This variable Zener diode circuit acts like a Zener diode with a breakdown voltage adjustable in a vast domain. The current through the voltage divider R1 and R2 must be higher current through the voltage divider should be higher compared with that of the transistors base. It depends only by R1 and has been set to 8 mA.
The breakdown voltage of this simulated Zener diode is adjustable between 5V and 45V. The circuit begins to stabilize at a current of 15 mA. The maximum current for a Zener is 4.7V/250mW is around 50 mA.
If you need to stabilize voltages above 15V, the transistor must be mounted on a heatsink. This measure must be taken if the current consumption of the circuit reaches 40-50 mA.