The compliances for this kind of current source are:
1) Maximum Operating Output Current Ic
2) Zener’s Minimum Operating Current
First, considering a lowest stable beta, the base will sink current from the resistor. To be sure that the zener’s voltage never drops off due to a decreasing impedance from the base, the maximum base current must always be less than a 10% of the zener’s minimum current.
The emitter resistor sets the operating current, the minimum resistor is chosen with the criteria explained above.
Choose an emitter and zener resistors that accomplish this two compliances respectively, and have fun.
PD: You can use this circuit as a Voltage Source taking the output voltage from the emitter, but that’s another conversation.
Hope it works for you.
