Zener Diode Series Resistor Calculator
A Zener diode in shunt configuration makes a simple voltage regulator. A series resistor sets the current and drops the difference between the input supply and the regulated Zener voltage. This calculator finds the required series resistor from the supply voltage, Zener voltage, load current, and the minimum Zener holding current you want to maintain. It also reports the power dissipated in the resistor and the worst-case power the Zener must handle when the load is removed. All values follow directly from Ohm's law and the power law.
Zener resistor formula
total current I = Iz + Iload (amps)
series resistor R = (Vin - Vz) / I
resistor power Pr = (Vin - Vz) * I
Zener power at no load Pz = Vz * I
Currents are entered in milliamps and converted to amps internally. At no load the entire resistor current flows through the diode, which sets its worst-case power.
Design notes
- The supply voltage must exceed the Zener voltage for the regulator to work.
- Choose a standard resistor value at or just below the computed value to keep current above the minimum.
- Rate the resistor for at least twice its calculated power dissipation.
- The Zener must handle the no-load power, which is its worst case.
- Shunt regulators are simple but lossy; for high current use a dedicated regulator IC.
Zener resistor sizing: frequently asked questions
How do you size the series resistor for a Zener diode?
The resistor drops the difference between the supply and the Zener voltage while carrying both the load current and the Zener's minimum holding current: R = (Vin - Vz) / (Iz + Iload). Choose Iz large enough to keep the diode in regulation across the supply and load range.
What power rating does the series resistor need?
The resistor dissipates P = (Vin - Vz)^2 / R, or equivalently the voltage across it times the current through it. Pick a resistor rated at least twice this value for headroom and reliability against heating.
How much power does the Zener diode dissipate?
At minimum load (worst case) the diode carries the full current through the resistor minus the load: P_zener = Vz * Iz_max. With no load, Iz_max is the whole resistor current, so the diode must be rated for Vz times that current.
Why must the supply voltage exceed the Zener voltage?
A Zener shunt regulator works only when the input is higher than the Zener voltage, so the series resistor has a positive voltage across it to set the current. If Vin is at or below Vz, no regulation current flows and the design is invalid.
What is the minimum Zener current?
Every Zener needs a minimum current (often a few milliamps, per the datasheet) to stay in its sharp breakdown region and regulate well. The series resistor is sized so this minimum still flows even at maximum load and minimum supply voltage.
Official sources
- NIST: SI units (volt, ohm, ampere, watt).
- NASA Glenn Research Center: Ohm's law and DC circuit fundamentals.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.