To find the value for R, enter the maximum and minimum input voltages (Vin max and Vin min), the maximum load current (Iout max), and the zener voltage.
While simple, this type of shunt regulator is normally used when the maximum load current is small (in the low milliamp range). Typical this circuit is used to generate a reference or bias voltage and is not used as the primary voltage regulator.
This is primarily due to the somewhat poor regulation with large load current changes (depending on the sharpness of the zener’s knee) and the fact that the resistor will always dissipate the same amount power regardless of the load current. In addition, without any load current, the power dissipated by the zener is the same the the power dissipated by the load when the load current is at its maximum.
In contrast, series regulators such as the 7805 or LM317 dissipate a very small amount of power when unloaded and are able to maintain tighter regulation. That said, at maximum load current, the power dissipated by the zener in this shunt regulator is the same as the power dissipated by a series regulator for the same Vin max input voltage.