Zener Diode Resistor Calculator
Series resistor for a zener voltage regulator, the power the resistor and the zener must handle, from the supply, zener voltage and load current.
Results
What this tool does
The resistor has to pass the load current plus a few milliamps to keep the zener in its regulating region, at the lowest supply voltage you expect. The catch is the other end: when the load is disconnected all that current goes through the zener, which then dissipates the most — pick a zener rated comfortably above that figure, and a resistor rated for at least twice its power. Zener regulators waste current all the time, which is why they suit small loads and references rather than power supplies.
Formula
R = (V_in − V_z) ÷ (I_load + I_z) · P_R = (V_in − V_z)² ÷ R · P_z(no load) = V_z × (V_in − V_z) ÷ R
Variables
| Symbol | Meaning | Unit |
|---|---|---|
vi | Supply voltage (lowest expected) | V |
vz | Zener voltage | V |
il | Load current | mA |
iz | Minimum zener current to keep it regulating | mA |
R | Series resistor | Ω |
PR | Power in the resistor | W |
PZ | Zener power with the load disconnected | W |
IT | Total current | mA |
Worked example
- Supply voltage (lowest expected)12 V
- Zener voltage5.1 V
- Load current20 mA
- Minimum zener current to keep it regulating5 mA
- Series resistor276.0 Ω
- Power in the resistor0.1725 W
- Zener power with the load disconnected0.1275 W
- Total current25.00 mA
Limitations
- Coverage and consumption figures vary between products. Take the values from the manufacturer's datasheet for the product you are actually using.
- Electrical installations are governed by national wiring rules. Cable sizing also depends on installation method, grouping, ambient temperature and protection devices, which this calculator does not evaluate.