the selected component datasheets before releasing a design.
Engineering tool
Zener Diode Resistor Calculator
Size a zener shunt regulator resistor and check current distribution, component power, and E24 selection.
Input voltage applied to the series resistor.
Nominal regulated voltage at the selected operating current.
Maximum current required by the load at Vz.
Bias current reserved for zener regulation under load.
Maximum rated zener dissipation before derating.
Recommended E24 series resistor
300 Ω
Rs = (Vin - Vz) / (Iload + Iz)
Result console
- Calculated series resistor
- 276Ω
- Nearest higher E24 resistor
- 300Ω
- Resistor voltage
- 6.9V
- Design total current
- 25mA
- Resistor power
- 172.5mW
- Recommended resistor rating
- 500mW
- Zener power
- 25.5mW
- Load power
- 102mW
- E24 estimated total current
- 23mA
- E24 estimated zener current
- 3mA
Formula reference
Zener Regulator Formulas
The resistor carries both load current and zener bias current. All calculations use volts, amperes, ohms, and watts internally.
Resistor voltage: Vr = Vin - VzTotal current: Itotal = Iload + IzSeries resistor: Rs = Vr / ItotalResistor power: Pr = Itotal² × RsZener power: Pz = Vz × IzLoad power: Pload = Vz × IloadVariable definitions
- Vin
- Supply voltage
- Vz
- Nominal zener voltage
- Iload
- Load current
- Iz
- Desired zener current
- Rs
- Series resistance
How to Use This Calculator
- Enter the supply voltage and nominal zener voltage.
- Enter maximum load current and the zener current needed for regulation.
- Enter the zener power rating, then calculate.
- Review the E24 value, actual estimated zener current, power, and warnings against worst-case datasheet limits.
Worked Example
12 V to 5.1 V Zener Regulator
Vin = 12 V, Vz = 5.1 V, Iload = 20 mA, Iz = 5 mA
Vr = 12 - 5.1 = 6.9 V; Itotal = 20 mA + 5 mA = 25 mA
Rs = 6.9 / 0.025 = 276 Ω; choose the next higher E24 value of 300 Ω
Pr = 0.025² × 276 = 0.1725 W; Pz = 5.1 × 0.005 = 25.5 mW; Pload = 102 mW
With 300 Ω, total current is about 23 mA and estimated zener current is 3 mA.
Engineering Notes
Check the knee current
A zener must usually operate above its datasheet knee current to regulate near its specified voltage.
Evaluate both load extremes
Maximum load checks regulation; minimum load often creates the highest zener current and power.
Include supply tolerance
Use maximum Vin for thermal stress and minimum Vin for available regulation current.
Derate component power
Select resistor and zener ratings with temperature, enclosure, PCB copper, and reliability margin.
Use the right regulator
A zener shunt regulator is inefficient for substantial load power or wide input-voltage ranges.
Common Mistakes
- Calculating from load current alone and omitting zener bias current.
- Checking nominal conditions without maximum input or minimum load.
- Assuming nominal zener voltage is exact at every current.
- Ignoring resistor, zener, and PCB temperature derating.
Support reference
FAQ
How do you calculate a resistor for a zener diode?
Subtract zener voltage from supply voltage, then divide by the sum of load current and desired zener current: Rs = (Vin - Vz) / (Iload + Iz).
How much current should flow through a zener diode?
Select enough current to keep the diode above its datasheet knee current while remaining below its current and power limits across supply and load tolerances.
How do you calculate zener diode power?
Multiply zener voltage by zener current: Pz = Vz × Iz. Check the worst case at maximum input voltage and minimum load current.
What happens if the zener resistor is too large?
Available current may be insufficient for both the load and zener, causing the zener to leave regulation and the output voltage to fall.
Can a zener diode regulator power any load?
No. Shunt regulators are generally suited to low-current references and simple loads. Higher power or efficient regulation usually requires an active linear or switching regulator.
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Browse the Diodes calculator category or the Engineering Reference Center.
This calculator provides nominal engineering estimates. Verify zener curves, dynamic resistance, tolerances, transient conditions, component power derating, and measured circuit behavior before production use.