Zener Shunt Regulator Calculator
Enter supply voltage, Zener voltage, Zener test current, and load current to get the series resistor value with its no-load and full-load bounds, plus worst-case power dissipation.
Use the Zener Shunt Regulator Calculator
Circuit Parameters
Enter circuit parameters and click Calculate
Resistor bounds and worst-case power appear here
Series Resistor Bounds
The full-load value is the resistor to use in the circuit. It keeps the Zener above its test current at full load; the no-load value shows how much extra current the same resistor pushes into the Zener when the load is disconnected.
Nearest Standard Resistor (E24 series, full-load bound)
Summary
A Zener shunt regulator holds a load near Vz by diverting extra current through the Zener diode whenever the load draws less than the resistor can supply. This calculator takes the supply voltage, Zener voltage, Zener test current, and load current, then computes the series resistor at both ends of the load range: the no-load bound (load disconnected, all current forced through the Zener) and the full-load bound (the tighter constraint that keeps the Zener above its test current). The smaller of the two is the resistor to actually use, and the calculator flags a negative or undefined result instead of printing a number that does not correspond to a real circuit.
How it works
- Enter the supply voltage (Vin), the unregulated DC feeding the circuit.
- Enter the Zener voltage (Vz) from the diode datasheet.
- Enter the Zener test current (Izt), the minimum current the Zener needs to stay in regulation.
- Enter the load current (IL) the circuit draws at full load.
- The calculator computes R at no load (IL = 0) and R at full load (IL as entered); the smaller value is the resistor to use.
- Worst-case Zener current and power are computed with the chosen resistor at no load, since that is when the Zener carries the most current.
Use cases
- Size the series resistor for a basic Zener shunt regulator before breadboarding it.
- Check whether a design point is even feasible before ordering parts.
- Find the worst-case Zener power dissipation when the load can be disconnected.
- Compare the no-load and full-load resistor bounds to see how much margin a design has.
- Verify an existing resistor choice against the datasheet Zener test current.
- Estimate the resistor power rating needed with a safety margin.