How to use the Zener Regulator Calculator
- 1. Enter input rangeMinimum and maximum input voltage.
- 2. Enter zener and loadVz, maximum and minimum load current, Iz(min) and the zener's watt rating.
- 3. Get Rs and powerSee the standard Rs value, zener dissipation and resistor power.
Zener regulator design formula
A zener in reverse breakdown holds its voltage nearly constant. The series resistor Rs must pass enough current to feed the load and keep the zener in breakdown, even at the lowest input and the highest load:
Rs = (Vin(min) − Vz) / (IL(max) + Iz(min))
The zener runs hottest at the highest input and the lowest load: Iz(max) = (Vin(max) − Vz)/Rs − IL(min) and Pz = Vz × Iz(max).
Worked example: Vin = 12 to 15 V, Vz = 5.1 V, IL up to 20 mA, Iz(min) = 5 mA. Rs = 6.9 / 0.025 = 276 Ω, so pick 270 Ω. With no load at 15 V, Iz = 9.9 / 270 = 36.7 mA and Pz = 5.1 × 0.0367 = 187 mW, which is fine for a 0.5 W zener. Rs dissipates 9.9² / 270 = 0.36 W, so use a 1 W resistor.
Exam questions and practical notes
- Find the range of load current over which the zener regulates.
- Find the minimum input voltage for regulation with a fixed Rs.
- Find the zener's maximum power dissipation.
- Find line regulation and load regulation using the zener resistance.
This tool uses the ideal model (constant Vz). Real zeners have a small dynamic resistance, so Vz rises slightly with current. For load currents above about 50 mA, a zener plus an emitter follower, or a linear regulator IC, wastes far less power.
Frequently asked questions
Why round Rs down and not up?
A smaller Rs passes more current, which keeps the zener in breakdown at the lowest input. Rounding up could starve the zener and lose regulation.
What is Iz(min)?
It is the smallest current that keeps the zener in breakdown, often called Izk. Datasheets give it; 1 to 5 mA is typical for small zeners.
What if the zener power is too high?
Use a higher wattage zener, reduce the input voltage range, or add a transistor so the zener only sets the voltage and the transistor carries the load current.
Is this tool free?
Yes. You enter your name and WhatsApp number once to see your result, and all other Uncodemy tools then open without the form.