Zener Diode Calculator
Series resistor and power ratings for a Zener voltage regulator.
About this tool
Enter the supply voltage, Zener voltage, load current and the Zener's minimum operating current. The tool gives the ideal resistor, the nearest standard value below it, and the power in the resistor and the Zener with suggested ratings.
Designing a Zener regulator
A Zener shunt regulator is a resistor in series from the supply and a Zener diode across the load. The resistor drops the excess voltage, and the Zener conducts just enough current to hold the output at its breakdown voltage. Choose the resistor so the Zener still has its minimum current (often 5 mA, or about 10% of the load) when the load draws the most: R = (Vs โ Vz) รท (I_load + I_z). For 12 V in, a 5.1 V Zener and a 20 mA load, R = 276 ฮฉ, so use the next standard value down, 270 ฮฉ. The resistor then dissipates about 176 mW, and the Zener's worst case is when the load is disconnected and it absorbs all 25.6 mA, about 130 mW. The tool suggests parts rated at twice the calculated power. Honest limits: Zener regulators waste power and their output varies with load and temperature, so for more than a few tens of milliamps a linear regulator IC or a switching regulator is better. If the supply varies, use its minimum for the resistor and its maximum for power. The LED resistor calculator and the Ohm's law calculator are related.
Frequently asked questions
How do I calculate a Zener resistor?
R = (supply voltage โ Zener voltage) รท (load current + minimum Zener current).
What power rating does the Zener need?
At least Vz ร the total resistor current (the no-load case), ideally with a 2ร margin.
What is the minimum Zener current?
Check the datasheet's test current; around 5 mA or 10% of the load is a common design value.
When should I use a regulator IC instead?
When the load current is more than a few tens of milliamps or you need tighter regulation and efficiency.
Is my data sent anywhere?
No. Everything is calculated in your browser, and nothing is stored or sent.