Zener Diode Voltage Regulator Sizer
Choose a series resistor for a simple Zener regulator. Check input and load extremes, keep enough current for regulation, and compare Zener and resistor dissipation against your derated power limits.
A resistor that works at nominal input can lose regulation at full load or overheat at light load. This tool screens a constant-voltage model; it does not certify a part or circuit.
Find a workable resistor range
For a positive DC supply, one series resistor, and a reverse-biased Zener across the load. Enter an operating-voltage envelope and power limits appropriate to the selected parts.
Resistor & operating checks
Submit the inputs to calculate.
Nominal resistor window
| Constraint | Nominal resistance |
|---|
Current & load headroom
Input / load corner checks
| Operating corner | Available IZ range | Minimum IZ check |
|---|
A DC sizing model—not a precision reference model
Output ripple, dynamic resistance, temperature drift, self-heating feedback, startup, source current limiting, surge behavior, and fault survival require separate analysis. A successful result means only that the entered steady-state constraints overlap.
The resistor supplies both branches
Load current and Zener current share one series path. Increasing load demand leaves less current for the Zener; reducing load demand shifts more current into it.
Too large or too small?
- A larger resistor reduces shunt current and dissipation, but may leave insufficient Zener current at low input and high load.
- A smaller resistor provides more current headroom, but increases continuous heating and can exceed the Zener current limit.
- No overlap means a single-resistor solution cannot satisfy the entered conditions. Review the supply range, load demand, actual part limits, or regulator topology.
IZ = IS − IL
PZ = VZ × IZ
PR = (VIN − VZ)² / R
The equations assume the Zener is operating in the intended reverse-breakdown region. If current is insufficient, they do not predict the actual output voltage.
How the resistor is selected
The calculations below use amperes, ohms, volts, and watts. The input form converts milliamperes to amperes. Voltage, load, and resistor bounds are treated independently.
Upper bound: retain regulation current
RLOW = RNOM × (1 − t)
RHIGH = RNOM × (1 + t)
RNOM,max = (VIN,min − VZ,max) / [(IL,max + IZ,min) × (1 + t)]
The upper bound reserves the entered minimum IZ at the least favorable input, load, voltage, and resistance combination. A zero or negative upper bound is not a usable resistor value.
Lower bounds: limit dissipation
RNOM,min,Z = max over v of {v × (VIN,max − v) / [PZ,allow + v × IL,min]} / (1 − t)
Here, v runs across the entered VZ band. The optional continuous-current limit adds another lower bound. The largest E24 value in the common window is selected to reduce current while retaining the required minimum IZ.
Peak power may lie inside the voltage band
For a fixed resistor at maximum input and minimum load, PZ(v) = v × [(VIN,max − v) / RLOW − IL,min]. This is a concave quadratic, so the maximum can occur between the voltage endpoints. The tool evaluates the interior peak as well as the bounds; it does not simply multiply maximum VZ by maximum IZ from different corners.
RNOM,min,I = max[0, (VIN,max − VZ,min) / (IZ,max + IL,min)] / (1 − t)
These extended bound calculations follow from the constant-voltage equations, not from a nonlinear diode simulation. Sizing searches E24 values from 0.1 Ω to 1 MΩ; check mode accepts 0.1 Ω to 10 MΩ. Numeric input limits are software limits, not component ratings.
Turn the calculation into a specification
Check the actual diode curves
Compare your expected IZ range with the voltage-current curves, test current, and temperature behavior of the exact ordering code. Use a precision reference or regulated supply when the required accuracy cannot be justified by that data.
Use derated continuous limits
Review ambient temperature, board copper or lead cooling, mounting, working voltage, and continuous power. Leave the margin required by your design policy. Pulse and surge ratings are not interchangeable with a steady-state limit.
Include sleep and disconnect states
Account for startup, maximum demand, standby, and an unplugged load. If a minimum load is essential, identify what guarantees it. Verify the final circuit under its supply and load extremes, including faults separately.
Zener regulator questions
Why not calculate R from nominal input voltage?
A nominal calculation describes only one point. The low-input, high-load condition sets the current headroom; the high-input, low-load condition sets heating. Resistor tolerance and the assumed Zener voltage band can reduce the overlap further.
Is a 5.1 V ±5% diode always between 4.845 V and 5.355 V?
Not necessarily. A voltage tolerance is specified under stated test conditions. Current, temperature, and self-heating can move the operating voltage. Use the entered bounds as an engineering assumption to verify, not as a guarantee created by this calculator.
Should IZ,min be the datasheet knee current?
Only if the device data supports your required output accuracy at that current. A knee-current test does not automatically establish useful regulation for your load. Review the voltage-current curve and specified test conditions before selecting the minimum.
Why does disconnecting the load heat the Zener?
In the constant-voltage model, the series resistor still supplies current. When the load stops taking its share, that current flows through the Zener. Set minimum load to zero when disconnect is an operating condition; the separate no-load check does not automatically extend a nonzero load range.
What does “No E24 candidate” mean?
Either the current and power constraints do not overlap, or no searched E24 value fits the narrow interval. Do not simply round to the nearest resistor. A custom value can be checked, but conflicting continuous bounds require a change to the design assumptions or circuit.
Can this tool size a TVS, TL431, or mains dropper?
No. The model is for one passive Zener in a positive DC shunt regulator. It does not address TVS pulse waveforms, TL431 bias and stability, capacitive or mains droppers, isolation, or surge protection. A dedicated regulator may be more suitable for tighter regulation or higher load power.
Continue your component review
Have a Zener or resistor part number?
Send the exact MPN, package, voltage or resistance, tolerance, quantity, and required date for an order-specific sourcing review.
Technical references
- Nexperia AN90031 — Zener diodes: physical basics, parameters and application examples. See the device parameters and DC voltage-stabilizer example.
- Vishay — Breakdown Voltage (Zener Voltage). Shunt-regulator current, power, and operating conditions.
- Vishay 1N4728A–1N4761A datasheet. An example of specified test currents, dynamic resistance, and rating conditions; not the source of the calculator defaults.